diff --git a/src/cpptoml/LICENSE b/src/cpptoml/LICENSE
deleted file mode 100644
index 8802c4fa5..000000000
--- a/src/cpptoml/LICENSE
+++ /dev/null
@@ -1,18 +0,0 @@
-Copyright (c) 2014 Chase Geigle
-
-Permission is hereby granted, free of charge, to any person obtaining a copy of
-this software and associated documentation files (the "Software"), to deal in
-the Software without restriction, including without limitation the rights to
-use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
-the Software, and to permit persons to whom the Software is furnished to do so,
-subject to the following conditions:
-
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
-FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
-COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
-IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
-CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/src/cpptoml/cpptoml.h b/src/cpptoml/cpptoml.h
deleted file mode 100644
index 5a00da3b4..000000000
--- a/src/cpptoml/cpptoml.h
+++ /dev/null
@@ -1,3668 +0,0 @@
-/**
- * @file cpptoml.h
- * @author Chase Geigle
- * @date May 2013
- */
-
-#ifndef CPPTOML_H
-#define CPPTOML_H
-
-#include <algorithm>
-#include <cassert>
-#include <clocale>
-#include <cstdint>
-#include <cstring>
-#include <fstream>
-#include <iomanip>
-#include <map>
-#include <memory>
-#include <sstream>
-#include <stdexcept>
-#include <string>
-#include <unordered_map>
-#include <vector>
-
-#if __cplusplus > 201103L
-#define CPPTOML_DEPRECATED(reason) [[deprecated(reason)]]
-#elif defined(__clang__)
-#define CPPTOML_DEPRECATED(reason) __attribute__((deprecated(reason)))
-#elif defined(__GNUG__)
-#define CPPTOML_DEPRECATED(reason) __attribute__((deprecated))
-#elif defined(_MSC_VER)
-#if _MSC_VER < 1910
-#define CPPTOML_DEPRECATED(reason) __declspec(deprecated)
-#else
-#define CPPTOML_DEPRECATED(reason) [[deprecated(reason)]]
-#endif
-#endif
-
-namespace cpptoml
-{
-class writer; // forward declaration
-class base;   // forward declaration
-#if defined(CPPTOML_USE_MAP)
-// a std::map will ensure that entries a sorted, albeit at a slight
-// performance penalty relative to the (default) unordered_map
-using string_to_base_map = std::map<std::string, std::shared_ptr<base>>;
-#else
-// by default an unordered_map is used for best performance as the
-// toml specification does not require entries to be sorted
-using string_to_base_map
-    = std::unordered_map<std::string, std::shared_ptr<base>>;
-#endif
-
-// if defined, `base` will retain type information in form of an enum class
-// such that static_cast can be used instead of dynamic_cast
-// #define CPPTOML_NO_RTTI
-
-template <class T>
-class option
-{
-  public:
-    option() : empty_{true}
-    {
-        // nothing
-    }
-
-    option(T value) : empty_{false}, value_(std::move(value))
-    {
-        // nothing
-    }
-
-    explicit operator bool() const
-    {
-        return !empty_;
-    }
-
-    const T& operator*() const
-    {
-        return value_;
-    }
-
-    const T* operator->() const
-    {
-        return &value_;
-    }
-
-    template <class U>
-    T value_or(U&& alternative) const
-    {
-        if (!empty_)
-            return value_;
-        return static_cast<T>(std::forward<U>(alternative));
-    }
-
-  private:
-    bool empty_;
-    T value_;
-};
-
-struct local_date
-{
-    int year = 0;
-    int month = 0;
-    int day = 0;
-};
-
-struct local_time
-{
-    int hour = 0;
-    int minute = 0;
-    int second = 0;
-    int microsecond = 0;
-};
-
-struct zone_offset
-{
-    int hour_offset = 0;
-    int minute_offset = 0;
-};
-
-struct local_datetime : local_date, local_time
-{
-};
-
-struct offset_datetime : local_datetime, zone_offset
-{
-    static inline struct offset_datetime from_zoned(const struct tm& t)
-    {
-        offset_datetime dt;
-        dt.year = t.tm_year + 1900;
-        dt.month = t.tm_mon + 1;
-        dt.day = t.tm_mday;
-        dt.hour = t.tm_hour;
-        dt.minute = t.tm_min;
-        dt.second = t.tm_sec;
-
-        char buf[16];
-        strftime(buf, 16, "%z", &t);
-
-        int offset = std::stoi(buf);
-        dt.hour_offset = offset / 100;
-        dt.minute_offset = offset % 100;
-        return dt;
-    }
-
-    CPPTOML_DEPRECATED("from_local has been renamed to from_zoned")
-    static inline struct offset_datetime from_local(const struct tm& t)
-    {
-        return from_zoned(t);
-    }
-
-    static inline struct offset_datetime from_utc(const struct tm& t)
-    {
-        offset_datetime dt;
-        dt.year = t.tm_year + 1900;
-        dt.month = t.tm_mon + 1;
-        dt.day = t.tm_mday;
-        dt.hour = t.tm_hour;
-        dt.minute = t.tm_min;
-        dt.second = t.tm_sec;
-        return dt;
-    }
-};
-
-CPPTOML_DEPRECATED("datetime has been renamed to offset_datetime")
-typedef offset_datetime datetime;
-
-class fill_guard
-{
-  public:
-    fill_guard(std::ostream& os) : os_(os), fill_{os.fill()}
-    {
-        // nothing
-    }
-
-    ~fill_guard()
-    {
-        os_.fill(fill_);
-    }
-
-  private:
-    std::ostream& os_;
-    std::ostream::char_type fill_;
-};
-
-inline std::ostream& operator<<(std::ostream& os, const local_date& dt)
-{
-    fill_guard g{os};
-    os.fill('0');
-
-    using std::setw;
-    os << setw(4) << dt.year << "-" << setw(2) << dt.month << "-" << setw(2)
-       << dt.day;
-
-    return os;
-}
-
-inline std::ostream& operator<<(std::ostream& os, const local_time& ltime)
-{
-    fill_guard g{os};
-    os.fill('0');
-
-    using std::setw;
-    os << setw(2) << ltime.hour << ":" << setw(2) << ltime.minute << ":"
-       << setw(2) << ltime.second;
-
-    if (ltime.microsecond > 0)
-    {
-        os << ".";
-        int power = 100000;
-        for (int curr_us = ltime.microsecond; curr_us; power /= 10)
-        {
-            auto num = curr_us / power;
-            os << num;
-            curr_us -= num * power;
-        }
-    }
-
-    return os;
-}
-
-inline std::ostream& operator<<(std::ostream& os, const zone_offset& zo)
-{
-    fill_guard g{os};
-    os.fill('0');
-
-    using std::setw;
-
-    if (zo.hour_offset != 0 || zo.minute_offset != 0)
-    {
-        if (zo.hour_offset > 0)
-        {
-            os << "+";
-        }
-        else
-        {
-            os << "-";
-        }
-        os << setw(2) << std::abs(zo.hour_offset) << ":" << setw(2)
-           << std::abs(zo.minute_offset);
-    }
-    else
-    {
-        os << "Z";
-    }
-
-    return os;
-}
-
-inline std::ostream& operator<<(std::ostream& os, const local_datetime& dt)
-{
-    return os << static_cast<const local_date&>(dt) << "T"
-              << static_cast<const local_time&>(dt);
-}
-
-inline std::ostream& operator<<(std::ostream& os, const offset_datetime& dt)
-{
-    return os << static_cast<const local_datetime&>(dt)
-              << static_cast<const zone_offset&>(dt);
-}
-
-template <class T, class... Ts>
-struct is_one_of;
-
-template <class T, class V>
-struct is_one_of<T, V> : std::is_same<T, V>
-{
-};
-
-template <class T, class V, class... Ts>
-struct is_one_of<T, V, Ts...>
-{
-    const static bool value
-        = std::is_same<T, V>::value || is_one_of<T, Ts...>::value;
-};
-
-template <class T>
-class value;
-
-template <class T>
-struct valid_value
-    : is_one_of<T, std::string, int64_t, double, bool, local_date, local_time,
-                local_datetime, offset_datetime>
-{
-};
-
-template <class T, class Enable = void>
-struct value_traits;
-
-template <class T>
-struct valid_value_or_string_convertible
-{
-
-    const static bool value = valid_value<typename std::decay<T>::type>::value
-                              || std::is_convertible<T, std::string>::value;
-};
-
-template <class T>
-struct value_traits<T, typename std::enable_if<
-                           valid_value_or_string_convertible<T>::value>::type>
-{
-    using value_type = typename std::conditional<
-        valid_value<typename std::decay<T>::type>::value,
-        typename std::decay<T>::type, std::string>::type;
-
-    using type = value<value_type>;
-
-    static value_type construct(T&& val)
-    {
-        return value_type(val);
-    }
-};
-
-template <class T>
-struct value_traits<
-    T,
-    typename std::enable_if<
-        !valid_value_or_string_convertible<T>::value
-        && std::is_floating_point<typename std::decay<T>::type>::value>::type>
-{
-    using value_type = typename std::decay<T>::type;
-
-    using type = value<double>;
-
-    static value_type construct(T&& val)
-    {
-        return value_type(val);
-    }
-};
-
-template <class T>
-struct value_traits<
-    T, typename std::enable_if<
-           !valid_value_or_string_convertible<T>::value
-           && !std::is_floating_point<typename std::decay<T>::type>::value
-           && std::is_signed<typename std::decay<T>::type>::value>::type>
-{
-    using value_type = int64_t;
-
-    using type = value<int64_t>;
-
-    static value_type construct(T&& val)
-    {
-        if (val < (std::numeric_limits<int64_t>::min)())
-            throw std::underflow_error{"constructed value cannot be "
-                                       "represented by a 64-bit signed "
-                                       "integer"};
-
-        if (val > (std::numeric_limits<int64_t>::max)())
-            throw std::overflow_error{"constructed value cannot be represented "
-                                      "by a 64-bit signed integer"};
-
-        return static_cast<int64_t>(val);
-    }
-};
-
-template <class T>
-struct value_traits<
-    T, typename std::enable_if<
-           !valid_value_or_string_convertible<T>::value
-           && std::is_unsigned<typename std::decay<T>::type>::value>::type>
-{
-    using value_type = int64_t;
-
-    using type = value<int64_t>;
-
-    static value_type construct(T&& val)
-    {
-        if (val > static_cast<uint64_t>((std::numeric_limits<int64_t>::max)()))
-            throw std::overflow_error{"constructed value cannot be represented "
-                                      "by a 64-bit signed integer"};
-
-        return static_cast<int64_t>(val);
-    }
-};
-
-class array;
-class table;
-class table_array;
-
-template <class T>
-struct array_of_trait
-{
-    using return_type = option<std::vector<T>>;
-};
-
-template <>
-struct array_of_trait<array>
-{
-    using return_type = option<std::vector<std::shared_ptr<array>>>;
-};
-
-template <class T>
-inline std::shared_ptr<typename value_traits<T>::type> make_value(T&& val);
-inline std::shared_ptr<array> make_array();
-
-namespace detail
-{
-template <class T>
-inline std::shared_ptr<T> make_element();
-}
-
-inline std::shared_ptr<table> make_table();
-inline std::shared_ptr<table_array> make_table_array(bool is_inline = false);
-
-#if defined(CPPTOML_NO_RTTI)
-/// Base type used to store underlying data type explicitly if RTTI is disabled
-enum class base_type
-{
-    NONE,
-    STRING,
-    LOCAL_TIME,
-    LOCAL_DATE,
-    LOCAL_DATETIME,
-    OFFSET_DATETIME,
-    INT,
-    FLOAT,
-    BOOL,
-    TABLE,
-    ARRAY,
-    TABLE_ARRAY
-};
-
-/// Type traits class to convert C++ types to enum member
-template <class T>
-struct base_type_traits;
-
-template <>
-struct base_type_traits<std::string>
-{
-    static const base_type type = base_type::STRING;
-};
-
-template <>
-struct base_type_traits<local_time>
-{
-    static const base_type type = base_type::LOCAL_TIME;
-};
-
-template <>
-struct base_type_traits<local_date>
-{
-    static const base_type type = base_type::LOCAL_DATE;
-};
-
-template <>
-struct base_type_traits<local_datetime>
-{
-    static const base_type type = base_type::LOCAL_DATETIME;
-};
-
-template <>
-struct base_type_traits<offset_datetime>
-{
-    static const base_type type = base_type::OFFSET_DATETIME;
-};
-
-template <>
-struct base_type_traits<int64_t>
-{
-    static const base_type type = base_type::INT;
-};
-
-template <>
-struct base_type_traits<double>
-{
-    static const base_type type = base_type::FLOAT;
-};
-
-template <>
-struct base_type_traits<bool>
-{
-    static const base_type type = base_type::BOOL;
-};
-
-template <>
-struct base_type_traits<table>
-{
-    static const base_type type = base_type::TABLE;
-};
-
-template <>
-struct base_type_traits<array>
-{
-    static const base_type type = base_type::ARRAY;
-};
-
-template <>
-struct base_type_traits<table_array>
-{
-    static const base_type type = base_type::TABLE_ARRAY;
-};
-#endif
-
-/**
- * A generic base TOML value used for type erasure.
- */
-class base : public std::enable_shared_from_this<base>
-{
-  public:
-    virtual ~base() = default;
-
-    virtual std::shared_ptr<base> clone() const = 0;
-
-    /**
-     * Determines if the given TOML element is a value.
-     */
-    virtual bool is_value() const
-    {
-        return false;
-    }
-
-    /**
-     * Determines if the given TOML element is a table.
-     */
-    virtual bool is_table() const
-    {
-        return false;
-    }
-
-    /**
-     * Converts the TOML element into a table.
-     */
-    std::shared_ptr<table> as_table()
-    {
-        if (is_table())
-            return std::static_pointer_cast<table>(shared_from_this());
-        return nullptr;
-    }
-    /**
-     * Determines if the TOML element is an array of "leaf" elements.
-     */
-    virtual bool is_array() const
-    {
-        return false;
-    }
-
-    /**
-     * Converts the TOML element to an array.
-     */
-    std::shared_ptr<array> as_array()
-    {
-        if (is_array())
-            return std::static_pointer_cast<array>(shared_from_this());
-        return nullptr;
-    }
-
-    /**
-     * Determines if the given TOML element is an array of tables.
-     */
-    virtual bool is_table_array() const
-    {
-        return false;
-    }
-
-    /**
-     * Converts the TOML element into a table array.
-     */
-    std::shared_ptr<table_array> as_table_array()
-    {
-        if (is_table_array())
-            return std::static_pointer_cast<table_array>(shared_from_this());
-        return nullptr;
-    }
-
-    /**
-     * Attempts to coerce the TOML element into a concrete TOML value
-     * of type T.
-     */
-    template <class T>
-    std::shared_ptr<value<T>> as();
-
-    template <class T>
-    std::shared_ptr<const value<T>> as() const;
-
-    template <class Visitor, class... Args>
-    void accept(Visitor&& visitor, Args&&... args) const;
-
-#if defined(CPPTOML_NO_RTTI)
-    base_type type() const
-    {
-        return type_;
-    }
-
-  protected:
-    base(const base_type t) : type_(t)
-    {
-        // nothing
-    }
-
-  private:
-    const base_type type_ = base_type::NONE;
-
-#else
-  protected:
-    base()
-    {
-        // nothing
-    }
-#endif
-};
-
-/**
- * A concrete TOML value representing the "leaves" of the "tree".
- */
-template <class T>
-class value : public base
-{
-    struct make_shared_enabler
-    {
-        // nothing; this is a private key accessible only to friends
-    };
-
-    template <class U>
-    friend std::shared_ptr<typename value_traits<U>::type>
-    cpptoml::make_value(U&& val);
-
-  public:
-    static_assert(valid_value<T>::value, "invalid value type");
-
-    std::shared_ptr<base> clone() const override;
-
-    value(const make_shared_enabler&, const T& val) : value(val)
-    {
-        // nothing; note that users cannot actually invoke this function
-        // because they lack access to the make_shared_enabler.
-    }
-
-    bool is_value() const override
-    {
-        return true;
-    }
-
-    /**
-     * Gets the data associated with this value.
-     */
-    T& get()
-    {
-        return data_;
-    }
-
-    /**
-     * Gets the data associated with this value. Const version.
-     */
-    const T& get() const
-    {
-        return data_;
-    }
-
-  private:
-    T data_;
-
-    /**
-     * Constructs a value from the given data.
-     */
-#if defined(CPPTOML_NO_RTTI)
-    value(const T& val) : base(base_type_traits<T>::type), data_(val)
-    {
-    }
-#else
-    value(const T& val) : data_(val)
-    {
-    }
-#endif
-
-    value(const value& val) = delete;
-    value& operator=(const value& val) = delete;
-};
-
-template <class T>
-std::shared_ptr<typename value_traits<T>::type> make_value(T&& val)
-{
-    using value_type = typename value_traits<T>::type;
-    using enabler = typename value_type::make_shared_enabler;
-    return std::make_shared<value_type>(
-        enabler{}, value_traits<T>::construct(std::forward<T>(val)));
-}
-
-template <class T>
-inline std::shared_ptr<value<T>> base::as()
-{
-#if defined(CPPTOML_NO_RTTI)
-    if (type() == base_type_traits<T>::type)
-        return std::static_pointer_cast<value<T>>(shared_from_this());
-    else
-        return nullptr;
-#else
-    return std::dynamic_pointer_cast<value<T>>(shared_from_this());
-#endif
-}
-
-// special case value<double> to allow getting an integer parameter as a
-// double value
-template <>
-inline std::shared_ptr<value<double>> base::as()
-{
-#if defined(CPPTOML_NO_RTTI)
-    if (type() == base_type::FLOAT)
-        return std::static_pointer_cast<value<double>>(shared_from_this());
-
-    if (type() == base_type::INT)
-    {
-        auto v = std::static_pointer_cast<value<int64_t>>(shared_from_this());
-        return make_value<double>(static_cast<double>(v->get()));
-    }
-#else
-    if (auto v = std::dynamic_pointer_cast<value<double>>(shared_from_this()))
-        return v;
-
-    if (auto v = std::dynamic_pointer_cast<value<int64_t>>(shared_from_this()))
-        return make_value<double>(static_cast<double>(v->get()));
-#endif
-
-    return nullptr;
-}
-
-template <class T>
-inline std::shared_ptr<const value<T>> base::as() const
-{
-#if defined(CPPTOML_NO_RTTI)
-    if (type() == base_type_traits<T>::type)
-        return std::static_pointer_cast<const value<T>>(shared_from_this());
-    else
-        return nullptr;
-#else
-    return std::dynamic_pointer_cast<const value<T>>(shared_from_this());
-#endif
-}
-
-// special case value<double> to allow getting an integer parameter as a
-// double value
-template <>
-inline std::shared_ptr<const value<double>> base::as() const
-{
-#if defined(CPPTOML_NO_RTTI)
-    if (type() == base_type::FLOAT)
-        return std::static_pointer_cast<const value<double>>(
-            shared_from_this());
-
-    if (type() == base_type::INT)
-    {
-        auto v = as<int64_t>();
-        // the below has to be a non-const value<double> due to a bug in
-        // libc++: https://llvm.org/bugs/show_bug.cgi?id=18843
-        return make_value<double>(static_cast<double>(v->get()));
-    }
-#else
-    if (auto v
-        = std::dynamic_pointer_cast<const value<double>>(shared_from_this()))
-        return v;
-
-    if (auto v = as<int64_t>())
-    {
-        // the below has to be a non-const value<double> due to a bug in
-        // libc++: https://llvm.org/bugs/show_bug.cgi?id=18843
-        return make_value<double>(static_cast<double>(v->get()));
-    }
-#endif
-
-    return nullptr;
-}
-
-/**
- * Exception class for array insertion errors.
- */
-class array_exception : public std::runtime_error
-{
-  public:
-    array_exception(const std::string& err) : std::runtime_error{err}
-    {
-    }
-};
-
-class array : public base
-{
-  public:
-    friend std::shared_ptr<array> make_array();
-
-    std::shared_ptr<base> clone() const override;
-
-    virtual bool is_array() const override
-    {
-        return true;
-    }
-
-    using size_type = std::size_t;
-
-    /**
-     * arrays can be iterated over
-     */
-    using iterator = std::vector<std::shared_ptr<base>>::iterator;
-
-    /**
-     * arrays can be iterated over.  Const version.
-     */
-    using const_iterator = std::vector<std::shared_ptr<base>>::const_iterator;
-
-    iterator begin()
-    {
-        return values_.begin();
-    }
-
-    const_iterator begin() const
-    {
-        return values_.begin();
-    }
-
-    iterator end()
-    {
-        return values_.end();
-    }
-
-    const_iterator end() const
-    {
-        return values_.end();
-    }
-
-    /**
-     * Obtains the array (vector) of base values.
-     */
-    std::vector<std::shared_ptr<base>>& get()
-    {
-        return values_;
-    }
-
-    /**
-     * Obtains the array (vector) of base values. Const version.
-     */
-    const std::vector<std::shared_ptr<base>>& get() const
-    {
-        return values_;
-    }
-
-    std::shared_ptr<base> at(size_t idx) const
-    {
-        return values_.at(idx);
-    }
-
-    /**
-     * Obtains an array of value<T>s. Note that elements may be
-     * nullptr if they cannot be converted to a value<T>.
-     */
-    template <class T>
-    std::vector<std::shared_ptr<value<T>>> array_of() const
-    {
-        std::vector<std::shared_ptr<value<T>>> result(values_.size());
-
-        std::transform(values_.begin(), values_.end(), result.begin(),
-                       [&](std::shared_ptr<base> v) { return v->as<T>(); });
-
-        return result;
-    }
-
-    /**
-     * Obtains a option<vector<T>>. The option will be empty if the array
-     * contains values that are not of type T.
-     */
-    template <class T>
-    inline typename array_of_trait<T>::return_type get_array_of() const
-    {
-        std::vector<T> result;
-        result.reserve(values_.size());
-
-        for (const auto& val : values_)
-        {
-            if (auto v = val->as<T>())
-                result.push_back(v->get());
-            else
-                return {};
-        }
-
-        return {std::move(result)};
-    }
-
-    /**
-     * Obtains an array of arrays. Note that elements may be nullptr
-     * if they cannot be converted to a array.
-     */
-    std::vector<std::shared_ptr<array>> nested_array() const
-    {
-        std::vector<std::shared_ptr<array>> result(values_.size());
-
-        std::transform(values_.begin(), values_.end(), result.begin(),
-                       [&](std::shared_ptr<base> v) -> std::shared_ptr<array> {
-                           if (v->is_array())
-                               return std::static_pointer_cast<array>(v);
-                           return std::shared_ptr<array>{};
-                       });
-
-        return result;
-    }
-
-    /**
-     * Add a value to the end of the array
-     */
-    template <class T>
-    void push_back(const std::shared_ptr<value<T>>& val)
-    {
-        if (values_.empty() || values_[0]->as<T>())
-        {
-            values_.push_back(val);
-        }
-        else
-        {
-            throw array_exception{"Arrays must be homogenous."};
-        }
-    }
-
-    /**
-     * Add an array to the end of the array
-     */
-    void push_back(const std::shared_ptr<array>& val)
-    {
-        if (values_.empty() || values_[0]->is_array())
-        {
-            values_.push_back(val);
-        }
-        else
-        {
-            throw array_exception{"Arrays must be homogenous."};
-        }
-    }
-
-    /**
-     * Convenience function for adding a simple element to the end
-     * of the array.
-     */
-    template <class T>
-    void push_back(T&& val, typename value_traits<T>::type* = 0)
-    {
-        push_back(make_value(std::forward<T>(val)));
-    }
-
-    /**
-     * Insert a value into the array
-     */
-    template <class T>
-    iterator insert(iterator position, const std::shared_ptr<value<T>>& value)
-    {
-        if (values_.empty() || values_[0]->as<T>())
-        {
-            return values_.insert(position, value);
-        }
-        else
-        {
-            throw array_exception{"Arrays must be homogenous."};
-        }
-    }
-
-    /**
-     * Insert an array into the array
-     */
-    iterator insert(iterator position, const std::shared_ptr<array>& value)
-    {
-        if (values_.empty() || values_[0]->is_array())
-        {
-            return values_.insert(position, value);
-        }
-        else
-        {
-            throw array_exception{"Arrays must be homogenous."};
-        }
-    }
-
-    /**
-     * Convenience function for inserting a simple element in the array
-     */
-    template <class T>
-    iterator insert(iterator position, T&& val,
-                    typename value_traits<T>::type* = 0)
-    {
-        return insert(position, make_value(std::forward<T>(val)));
-    }
-
-    /**
-     * Erase an element from the array
-     */
-    iterator erase(iterator position)
-    {
-        return values_.erase(position);
-    }
-
-    /**
-     * Clear the array
-     */
-    void clear()
-    {
-        values_.clear();
-    }
-
-    /**
-     * Reserve space for n values.
-     */
-    void reserve(size_type n)
-    {
-        values_.reserve(n);
-    }
-
-  private:
-#if defined(CPPTOML_NO_RTTI)
-    array() : base(base_type::ARRAY)
-    {
-        // empty
-    }
-#else
-    array() = default;
-#endif
-
-    template <class InputIterator>
-    array(InputIterator begin, InputIterator end) : values_{begin, end}
-    {
-        // nothing
-    }
-
-    array(const array& obj) = delete;
-    array& operator=(const array& obj) = delete;
-
-    std::vector<std::shared_ptr<base>> values_;
-};
-
-inline std::shared_ptr<array> make_array()
-{
-    struct make_shared_enabler : public array
-    {
-        make_shared_enabler()
-        {
-            // nothing
-        }
-    };
-
-    return std::make_shared<make_shared_enabler>();
-}
-
-namespace detail
-{
-template <>
-inline std::shared_ptr<array> make_element<array>()
-{
-    return make_array();
-}
-} // namespace detail
-
-/**
- * Obtains a option<vector<T>>. The option will be empty if the array
- * contains values that are not of type T.
- */
-template <>
-inline typename array_of_trait<array>::return_type
-array::get_array_of<array>() const
-{
-    std::vector<std::shared_ptr<array>> result;
-    result.reserve(values_.size());
-
-    for (const auto& val : values_)
-    {
-        if (auto v = val->as_array())
-            result.push_back(v);
-        else
-            return {};
-    }
-
-    return {std::move(result)};
-}
-
-class table;
-
-class table_array : public base
-{
-    friend class table;
-    friend std::shared_ptr<table_array> make_table_array(bool);
-
-  public:
-    std::shared_ptr<base> clone() const override;
-
-    using size_type = std::size_t;
-
-    /**
-     * arrays can be iterated over
-     */
-    using iterator = std::vector<std::shared_ptr<table>>::iterator;
-
-    /**
-     * arrays can be iterated over.  Const version.
-     */
-    using const_iterator = std::vector<std::shared_ptr<table>>::const_iterator;
-
-    iterator begin()
-    {
-        return array_.begin();
-    }
-
-    const_iterator begin() const
-    {
-        return array_.begin();
-    }
-
-    iterator end()
-    {
-        return array_.end();
-    }
-
-    const_iterator end() const
-    {
-        return array_.end();
-    }
-
-    virtual bool is_table_array() const override
-    {
-        return true;
-    }
-
-    std::vector<std::shared_ptr<table>>& get()
-    {
-        return array_;
-    }
-
-    const std::vector<std::shared_ptr<table>>& get() const
-    {
-        return array_;
-    }
-
-    /**
-     * Add a table to the end of the array
-     */
-    void push_back(const std::shared_ptr<table>& val)
-    {
-        array_.push_back(val);
-    }
-
-    /**
-     * Insert a table into the array
-     */
-    iterator insert(iterator position, const std::shared_ptr<table>& value)
-    {
-        return array_.insert(position, value);
-    }
-
-    /**
-     * Erase an element from the array
-     */
-    iterator erase(iterator position)
-    {
-        return array_.erase(position);
-    }
-
-    /**
-     * Clear the array
-     */
-    void clear()
-    {
-        array_.clear();
-    }
-
-    /**
-     * Reserve space for n tables.
-     */
-    void reserve(size_type n)
-    {
-        array_.reserve(n);
-    }
-
-    /**
-     * Whether or not the table array is declared inline. This mostly
-     * matters for parsing, where statically defined arrays cannot be
-     * appended to using the array-of-table syntax.
-     */
-    bool is_inline() const
-    {
-        return is_inline_;
-    }
-
-  private:
-#if defined(CPPTOML_NO_RTTI)
-    table_array(bool is_inline = false)
-        : base(base_type::TABLE_ARRAY), is_inline_(is_inline)
-    {
-        // nothing
-    }
-#else
-    table_array(bool is_inline = false) : is_inline_(is_inline)
-    {
-        // nothing
-    }
-#endif
-
-    table_array(const table_array& obj) = delete;
-    table_array& operator=(const table_array& rhs) = delete;
-
-    std::vector<std::shared_ptr<table>> array_;
-    const bool is_inline_ = false;
-};
-
-inline std::shared_ptr<table_array> make_table_array(bool is_inline)
-{
-    struct make_shared_enabler : public table_array
-    {
-        make_shared_enabler(bool mse_is_inline) : table_array(mse_is_inline)
-        {
-            // nothing
-        }
-    };
-
-    return std::make_shared<make_shared_enabler>(is_inline);
-}
-
-namespace detail
-{
-template <>
-inline std::shared_ptr<table_array> make_element<table_array>()
-{
-    return make_table_array(true);
-}
-} // namespace detail
-
-// The below are overloads for fetching specific value types out of a value
-// where special casting behavior (like bounds checking) is desired
-
-template <class T>
-typename std::enable_if<!std::is_floating_point<T>::value
-                            && std::is_signed<T>::value,
-                        option<T>>::type
-get_impl(const std::shared_ptr<base>& elem)
-{
-    if (auto v = elem->as<int64_t>())
-    {
-        if (v->get() < (std::numeric_limits<T>::min)())
-            throw std::underflow_error{
-                "T cannot represent the value requested in get"};
-
-        if (v->get() > (std::numeric_limits<T>::max)())
-            throw std::overflow_error{
-                "T cannot represent the value requested in get"};
-
-        return {static_cast<T>(v->get())};
-    }
-    else
-    {
-        return {};
-    }
-}
-
-template <class T>
-typename std::enable_if<!std::is_same<T, bool>::value
-                            && std::is_unsigned<T>::value,
-                        option<T>>::type
-get_impl(const std::shared_ptr<base>& elem)
-{
-    if (auto v = elem->as<int64_t>())
-    {
-        if (v->get() < 0)
-            throw std::underflow_error{"T cannot store negative value in get"};
-
-        if (static_cast<uint64_t>(v->get()) > (std::numeric_limits<T>::max)())
-            throw std::overflow_error{
-                "T cannot represent the value requested in get"};
-
-        return {static_cast<T>(v->get())};
-    }
-    else
-    {
-        return {};
-    }
-}
-
-template <class T>
-typename std::enable_if<!std::is_integral<T>::value
-                            || std::is_same<T, bool>::value,
-                        option<T>>::type
-get_impl(const std::shared_ptr<base>& elem)
-{
-    if (auto v = elem->as<T>())
-    {
-        return {v->get()};
-    }
-    else
-    {
-        return {};
-    }
-}
-
-/**
- * Represents a TOML keytable.
- */
-class table : public base
-{
-  public:
-    friend class table_array;
-    friend std::shared_ptr<table> make_table();
-
-    std::shared_ptr<base> clone() const override;
-
-    /**
-     * tables can be iterated over.
-     */
-    using iterator = string_to_base_map::iterator;
-
-    /**
-     * tables can be iterated over. Const version.
-     */
-    using const_iterator = string_to_base_map::const_iterator;
-
-    iterator begin()
-    {
-        return map_.begin();
-    }
-
-    const_iterator begin() const
-    {
-        return map_.begin();
-    }
-
-    iterator end()
-    {
-        return map_.end();
-    }
-
-    const_iterator end() const
-    {
-        return map_.end();
-    }
-
-    bool is_table() const override
-    {
-        return true;
-    }
-
-    bool empty() const
-    {
-        return map_.empty();
-    }
-
-    /**
-     * Determines if this key table contains the given key.
-     */
-    bool contains(const std::string& key) const
-    {
-        return map_.find(key) != map_.end();
-    }
-
-    /**
-     * Determines if this key table contains the given key. Will
-     * resolve "qualified keys". Qualified keys are the full access
-     * path separated with dots like "grandparent.parent.child".
-     */
-    bool contains_qualified(const std::string& key) const
-    {
-        return resolve_qualified(key);
-    }
-
-    /**
-     * Obtains the base for a given key.
-     * @throw std::out_of_range if the key does not exist
-     */
-    std::shared_ptr<base> get(const std::string& key) const
-    {
-        return map_.at(key);
-    }
-
-    /**
-     * Obtains the base for a given key. Will resolve "qualified
-     * keys". Qualified keys are the full access path separated with
-     * dots like "grandparent.parent.child".
-     *
-     * @throw std::out_of_range if the key does not exist
-     */
-    std::shared_ptr<base> get_qualified(const std::string& key) const
-    {
-        std::shared_ptr<base> p;
-        resolve_qualified(key, &p);
-        return p;
-    }
-
-    /**
-     * Obtains a table for a given key, if possible.
-     */
-    std::shared_ptr<table> get_table(const std::string& key) const
-    {
-        if (contains(key) && get(key)->is_table())
-            return std::static_pointer_cast<table>(get(key));
-        return nullptr;
-    }
-
-    /**
-     * Obtains a table for a given key, if possible. Will resolve
-     * "qualified keys".
-     */
-    std::shared_ptr<table> get_table_qualified(const std::string& key) const
-    {
-        if (contains_qualified(key) && get_qualified(key)->is_table())
-            return std::static_pointer_cast<table>(get_qualified(key));
-        return nullptr;
-    }
-
-    /**
-     * Obtains an array for a given key.
-     */
-    std::shared_ptr<array> get_array(const std::string& key) const
-    {
-        if (!contains(key))
-            return nullptr;
-        return get(key)->as_array();
-    }
-
-    /**
-     * Obtains an array for a given key. Will resolve "qualified keys".
-     */
-    std::shared_ptr<array> get_array_qualified(const std::string& key) const
-    {
-        if (!contains_qualified(key))
-            return nullptr;
-        return get_qualified(key)->as_array();
-    }
-
-    /**
-     * Obtains a table_array for a given key, if possible.
-     */
-    std::shared_ptr<table_array> get_table_array(const std::string& key) const
-    {
-        if (!contains(key))
-            return nullptr;
-        return get(key)->as_table_array();
-    }
-
-    /**
-     * Obtains a table_array for a given key, if possible. Will resolve
-     * "qualified keys".
-     */
-    std::shared_ptr<table_array>
-    get_table_array_qualified(const std::string& key) const
-    {
-        if (!contains_qualified(key))
-            return nullptr;
-        return get_qualified(key)->as_table_array();
-    }
-
-    /**
-     * Helper function that attempts to get a value corresponding
-     * to the template parameter from a given key.
-     */
-    template <class T>
-    option<T> get_as(const std::string& key) const
-    {
-        try
-        {
-            return get_impl<T>(get(key));
-        }
-        catch (const std::out_of_range&)
-        {
-            return {};
-        }
-    }
-
-    /**
-     * Helper function that attempts to get a value corresponding
-     * to the template parameter from a given key. Will resolve "qualified
-     * keys".
-     */
-    template <class T>
-    option<T> get_qualified_as(const std::string& key) const
-    {
-        try
-        {
-            return get_impl<T>(get_qualified(key));
-        }
-        catch (const std::out_of_range&)
-        {
-            return {};
-        }
-    }
-
-    /**
-     * Helper function that attempts to get an array of values of a given
-     * type corresponding to the template parameter for a given key.
-     *
-     * If the key doesn't exist, doesn't exist as an array type, or one or
-     * more keys inside the array type are not of type T, an empty option
-     * is returned. Otherwise, an option containing a vector of the values
-     * is returned.
-     */
-    template <class T>
-    inline typename array_of_trait<T>::return_type
-    get_array_of(const std::string& key) const
-    {
-        if (auto v = get_array(key))
-        {
-            std::vector<T> result;
-            result.reserve(v->get().size());
-
-            for (const auto& b : v->get())
-            {
-                if (auto val = b->as<T>())
-                    result.push_back(val->get());
-                else
-                    return {};
-            }
-            return {std::move(result)};
-        }
-
-        return {};
-    }
-
-    /**
-     * Helper function that attempts to get an array of values of a given
-     * type corresponding to the template parameter for a given key. Will
-     * resolve "qualified keys".
-     *
-     * If the key doesn't exist, doesn't exist as an array type, or one or
-     * more keys inside the array type are not of type T, an empty option
-     * is returned. Otherwise, an option containing a vector of the values
-     * is returned.
-     */
-    template <class T>
-    inline typename array_of_trait<T>::return_type
-    get_qualified_array_of(const std::string& key) const
-    {
-        if (auto v = get_array_qualified(key))
-        {
-            std::vector<T> result;
-            result.reserve(v->get().size());
-
-            for (const auto& b : v->get())
-            {
-                if (auto val = b->as<T>())
-                    result.push_back(val->get());
-                else
-                    return {};
-            }
-            return {std::move(result)};
-        }
-
-        return {};
-    }
-
-    /**
-     * Adds an element to the keytable.
-     */
-    void insert(const std::string& key, const std::shared_ptr<base>& value)
-    {
-        map_[key] = value;
-    }
-
-    /**
-     * Convenience shorthand for adding a simple element to the
-     * keytable.
-     */
-    template <class T>
-    void insert(const std::string& key, T&& val,
-                typename value_traits<T>::type* = 0)
-    {
-        insert(key, make_value(std::forward<T>(val)));
-    }
-
-    /**
-     * Removes an element from the table.
-     */
-    void erase(const std::string& key)
-    {
-        map_.erase(key);
-    }
-
-  private:
-#if defined(CPPTOML_NO_RTTI)
-    table() : base(base_type::TABLE)
-    {
-        // nothing
-    }
-#else
-    table()
-    {
-        // nothing
-    }
-#endif
-
-    table(const table& obj) = delete;
-    table& operator=(const table& rhs) = delete;
-
-    std::vector<std::string> split(const std::string& value,
-                                   char separator) const
-    {
-        std::vector<std::string> result;
-        std::string::size_type p = 0;
-        std::string::size_type q;
-        while ((q = value.find(separator, p)) != std::string::npos)
-        {
-            result.emplace_back(value, p, q - p);
-            p = q + 1;
-        }
-        result.emplace_back(value, p);
-        return result;
-    }
-
-    // If output parameter p is specified, fill it with the pointer to the
-    // specified entry and throw std::out_of_range if it couldn't be found.
-    //
-    // Otherwise, just return true if the entry could be found or false
-    // otherwise and do not throw.
-    bool resolve_qualified(const std::string& key,
-                           std::shared_ptr<base>* p = nullptr) const
-    {
-        auto parts = split(key, '.');
-        auto last_key = parts.back();
-        parts.pop_back();
-
-        auto cur_table = this;
-        for (const auto& part : parts)
-        {
-            cur_table = cur_table->get_table(part).get();
-            if (!cur_table)
-            {
-                if (!p)
-                    return false;
-
-                throw std::out_of_range{key + " is not a valid key"};
-            }
-        }
-
-        if (!p)
-            return cur_table->map_.count(last_key) != 0;
-
-        *p = cur_table->map_.at(last_key);
-        return true;
-    }
-
-    string_to_base_map map_;
-};
-
-/**
- * Helper function that attempts to get an array of arrays for a given
- * key.
- *
- * If the key doesn't exist, doesn't exist as an array type, or one or
- * more keys inside the array type are not of type T, an empty option
- * is returned. Otherwise, an option containing a vector of the values
- * is returned.
- */
-template <>
-inline typename array_of_trait<array>::return_type
-table::get_array_of<array>(const std::string& key) const
-{
-    if (auto v = get_array(key))
-    {
-        std::vector<std::shared_ptr<array>> result;
-        result.reserve(v->get().size());
-
-        for (const auto& b : v->get())
-        {
-            if (auto val = b->as_array())
-                result.push_back(val);
-            else
-                return {};
-        }
-
-        return {std::move(result)};
-    }
-
-    return {};
-}
-
-/**
- * Helper function that attempts to get an array of arrays for a given
- * key. Will resolve "qualified keys".
- *
- * If the key doesn't exist, doesn't exist as an array type, or one or
- * more keys inside the array type are not of type T, an empty option
- * is returned. Otherwise, an option containing a vector of the values
- * is returned.
- */
-template <>
-inline typename array_of_trait<array>::return_type
-table::get_qualified_array_of<array>(const std::string& key) const
-{
-    if (auto v = get_array_qualified(key))
-    {
-        std::vector<std::shared_ptr<array>> result;
-        result.reserve(v->get().size());
-
-        for (const auto& b : v->get())
-        {
-            if (auto val = b->as_array())
-                result.push_back(val);
-            else
-                return {};
-        }
-
-        return {std::move(result)};
-    }
-
-    return {};
-}
-
-std::shared_ptr<table> make_table()
-{
-    struct make_shared_enabler : public table
-    {
-        make_shared_enabler()
-        {
-            // nothing
-        }
-    };
-
-    return std::make_shared<make_shared_enabler>();
-}
-
-namespace detail
-{
-template <>
-inline std::shared_ptr<table> make_element<table>()
-{
-    return make_table();
-}
-} // namespace detail
-
-template <class T>
-std::shared_ptr<base> value<T>::clone() const
-{
-    return make_value(data_);
-}
-
-inline std::shared_ptr<base> array::clone() const
-{
-    auto result = make_array();
-    result->reserve(values_.size());
-    for (const auto& ptr : values_)
-        result->values_.push_back(ptr->clone());
-    return result;
-}
-
-inline std::shared_ptr<base> table_array::clone() const
-{
-    auto result = make_table_array(is_inline());
-    result->reserve(array_.size());
-    for (const auto& ptr : array_)
-        result->array_.push_back(ptr->clone()->as_table());
-    return result;
-}
-
-inline std::shared_ptr<base> table::clone() const
-{
-    auto result = make_table();
-    for (const auto& pr : map_)
-        result->insert(pr.first, pr.second->clone());
-    return result;
-}
-
-/**
- * Exception class for all TOML parsing errors.
- */
-class parse_exception : public std::runtime_error
-{
-  public:
-    parse_exception(const std::string& err) : std::runtime_error{err}
-    {
-    }
-
-    parse_exception(const std::string& err, std::size_t line_number)
-        : std::runtime_error{err + " at line " + std::to_string(line_number)}
-    {
-    }
-};
-
-inline bool is_number(char c)
-{
-    return c >= '0' && c <= '9';
-}
-
-inline bool is_hex(char c)
-{
-    return is_number(c) || (c >= 'a' && c <= 'f') || (c >= 'A' && c <= 'F');
-}
-
-/**
- * Helper object for consuming expected characters.
- */
-template <class OnError>
-class consumer
-{
-  public:
-    consumer(std::string::iterator& it, const std::string::iterator& end,
-             OnError&& on_error)
-        : it_(it), end_(end), on_error_(std::forward<OnError>(on_error))
-    {
-        // nothing
-    }
-
-    void operator()(char c)
-    {
-        if (it_ == end_ || *it_ != c)
-            on_error_();
-        ++it_;
-    }
-
-    template <std::size_t N>
-    void operator()(const char (&str)[N])
-    {
-        std::for_each(std::begin(str), std::end(str) - 1,
-                      [&](char c) { (*this)(c); });
-    }
-
-    void eat_or(char a, char b)
-    {
-        if (it_ == end_ || (*it_ != a && *it_ != b))
-            on_error_();
-        ++it_;
-    }
-
-    int eat_digits(int len)
-    {
-        int val = 0;
-        for (int i = 0; i < len; ++i)
-        {
-            if (!is_number(*it_) || it_ == end_)
-                on_error_();
-            val = 10 * val + (*it_++ - '0');
-        }
-        return val;
-    }
-
-    void error()
-    {
-        on_error_();
-    }
-
-  private:
-    std::string::iterator& it_;
-    const std::string::iterator& end_;
-    OnError on_error_;
-};
-
-template <class OnError>
-consumer<OnError> make_consumer(std::string::iterator& it,
-                                const std::string::iterator& end,
-                                OnError&& on_error)
-{
-    return consumer<OnError>(it, end, std::forward<OnError>(on_error));
-}
-
-// replacement for std::getline to handle incorrectly line-ended files
-// https://stackoverflow.com/questions/6089231/getting-std-ifstream-to-handle-lf-cr-and-crlf
-namespace detail
-{
-inline std::istream& getline(std::istream& input, std::string& line)
-{
-    line.clear();
-
-    std::istream::sentry sentry{input, true};
-    auto sb = input.rdbuf();
-
-    while (true)
-    {
-        auto c = sb->sbumpc();
-        if (c == '\r')
-        {
-            if (sb->sgetc() == '\n')
-                c = sb->sbumpc();
-        }
-
-        if (c == '\n')
-            return input;
-
-        if (c == std::istream::traits_type::eof())
-        {
-            if (line.empty())
-                input.setstate(std::ios::eofbit);
-            return input;
-        }
-
-        line.push_back(static_cast<char>(c));
-    }
-}
-} // namespace detail
-
-/**
- * The parser class.
- */
-class parser
-{
-  public:
-    /**
-     * Parsers are constructed from streams.
-     */
-    parser(std::istream& stream) : input_(stream)
-    {
-        // nothing
-    }
-
-    parser& operator=(const parser& parser) = delete;
-
-    /**
-     * Parses the stream this parser was created on until EOF.
-     * @throw parse_exception if there are errors in parsing
-     */
-    std::shared_ptr<table> parse()
-    {
-        std::shared_ptr<table> root = make_table();
-
-        table* curr_table = root.get();
-
-        while (detail::getline(input_, line_))
-        {
-            line_number_++;
-            auto it = line_.begin();
-            auto end = line_.end();
-            consume_whitespace(it, end);
-            if (it == end || *it == '#')
-                continue;
-            if (*it == '[')
-            {
-                curr_table = root.get();
-                parse_table(it, end, curr_table);
-            }
-            else
-            {
-                parse_key_value(it, end, curr_table);
-                consume_whitespace(it, end);
-                eol_or_comment(it, end);
-            }
-        }
-        return root;
-    }
-
-  private:
-#if defined _MSC_VER
-    __declspec(noreturn)
-#elif defined __GNUC__
-    __attribute__((noreturn))
-#endif
-        void throw_parse_exception(const std::string& err)
-    {
-        throw parse_exception{err, line_number_};
-    }
-
-    void parse_table(std::string::iterator& it,
-                     const std::string::iterator& end, table*& curr_table)
-    {
-        // remove the beginning keytable marker
-        ++it;
-        if (it == end)
-            throw_parse_exception("Unexpected end of table");
-        if (*it == '[')
-            parse_table_array(it, end, curr_table);
-        else
-            parse_single_table(it, end, curr_table);
-    }
-
-    void parse_single_table(std::string::iterator& it,
-                            const std::string::iterator& end,
-                            table*& curr_table)
-    {
-        if (it == end || *it == ']')
-            throw_parse_exception("Table name cannot be empty");
-
-        std::string full_table_name;
-        bool inserted = false;
-
-        auto key_end = [](char c) { return c == ']'; };
-
-        auto key_part_handler = [&](const std::string& part) {
-            if (part.empty())
-                throw_parse_exception("Empty component of table name");
-
-            if (!full_table_name.empty())
-                full_table_name += '.';
-            full_table_name += part;
-
-            if (curr_table->contains(part))
-            {
-#if !defined(__PGI)
-                auto b = curr_table->get(part);
-#else
-                // Workaround for PGI compiler
-                std::shared_ptr<base> b = curr_table->get(part);
-#endif
-                if (b->is_table())
-                    curr_table = static_cast<table*>(b.get());
-                else if (b->is_table_array())
-                    curr_table = std::static_pointer_cast<table_array>(b)
-                                     ->get()
-                                     .back()
-                                     .get();
-                else
-                    throw_parse_exception("Key " + full_table_name
-                                          + "already exists as a value");
-            }
-            else
-            {
-                inserted = true;
-                curr_table->insert(part, make_table());
-                curr_table = static_cast<table*>(curr_table->get(part).get());
-            }
-        };
-
-        key_part_handler(parse_key(it, end, key_end, key_part_handler));
-
-        if (it == end)
-            throw_parse_exception(
-                "Unterminated table declaration; did you forget a ']'?");
-
-        if (*it != ']')
-        {
-            std::string errmsg{"Unexpected character in table definition: "};
-            errmsg += '"';
-            errmsg += *it;
-            errmsg += '"';
-            throw_parse_exception(errmsg);
-        }
-
-        // table already existed
-        if (!inserted)
-        {
-            auto is_value
-                = [](const std::pair<const std::string&,
-                                     const std::shared_ptr<base>&>& p) {
-                      return p.second->is_value();
-                  };
-
-            // if there are any values, we can't add values to this table
-            // since it has already been defined. If there aren't any
-            // values, then it was implicitly created by something like
-            // [a.b]
-            if (curr_table->empty()
-                || std::any_of(curr_table->begin(), curr_table->end(),
-                               is_value))
-            {
-                throw_parse_exception("Redefinition of table "
-                                      + full_table_name);
-            }
-        }
-
-        ++it;
-        consume_whitespace(it, end);
-        eol_or_comment(it, end);
-    }
-
-    void parse_table_array(std::string::iterator& it,
-                           const std::string::iterator& end, table*& curr_table)
-    {
-        ++it;
-        if (it == end || *it == ']')
-            throw_parse_exception("Table array name cannot be empty");
-
-        auto key_end = [](char c) { return c == ']'; };
-
-        std::string full_ta_name;
-        auto key_part_handler = [&](const std::string& part) {
-            if (part.empty())
-                throw_parse_exception("Empty component of table array name");
-
-            if (!full_ta_name.empty())
-                full_ta_name += '.';
-            full_ta_name += part;
-
-            if (curr_table->contains(part))
-            {
-#if !defined(__PGI)
-                auto b = curr_table->get(part);
-#else
-                // Workaround for PGI compiler
-                std::shared_ptr<base> b = curr_table->get(part);
-#endif
-
-                // if this is the end of the table array name, add an
-                // element to the table array that we just looked up,
-                // provided it was not declared inline
-                if (it != end && *it == ']')
-                {
-                    if (!b->is_table_array())
-                    {
-                        throw_parse_exception("Key " + full_ta_name
-                                              + " is not a table array");
-                    }
-
-                    auto v = b->as_table_array();
-
-                    if (v->is_inline())
-                    {
-                        throw_parse_exception("Static array " + full_ta_name
-                                              + " cannot be appended to");
-                    }
-
-                    v->get().push_back(make_table());
-                    curr_table = v->get().back().get();
-                }
-                // otherwise, just keep traversing down the key name
-                else
-                {
-                    if (b->is_table())
-                        curr_table = static_cast<table*>(b.get());
-                    else if (b->is_table_array())
-                        curr_table = std::static_pointer_cast<table_array>(b)
-                                         ->get()
-                                         .back()
-                                         .get();
-                    else
-                        throw_parse_exception("Key " + full_ta_name
-                                              + " already exists as a value");
-                }
-            }
-            else
-            {
-                // if this is the end of the table array name, add a new
-                // table array and a new table inside that array for us to
-                // add keys to next
-                if (it != end && *it == ']')
-                {
-                    curr_table->insert(part, make_table_array());
-                    auto arr = std::static_pointer_cast<table_array>(
-                        curr_table->get(part));
-                    arr->get().push_back(make_table());
-                    curr_table = arr->get().back().get();
-                }
-                // otherwise, create the implicitly defined table and move
-                // down to it
-                else
-                {
-                    curr_table->insert(part, make_table());
-                    curr_table
-                        = static_cast<table*>(curr_table->get(part).get());
-                }
-            }
-        };
-
-        key_part_handler(parse_key(it, end, key_end, key_part_handler));
-
-        // consume the last "]]"
-        auto eat = make_consumer(it, end, [this]() {
-            throw_parse_exception("Unterminated table array name");
-        });
-        eat(']');
-        eat(']');
-
-        consume_whitespace(it, end);
-        eol_or_comment(it, end);
-    }
-
-    void parse_key_value(std::string::iterator& it, std::string::iterator& end,
-                         table* curr_table)
-    {
-        auto key_end = [](char c) { return c == '='; };
-
-        auto key_part_handler = [&](const std::string& part) {
-            // two cases: this key part exists already, in which case it must
-            // be a table, or it doesn't exist in which case we must create
-            // an implicitly defined table
-            if (curr_table->contains(part))
-            {
-                auto val = curr_table->get(part);
-                if (val->is_table())
-                {
-                    curr_table = static_cast<table*>(val.get());
-                }
-                else
-                {
-                    throw_parse_exception("Key " + part
-                                          + " already exists as a value");
-                }
-            }
-            else
-            {
-                auto newtable = make_table();
-                curr_table->insert(part, newtable);
-                curr_table = newtable.get();
-            }
-        };
-
-        auto key = parse_key(it, end, key_end, key_part_handler);
-
-        if (curr_table->contains(key))
-            throw_parse_exception("Key " + key + " already present");
-        if (it == end || *it != '=')
-            throw_parse_exception("Value must follow after a '='");
-        ++it;
-        consume_whitespace(it, end);
-        curr_table->insert(key, parse_value(it, end));
-        consume_whitespace(it, end);
-    }
-
-    template <class KeyEndFinder, class KeyPartHandler>
-    std::string
-    parse_key(std::string::iterator& it, const std::string::iterator& end,
-              KeyEndFinder&& key_end, KeyPartHandler&& key_part_handler)
-    {
-        // parse the key as a series of one or more simple-keys joined with '.'
-        while (it != end && !key_end(*it))
-        {
-            auto part = parse_simple_key(it, end);
-            consume_whitespace(it, end);
-
-            if (it == end || key_end(*it))
-            {
-                return part;
-            }
-
-            if (*it != '.')
-            {
-                std::string errmsg{"Unexpected character in key: "};
-                errmsg += '"';
-                errmsg += *it;
-                errmsg += '"';
-                throw_parse_exception(errmsg);
-            }
-
-            key_part_handler(part);
-
-            // consume the dot
-            ++it;
-        }
-
-        throw_parse_exception("Unexpected end of key");
-    }
-
-    std::string parse_simple_key(std::string::iterator& it,
-                                 const std::string::iterator& end)
-    {
-        consume_whitespace(it, end);
-
-        if (it == end)
-            throw_parse_exception("Unexpected end of key (blank key?)");
-
-        if (*it == '"' || *it == '\'')
-        {
-            return string_literal(it, end, *it);
-        }
-        else
-        {
-            auto bke = std::find_if(it, end, [](char c) {
-                return c == '.' || c == '=' || c == ']';
-            });
-            return parse_bare_key(it, bke);
-        }
-    }
-
-    std::string parse_bare_key(std::string::iterator& it,
-                               const std::string::iterator& end)
-    {
-        if (it == end)
-        {
-            throw_parse_exception("Bare key missing name");
-        }
-
-        auto key_end = end;
-        --key_end;
-        consume_backwards_whitespace(key_end, it);
-        ++key_end;
-        std::string key{it, key_end};
-
-        if (std::find(it, key_end, '#') != key_end)
-        {
-            throw_parse_exception("Bare key " + key + " cannot contain #");
-        }
-
-        if (std::find_if(it, key_end,
-                         [](char c) { return c == ' ' || c == '\t'; })
-            != key_end)
-        {
-            throw_parse_exception("Bare key " + key
-                                  + " cannot contain whitespace");
-        }
-
-        if (std::find_if(it, key_end,
-                         [](char c) { return c == '[' || c == ']'; })
-            != key_end)
-        {
-            throw_parse_exception("Bare key " + key
-                                  + " cannot contain '[' or ']'");
-        }
-
-        it = end;
-        return key;
-    }
-
-    enum class parse_type
-    {
-        STRING = 1,
-        LOCAL_TIME,
-        LOCAL_DATE,
-        LOCAL_DATETIME,
-        OFFSET_DATETIME,
-        INT,
-        FLOAT,
-        BOOL,
-        ARRAY,
-        INLINE_TABLE
-    };
-
-    std::shared_ptr<base> parse_value(std::string::iterator& it,
-                                      std::string::iterator& end)
-    {
-        parse_type type = determine_value_type(it, end);
-        switch (type)
-        {
-            case parse_type::STRING:
-                return parse_string(it, end);
-            case parse_type::LOCAL_TIME:
-                return parse_time(it, end);
-            case parse_type::LOCAL_DATE:
-            case parse_type::LOCAL_DATETIME:
-            case parse_type::OFFSET_DATETIME:
-                return parse_date(it, end);
-            case parse_type::INT:
-            case parse_type::FLOAT:
-                return parse_number(it, end);
-            case parse_type::BOOL:
-                return parse_bool(it, end);
-            case parse_type::ARRAY:
-                return parse_array(it, end);
-            case parse_type::INLINE_TABLE:
-                return parse_inline_table(it, end);
-            default:
-                throw_parse_exception("Failed to parse value");
-        }
-    }
-
-    parse_type determine_value_type(const std::string::iterator& it,
-                                    const std::string::iterator& end)
-    {
-        if (it == end)
-        {
-            throw_parse_exception("Failed to parse value type");
-        }
-        if (*it == '"' || *it == '\'')
-        {
-            return parse_type::STRING;
-        }
-        else if (is_time(it, end))
-        {
-            return parse_type::LOCAL_TIME;
-        }
-        else if (auto dtype = date_type(it, end))
-        {
-            return *dtype;
-        }
-        else if (is_number(*it) || *it == '-' || *it == '+'
-                 || (*it == 'i' && it + 1 != end && it[1] == 'n'
-                     && it + 2 != end && it[2] == 'f')
-                 || (*it == 'n' && it + 1 != end && it[1] == 'a'
-                     && it + 2 != end && it[2] == 'n'))
-        {
-            return determine_number_type(it, end);
-        }
-        else if (*it == 't' || *it == 'f')
-        {
-            return parse_type::BOOL;
-        }
-        else if (*it == '[')
-        {
-            return parse_type::ARRAY;
-        }
-        else if (*it == '{')
-        {
-            return parse_type::INLINE_TABLE;
-        }
-        throw_parse_exception("Failed to parse value type");
-    }
-
-    parse_type determine_number_type(const std::string::iterator& it,
-                                     const std::string::iterator& end)
-    {
-        // determine if we are an integer or a float
-        auto check_it = it;
-        if (*check_it == '-' || *check_it == '+')
-            ++check_it;
-
-        if (check_it == end)
-            throw_parse_exception("Malformed number");
-
-        if (*check_it == 'i' || *check_it == 'n')
-            return parse_type::FLOAT;
-
-        while (check_it != end && is_number(*check_it))
-            ++check_it;
-        if (check_it != end && *check_it == '.')
-        {
-            ++check_it;
-            while (check_it != end && is_number(*check_it))
-                ++check_it;
-            return parse_type::FLOAT;
-        }
-        else
-        {
-            return parse_type::INT;
-        }
-    }
-
-    std::shared_ptr<value<std::string>> parse_string(std::string::iterator& it,
-                                                     std::string::iterator& end)
-    {
-        auto delim = *it;
-        assert(delim == '"' || delim == '\'');
-
-        // end is non-const here because we have to be able to potentially
-        // parse multiple lines in a string, not just one
-        auto check_it = it;
-        ++check_it;
-        if (check_it != end && *check_it == delim)
-        {
-            ++check_it;
-            if (check_it != end && *check_it == delim)
-            {
-                it = ++check_it;
-                return parse_multiline_string(it, end, delim);
-            }
-        }
-        return make_value<std::string>(string_literal(it, end, delim));
-    }
-
-    std::shared_ptr<value<std::string>>
-    parse_multiline_string(std::string::iterator& it,
-                           std::string::iterator& end, char delim)
-    {
-        std::stringstream ss;
-
-        auto is_ws = [](char c) { return c == ' ' || c == '\t'; };
-
-        bool consuming = false;
-        std::shared_ptr<value<std::string>> ret;
-
-        auto handle_line = [&](std::string::iterator& local_it,
-                               std::string::iterator& local_end) {
-            if (consuming)
-            {
-                local_it = std::find_if_not(local_it, local_end, is_ws);
-
-                // whole line is whitespace
-                if (local_it == local_end)
-                    return;
-            }
-
-            consuming = false;
-
-            while (local_it != local_end)
-            {
-                // handle escaped characters
-                if (delim == '"' && *local_it == '\\')
-                {
-                    auto check = local_it;
-                    // check if this is an actual escape sequence or a
-                    // whitespace escaping backslash
-                    ++check;
-                    consume_whitespace(check, local_end);
-                    if (check == local_end)
-                    {
-                        consuming = true;
-                        break;
-                    }
-
-                    ss << parse_escape_code(local_it, local_end);
-                    continue;
-                }
-
-                // if we can end the string
-                if (std::distance(local_it, local_end) >= 3)
-                {
-                    auto check = local_it;
-                    // check for """
-                    if (*check++ == delim && *check++ == delim
-                        && *check++ == delim)
-                    {
-                        local_it = check;
-                        ret = make_value<std::string>(ss.str());
-                        break;
-                    }
-                }
-
-                ss << *local_it++;
-            }
-        };
-
-        // handle the remainder of the current line
-        handle_line(it, end);
-        if (ret)
-            return ret;
-
-        // start eating lines
-        while (detail::getline(input_, line_))
-        {
-            ++line_number_;
-
-            it = line_.begin();
-            end = line_.end();
-
-            handle_line(it, end);
-
-            if (ret)
-                return ret;
-
-            if (!consuming)
-                ss << std::endl;
-        }
-
-        throw_parse_exception("Unterminated multi-line basic string");
-    }
-
-    std::string string_literal(std::string::iterator& it,
-                               const std::string::iterator& end, char delim)
-    {
-        ++it;
-        std::string val;
-        while (it != end)
-        {
-            // handle escaped characters
-            if (delim == '"' && *it == '\\')
-            {
-                val += parse_escape_code(it, end);
-            }
-            else if (*it == delim)
-            {
-                ++it;
-                consume_whitespace(it, end);
-                return val;
-            }
-            else
-            {
-                val += *it++;
-            }
-        }
-        throw_parse_exception("Unterminated string literal");
-    }
-
-    std::string parse_escape_code(std::string::iterator& it,
-                                  const std::string::iterator& end)
-    {
-        ++it;
-        if (it == end)
-            throw_parse_exception("Invalid escape sequence");
-        char value;
-        if (*it == 'b')
-        {
-            value = '\b';
-        }
-        else if (*it == 't')
-        {
-            value = '\t';
-        }
-        else if (*it == 'n')
-        {
-            value = '\n';
-        }
-        else if (*it == 'f')
-        {
-            value = '\f';
-        }
-        else if (*it == 'r')
-        {
-            value = '\r';
-        }
-        else if (*it == '"')
-        {
-            value = '"';
-        }
-        else if (*it == '\\')
-        {
-            value = '\\';
-        }
-        else if (*it == 'u' || *it == 'U')
-        {
-            return parse_unicode(it, end);
-        }
-        else
-        {
-            throw_parse_exception("Invalid escape sequence");
-        }
-        ++it;
-        return std::string(1, value);
-    }
-
-    std::string parse_unicode(std::string::iterator& it,
-                              const std::string::iterator& end)
-    {
-        bool large = *it++ == 'U';
-        auto codepoint = parse_hex(it, end, large ? 0x10000000 : 0x1000);
-
-        if ((codepoint > 0xd7ff && codepoint < 0xe000) || codepoint > 0x10ffff)
-        {
-            throw_parse_exception(
-                "Unicode escape sequence is not a Unicode scalar value");
-        }
-
-        std::string result;
-        // See Table 3-6 of the Unicode standard
-        if (codepoint <= 0x7f)
-        {
-            // 1-byte codepoints: 00000000 0xxxxxxx
-            // repr: 0xxxxxxx
-            result += static_cast<char>(codepoint & 0x7f);
-        }
-        else if (codepoint <= 0x7ff)
-        {
-            // 2-byte codepoints: 00000yyy yyxxxxxx
-            // repr: 110yyyyy 10xxxxxx
-            //
-            // 0x1f = 00011111
-            // 0xc0 = 11000000
-            //
-            result += static_cast<char>(0xc0 | ((codepoint >> 6) & 0x1f));
-            //
-            // 0x80 = 10000000
-            // 0x3f = 00111111
-            //
-            result += static_cast<char>(0x80 | (codepoint & 0x3f));
-        }
-        else if (codepoint <= 0xffff)
-        {
-            // 3-byte codepoints: zzzzyyyy yyxxxxxx
-            // repr: 1110zzzz 10yyyyyy 10xxxxxx
-            //
-            // 0xe0 = 11100000
-            // 0x0f = 00001111
-            //
-            result += static_cast<char>(0xe0 | ((codepoint >> 12) & 0x0f));
-            result += static_cast<char>(0x80 | ((codepoint >> 6) & 0x1f));
-            result += static_cast<char>(0x80 | (codepoint & 0x3f));
-        }
-        else
-        {
-            // 4-byte codepoints: 000uuuuu zzzzyyyy yyxxxxxx
-            // repr: 11110uuu 10uuzzzz 10yyyyyy 10xxxxxx
-            //
-            // 0xf0 = 11110000
-            // 0x07 = 00000111
-            //
-            result += static_cast<char>(0xf0 | ((codepoint >> 18) & 0x07));
-            result += static_cast<char>(0x80 | ((codepoint >> 12) & 0x3f));
-            result += static_cast<char>(0x80 | ((codepoint >> 6) & 0x3f));
-            result += static_cast<char>(0x80 | (codepoint & 0x3f));
-        }
-        return result;
-    }
-
-    uint32_t parse_hex(std::string::iterator& it,
-                       const std::string::iterator& end, uint32_t place)
-    {
-        uint32_t value = 0;
-        while (place > 0)
-        {
-            if (it == end)
-                throw_parse_exception("Unexpected end of unicode sequence");
-
-            if (!is_hex(*it))
-                throw_parse_exception("Invalid unicode escape sequence");
-
-            value += place * hex_to_digit(*it++);
-            place /= 16;
-        }
-        return value;
-    }
-
-    uint32_t hex_to_digit(char c)
-    {
-        if (is_number(c))
-            return static_cast<uint32_t>(c - '0');
-        return 10
-               + static_cast<uint32_t>(c
-                                       - ((c >= 'a' && c <= 'f') ? 'a' : 'A'));
-    }
-
-    std::shared_ptr<base> parse_number(std::string::iterator& it,
-                                       const std::string::iterator& end)
-    {
-        auto check_it = it;
-        auto check_end = find_end_of_number(it, end);
-
-        auto eat_sign = [&]() {
-            if (check_it != end && (*check_it == '-' || *check_it == '+'))
-                ++check_it;
-        };
-
-        auto check_no_leading_zero = [&]() {
-            if (check_it != end && *check_it == '0' && check_it + 1 != check_end
-                && check_it[1] != '.')
-            {
-                throw_parse_exception("Numbers may not have leading zeros");
-            }
-        };
-
-        auto eat_digits = [&](bool (*check_char)(char)) {
-            auto beg = check_it;
-            while (check_it != end && check_char(*check_it))
-            {
-                ++check_it;
-                if (check_it != end && *check_it == '_')
-                {
-                    ++check_it;
-                    if (check_it == end || !check_char(*check_it))
-                        throw_parse_exception("Malformed number");
-                }
-            }
-
-            if (check_it == beg)
-                throw_parse_exception("Malformed number");
-        };
-
-        auto eat_hex = [&]() { eat_digits(&is_hex); };
-
-        auto eat_numbers = [&]() { eat_digits(&is_number); };
-
-        if (check_it != end && *check_it == '0' && check_it + 1 != check_end
-            && (check_it[1] == 'x' || check_it[1] == 'o' || check_it[1] == 'b'))
-        {
-            ++check_it;
-            char base = *check_it;
-            ++check_it;
-            if (base == 'x')
-            {
-                eat_hex();
-                return parse_int(it, check_it, 16);
-            }
-            else if (base == 'o')
-            {
-                auto start = check_it;
-                eat_numbers();
-                auto val = parse_int(start, check_it, 8, "0");
-                it = start;
-                return val;
-            }
-            else // if (base == 'b')
-            {
-                auto start = check_it;
-                eat_numbers();
-                auto val = parse_int(start, check_it, 2);
-                it = start;
-                return val;
-            }
-        }
-
-        eat_sign();
-        check_no_leading_zero();
-
-        if (check_it != end && check_it + 1 != end && check_it + 2 != end)
-        {
-            if (check_it[0] == 'i' && check_it[1] == 'n' && check_it[2] == 'f')
-            {
-                auto val = std::numeric_limits<double>::infinity();
-                if (*it == '-')
-                    val = -val;
-                it = check_it + 3;
-                return make_value(val);
-            }
-            else if (check_it[0] == 'n' && check_it[1] == 'a'
-                     && check_it[2] == 'n')
-            {
-                auto val = std::numeric_limits<double>::quiet_NaN();
-                if (*it == '-')
-                    val = -val;
-                it = check_it + 3;
-                return make_value(val);
-            }
-        }
-
-        eat_numbers();
-
-        if (check_it != end
-            && (*check_it == '.' || *check_it == 'e' || *check_it == 'E'))
-        {
-            bool is_exp = *check_it == 'e' || *check_it == 'E';
-
-            ++check_it;
-            if (check_it == end)
-                throw_parse_exception("Floats must have trailing digits");
-
-            auto eat_exp = [&]() {
-                eat_sign();
-                check_no_leading_zero();
-                eat_numbers();
-            };
-
-            if (is_exp)
-                eat_exp();
-            else
-                eat_numbers();
-
-            if (!is_exp && check_it != end
-                && (*check_it == 'e' || *check_it == 'E'))
-            {
-                ++check_it;
-                eat_exp();
-            }
-
-            return parse_float(it, check_it);
-        }
-        else
-        {
-            return parse_int(it, check_it);
-        }
-    }
-
-    std::shared_ptr<value<int64_t>> parse_int(std::string::iterator& it,
-                                              const std::string::iterator& end,
-                                              int base = 10,
-                                              const char* prefix = "")
-    {
-        std::string v{it, end};
-        v = prefix + v;
-        v.erase(std::remove(v.begin(), v.end(), '_'), v.end());
-        it = end;
-        try
-        {
-            return make_value<int64_t>(std::stoll(v, nullptr, base));
-        }
-        catch (const std::invalid_argument& ex)
-        {
-            throw_parse_exception("Malformed number (invalid argument: "
-                                  + std::string{ex.what()} + ")");
-        }
-        catch (const std::out_of_range& ex)
-        {
-            throw_parse_exception("Malformed number (out of range: "
-                                  + std::string{ex.what()} + ")");
-        }
-    }
-
-    std::shared_ptr<value<double>> parse_float(std::string::iterator& it,
-                                               const std::string::iterator& end)
-    {
-        std::string v{it, end};
-        v.erase(std::remove(v.begin(), v.end(), '_'), v.end());
-        it = end;
-        char decimal_point = std::localeconv()->decimal_point[0];
-        std::replace(v.begin(), v.end(), '.', decimal_point);
-        try
-        {
-            return make_value<double>(std::stod(v));
-        }
-        catch (const std::invalid_argument& ex)
-        {
-            throw_parse_exception("Malformed number (invalid argument: "
-                                  + std::string{ex.what()} + ")");
-        }
-        catch (const std::out_of_range& ex)
-        {
-            throw_parse_exception("Malformed number (out of range: "
-                                  + std::string{ex.what()} + ")");
-        }
-    }
-
-    std::shared_ptr<value<bool>> parse_bool(std::string::iterator& it,
-                                            const std::string::iterator& end)
-    {
-        auto eat = make_consumer(it, end, [this]() {
-            throw_parse_exception("Attempted to parse invalid boolean value");
-        });
-
-        if (*it == 't')
-        {
-            eat("true");
-            return make_value<bool>(true);
-        }
-        else if (*it == 'f')
-        {
-            eat("false");
-            return make_value<bool>(false);
-        }
-
-        eat.error();
-        return nullptr;
-    }
-
-    std::string::iterator find_end_of_number(std::string::iterator it,
-                                             std::string::iterator end)
-    {
-        auto ret = std::find_if(it, end, [](char c) {
-            return !is_number(c) && c != '_' && c != '.' && c != 'e' && c != 'E'
-                   && c != '-' && c != '+' && c != 'x' && c != 'o' && c != 'b';
-        });
-        if (ret != end && ret + 1 != end && ret + 2 != end)
-        {
-            if ((ret[0] == 'i' && ret[1] == 'n' && ret[2] == 'f')
-                || (ret[0] == 'n' && ret[1] == 'a' && ret[2] == 'n'))
-            {
-                ret = ret + 3;
-            }
-        }
-        return ret;
-    }
-
-    std::string::iterator find_end_of_date(std::string::iterator it,
-                                           std::string::iterator end)
-    {
-        auto end_of_date = std::find_if(it, end, [](char c) {
-            return !is_number(c) && c != '-';
-        });
-        if (end_of_date != end && *end_of_date == ' ' && end_of_date + 1 != end
-            && is_number(end_of_date[1]))
-            end_of_date++;
-        return std::find_if(end_of_date, end, [](char c) {
-            return !is_number(c) && c != 'T' && c != 'Z' && c != ':'
-                   && c != '-' && c != '+' && c != '.';
-        });
-    }
-
-    std::string::iterator find_end_of_time(std::string::iterator it,
-                                           std::string::iterator end)
-    {
-        return std::find_if(it, end, [](char c) {
-            return !is_number(c) && c != ':' && c != '.';
-        });
-    }
-
-    local_time read_time(std::string::iterator& it,
-                         const std::string::iterator& end)
-    {
-        auto time_end = find_end_of_time(it, end);
-
-        auto eat = make_consumer(
-            it, time_end, [&]() { throw_parse_exception("Malformed time"); });
-
-        local_time ltime;
-
-        ltime.hour = eat.eat_digits(2);
-        eat(':');
-        ltime.minute = eat.eat_digits(2);
-        eat(':');
-        ltime.second = eat.eat_digits(2);
-
-        int power = 100000;
-        if (it != time_end && *it == '.')
-        {
-            ++it;
-            while (it != time_end && is_number(*it))
-            {
-                ltime.microsecond += power * (*it++ - '0');
-                power /= 10;
-            }
-        }
-
-        if (it != time_end)
-            throw_parse_exception("Malformed time");
-
-        return ltime;
-    }
-
-    std::shared_ptr<value<local_time>>
-    parse_time(std::string::iterator& it, const std::string::iterator& end)
-    {
-        return make_value(read_time(it, end));
-    }
-
-    std::shared_ptr<base> parse_date(std::string::iterator& it,
-                                     const std::string::iterator& end)
-    {
-        auto date_end = find_end_of_date(it, end);
-
-        auto eat = make_consumer(
-            it, date_end, [&]() { throw_parse_exception("Malformed date"); });
-
-        local_date ldate;
-        ldate.year = eat.eat_digits(4);
-        eat('-');
-        ldate.month = eat.eat_digits(2);
-        eat('-');
-        ldate.day = eat.eat_digits(2);
-
-        if (it == date_end)
-            return make_value(ldate);
-
-        eat.eat_or('T', ' ');
-
-        local_datetime ldt;
-        static_cast<local_date&>(ldt) = ldate;
-        static_cast<local_time&>(ldt) = read_time(it, date_end);
-
-        if (it == date_end)
-            return make_value(ldt);
-
-        offset_datetime dt;
-        static_cast<local_datetime&>(dt) = ldt;
-
-        int hoff = 0;
-        int moff = 0;
-        if (*it == '+' || *it == '-')
-        {
-            auto plus = *it == '+';
-            ++it;
-
-            hoff = eat.eat_digits(2);
-            dt.hour_offset = (plus) ? hoff : -hoff;
-            eat(':');
-            moff = eat.eat_digits(2);
-            dt.minute_offset = (plus) ? moff : -moff;
-        }
-        else if (*it == 'Z')
-        {
-            ++it;
-        }
-
-        if (it != date_end)
-            throw_parse_exception("Malformed date");
-
-        return make_value(dt);
-    }
-
-    std::shared_ptr<base> parse_array(std::string::iterator& it,
-                                      std::string::iterator& end)
-    {
-        // this gets ugly because of the "homogeneity" restriction:
-        // arrays can either be of only one type, or contain arrays
-        // (each of those arrays could be of different types, though)
-        //
-        // because of the latter portion, we don't really have a choice
-        // but to represent them as arrays of base values...
-        ++it;
-
-        // ugh---have to read the first value to determine array type...
-        skip_whitespace_and_comments(it, end);
-
-        // edge case---empty array
-        if (*it == ']')
-        {
-            ++it;
-            return make_array();
-        }
-
-        auto val_end = std::find_if(
-            it, end, [](char c) { return c == ',' || c == ']' || c == '#'; });
-        parse_type type = determine_value_type(it, val_end);
-        switch (type)
-        {
-            case parse_type::STRING:
-                return parse_value_array<std::string>(it, end);
-            case parse_type::LOCAL_TIME:
-                return parse_value_array<local_time>(it, end);
-            case parse_type::LOCAL_DATE:
-                return parse_value_array<local_date>(it, end);
-            case parse_type::LOCAL_DATETIME:
-                return parse_value_array<local_datetime>(it, end);
-            case parse_type::OFFSET_DATETIME:
-                return parse_value_array<offset_datetime>(it, end);
-            case parse_type::INT:
-                return parse_value_array<int64_t>(it, end);
-            case parse_type::FLOAT:
-                return parse_value_array<double>(it, end);
-            case parse_type::BOOL:
-                return parse_value_array<bool>(it, end);
-            case parse_type::ARRAY:
-                return parse_object_array<array>(&parser::parse_array, '[', it,
-                                                 end);
-            case parse_type::INLINE_TABLE:
-                return parse_object_array<table_array>(
-                    &parser::parse_inline_table, '{', it, end);
-            default:
-                throw_parse_exception("Unable to parse array");
-        }
-    }
-
-    template <class Value>
-    std::shared_ptr<array> parse_value_array(std::string::iterator& it,
-                                             std::string::iterator& end)
-    {
-        auto arr = make_array();
-        while (it != end && *it != ']')
-        {
-            auto val = parse_value(it, end);
-            if (auto v = val->as<Value>())
-                arr->get().push_back(val);
-            else
-                throw_parse_exception("Arrays must be homogeneous");
-            skip_whitespace_and_comments(it, end);
-            if (*it != ',')
-                break;
-            ++it;
-            skip_whitespace_and_comments(it, end);
-        }
-        if (it != end)
-            ++it;
-        return arr;
-    }
-
-    template <class Object, class Function>
-    std::shared_ptr<Object> parse_object_array(Function&& fun, char delim,
-                                               std::string::iterator& it,
-                                               std::string::iterator& end)
-    {
-        auto arr = detail::make_element<Object>();
-
-        while (it != end && *it != ']')
-        {
-            if (*it != delim)
-                throw_parse_exception("Unexpected character in array");
-
-            arr->get().push_back(((*this).*fun)(it, end));
-            skip_whitespace_and_comments(it, end);
-
-            if (it == end || *it != ',')
-                break;
-
-            ++it;
-            skip_whitespace_and_comments(it, end);
-        }
-
-        if (it == end || *it != ']')
-            throw_parse_exception("Unterminated array");
-
-        ++it;
-        return arr;
-    }
-
-    std::shared_ptr<table> parse_inline_table(std::string::iterator& it,
-                                              std::string::iterator& end)
-    {
-        auto tbl = make_table();
-        do
-        {
-            ++it;
-            if (it == end)
-                throw_parse_exception("Unterminated inline table");
-
-            consume_whitespace(it, end);
-            if (it != end && *it != '}')
-            {
-                parse_key_value(it, end, tbl.get());
-                consume_whitespace(it, end);
-            }
-        } while (*it == ',');
-
-        if (it == end || *it != '}')
-            throw_parse_exception("Unterminated inline table");
-
-        ++it;
-        consume_whitespace(it, end);
-
-        return tbl;
-    }
-
-    void skip_whitespace_and_comments(std::string::iterator& start,
-                                      std::string::iterator& end)
-    {
-        consume_whitespace(start, end);
-        while (start == end || *start == '#')
-        {
-            if (!detail::getline(input_, line_))
-                throw_parse_exception("Unclosed array");
-            line_number_++;
-            start = line_.begin();
-            end = line_.end();
-            consume_whitespace(start, end);
-        }
-    }
-
-    void consume_whitespace(std::string::iterator& it,
-                            const std::string::iterator& end)
-    {
-        while (it != end && (*it == ' ' || *it == '\t'))
-            ++it;
-    }
-
-    void consume_backwards_whitespace(std::string::iterator& back,
-                                      const std::string::iterator& front)
-    {
-        while (back != front && (*back == ' ' || *back == '\t'))
-            --back;
-    }
-
-    void eol_or_comment(const std::string::iterator& it,
-                        const std::string::iterator& end)
-    {
-        if (it != end && *it != '#')
-            throw_parse_exception("Unidentified trailing character '"
-                                  + std::string{*it}
-                                  + "'---did you forget a '#'?");
-    }
-
-    bool is_time(const std::string::iterator& it,
-                 const std::string::iterator& end)
-    {
-        auto time_end = find_end_of_time(it, end);
-        auto len = std::distance(it, time_end);
-
-        if (len < 8)
-            return false;
-
-        if (it[2] != ':' || it[5] != ':')
-            return false;
-
-        if (len > 8)
-            return it[8] == '.' && len > 9;
-
-        return true;
-    }
-
-    option<parse_type> date_type(const std::string::iterator& it,
-                                 const std::string::iterator& end)
-    {
-        auto date_end = find_end_of_date(it, end);
-        auto len = std::distance(it, date_end);
-
-        if (len < 10)
-            return {};
-
-        if (it[4] != '-' || it[7] != '-')
-            return {};
-
-        if (len >= 19 && (it[10] == 'T' || it[10] == ' ')
-            && is_time(it + 11, date_end))
-        {
-            // datetime type
-            auto time_end = find_end_of_time(it + 11, date_end);
-            if (time_end == date_end)
-                return {parse_type::LOCAL_DATETIME};
-            else
-                return {parse_type::OFFSET_DATETIME};
-        }
-        else if (len == 10)
-        {
-            // just a regular date
-            return {parse_type::LOCAL_DATE};
-        }
-
-        return {};
-    }
-
-    std::istream& input_;
-    std::string line_;
-    std::size_t line_number_ = 0;
-};
-
-/**
- * Utility function to parse a file as a TOML file. Returns the root table.
- * Throws a parse_exception if the file cannot be opened.
- */
-inline std::shared_ptr<table> parse_file(const std::string& filename)
-{
-#if defined(BOOST_NOWIDE_FSTREAM_INCLUDED_HPP)
-    boost::nowide::ifstream file{filename.c_str()};
-#elif defined(NOWIDE_FSTREAM_INCLUDED_HPP)
-    nowide::ifstream file{filename.c_str()};
-#else
-    std::ifstream file{filename};
-#endif
-    if (!file.is_open())
-        throw parse_exception{filename + " could not be opened for parsing"};
-    parser p{file};
-    return p.parse();
-}
-
-template <class... Ts>
-struct value_accept;
-
-template <>
-struct value_accept<>
-{
-    template <class Visitor, class... Args>
-    static void accept(const base&, Visitor&&, Args&&...)
-    {
-        // nothing
-    }
-};
-
-template <class T, class... Ts>
-struct value_accept<T, Ts...>
-{
-    template <class Visitor, class... Args>
-    static void accept(const base& b, Visitor&& visitor, Args&&... args)
-    {
-        if (auto v = b.as<T>())
-        {
-            visitor.visit(*v, std::forward<Args>(args)...);
-        }
-        else
-        {
-            value_accept<Ts...>::accept(b, std::forward<Visitor>(visitor),
-                                        std::forward<Args>(args)...);
-        }
-    }
-};
-
-/**
- * base implementation of accept() that calls visitor.visit() on the concrete
- * class.
- */
-template <class Visitor, class... Args>
-void base::accept(Visitor&& visitor, Args&&... args) const
-{
-    if (is_value())
-    {
-        using value_acceptor
-            = value_accept<std::string, int64_t, double, bool, local_date,
-                           local_time, local_datetime, offset_datetime>;
-        value_acceptor::accept(*this, std::forward<Visitor>(visitor),
-                               std::forward<Args>(args)...);
-    }
-    else if (is_table())
-    {
-        visitor.visit(static_cast<const table&>(*this),
-                      std::forward<Args>(args)...);
-    }
-    else if (is_array())
-    {
-        visitor.visit(static_cast<const array&>(*this),
-                      std::forward<Args>(args)...);
-    }
-    else if (is_table_array())
-    {
-        visitor.visit(static_cast<const table_array&>(*this),
-                      std::forward<Args>(args)...);
-    }
-}
-
-/**
- * Writer that can be passed to accept() functions of cpptoml objects and
- * will output valid TOML to a stream.
- */
-class toml_writer
-{
-  public:
-    /**
-     * Construct a toml_writer that will write to the given stream
-     */
-    toml_writer(std::ostream& s, const std::string& indent_space = "\t")
-        : stream_(s), indent_(indent_space), has_naked_endline_(false)
-    {
-        // nothing
-    }
-
-  public:
-    /**
-     * Output a base value of the TOML tree.
-     */
-    template <class T>
-    void visit(const value<T>& v, bool = false)
-    {
-        write(v);
-    }
-
-    /**
-     * Output a table element of the TOML tree
-     */
-    void visit(const table& t, bool in_array = false)
-    {
-        write_table_header(in_array);
-        std::vector<std::string> values;
-        std::vector<std::string> tables;
-
-        for (const auto& i : t)
-        {
-            if (i.second->is_table() || i.second->is_table_array())
-            {
-                tables.push_back(i.first);
-            }
-            else
-            {
-                values.push_back(i.first);
-            }
-        }
-
-        for (unsigned int i = 0; i < values.size(); ++i)
-        {
-            path_.push_back(values[i]);
-
-            if (i > 0)
-                endline();
-
-            write_table_item_header(*t.get(values[i]));
-            t.get(values[i])->accept(*this, false);
-            path_.pop_back();
-        }
-
-        for (unsigned int i = 0; i < tables.size(); ++i)
-        {
-            path_.push_back(tables[i]);
-
-            if (values.size() > 0 || i > 0)
-                endline();
-
-            write_table_item_header(*t.get(tables[i]));
-            t.get(tables[i])->accept(*this, false);
-            path_.pop_back();
-        }
-
-        endline();
-    }
-
-    /**
-     * Output an array element of the TOML tree
-     */
-    void visit(const array& a, bool = false)
-    {
-        write("[");
-
-        for (unsigned int i = 0; i < a.get().size(); ++i)
-        {
-            if (i > 0)
-                write(", ");
-
-            if (a.get()[i]->is_array())
-            {
-                a.get()[i]->as_array()->accept(*this, true);
-            }
-            else
-            {
-                a.get()[i]->accept(*this, true);
-            }
-        }
-
-        write("]");
-    }
-
-    /**
-     * Output a table_array element of the TOML tree
-     */
-    void visit(const table_array& t, bool = false)
-    {
-        for (unsigned int j = 0; j < t.get().size(); ++j)
-        {
-            if (j > 0)
-                endline();
-
-            t.get()[j]->accept(*this, true);
-        }
-
-        endline();
-    }
-
-    /**
-     * Escape a string for output.
-     */
-    static std::string escape_string(const std::string& str)
-    {
-        std::string res;
-        for (auto it = str.begin(); it != str.end(); ++it)
-        {
-            if (*it == '\b')
-            {
-                res += "\\b";
-            }
-            else if (*it == '\t')
-            {
-                res += "\\t";
-            }
-            else if (*it == '\n')
-            {
-                res += "\\n";
-            }
-            else if (*it == '\f')
-            {
-                res += "\\f";
-            }
-            else if (*it == '\r')
-            {
-                res += "\\r";
-            }
-            else if (*it == '"')
-            {
-                res += "\\\"";
-            }
-            else if (*it == '\\')
-            {
-                res += "\\\\";
-            }
-            else if (static_cast<uint32_t>(*it) <= UINT32_C(0x001f))
-            {
-                res += "\\u";
-                std::stringstream ss;
-                ss << std::hex << static_cast<uint32_t>(*it);
-                res += ss.str();
-            }
-            else
-            {
-                res += *it;
-            }
-        }
-        return res;
-    }
-
-  protected:
-    /**
-     * Write out a string.
-     */
-    void write(const value<std::string>& v)
-    {
-        write("\"");
-        write(escape_string(v.get()));
-        write("\"");
-    }
-
-    /**
-     * Write out a double.
-     */
-    void write(const value<double>& v)
-    {
-        std::stringstream ss;
-        ss << std::showpoint
-           << std::setprecision(std::numeric_limits<double>::max_digits10)
-           << v.get();
-
-        auto double_str = ss.str();
-        auto pos = double_str.find("e0");
-        if (pos != std::string::npos)
-            double_str.replace(pos, 2, "e");
-        pos = double_str.find("e-0");
-        if (pos != std::string::npos)
-            double_str.replace(pos, 3, "e-");
-
-        stream_ << double_str;
-        has_naked_endline_ = false;
-    }
-
-    /**
-     * Write out an integer, local_date, local_time, local_datetime, or
-     * offset_datetime.
-     */
-    template <class T>
-    typename std::enable_if<
-        is_one_of<T, int64_t, local_date, local_time, local_datetime,
-                  offset_datetime>::value>::type
-    write(const value<T>& v)
-    {
-        write(v.get());
-    }
-
-    /**
-     * Write out a boolean.
-     */
-    void write(const value<bool>& v)
-    {
-        write((v.get() ? "true" : "false"));
-    }
-
-    /**
-     * Write out the header of a table.
-     */
-    void write_table_header(bool in_array = false)
-    {
-        if (!path_.empty())
-        {
-            indent();
-
-            write("[");
-
-            if (in_array)
-            {
-                write("[");
-            }
-
-            for (unsigned int i = 0; i < path_.size(); ++i)
-            {
-                if (i > 0)
-                {
-                    write(".");
-                }
-
-                if (path_[i].find_first_not_of("ABCDEFGHIJKLMNOPQRSTUVWXYZabcde"
-                                               "fghijklmnopqrstuvwxyz0123456789"
-                                               "_-")
-                    == std::string::npos)
-                {
-                    write(path_[i]);
-                }
-                else
-                {
-                    write("\"");
-                    write(escape_string(path_[i]));
-                    write("\"");
-                }
-            }
-
-            if (in_array)
-            {
-                write("]");
-            }
-
-            write("]");
-            endline();
-        }
-    }
-
-    /**
-     * Write out the identifier for an item in a table.
-     */
-    void write_table_item_header(const base& b)
-    {
-        if (!b.is_table() && !b.is_table_array())
-        {
-            indent();
-
-            if (path_.back().find_first_not_of("ABCDEFGHIJKLMNOPQRSTUVWXYZabcde"
-                                               "fghijklmnopqrstuvwxyz0123456789"
-                                               "_-")
-                == std::string::npos)
-            {
-                write(path_.back());
-            }
-            else
-            {
-                write("\"");
-                write(escape_string(path_.back()));
-                write("\"");
-            }
-
-            write(" = ");
-        }
-    }
-
-  private:
-    /**
-     * Indent the proper number of tabs given the size of
-     * the path.
-     */
-    void indent()
-    {
-        for (std::size_t i = 1; i < path_.size(); ++i)
-            write(indent_);
-    }
-
-    /**
-     * Write a value out to the stream.
-     */
-    template <class T>
-    void write(const T& v)
-    {
-        stream_ << v;
-        has_naked_endline_ = false;
-    }
-
-    /**
-     * Write an endline out to the stream
-     */
-    void endline()
-    {
-        if (!has_naked_endline_)
-        {
-            stream_ << "\n";
-            has_naked_endline_ = true;
-        }
-    }
-
-  private:
-    std::ostream& stream_;
-    const std::string indent_;
-    std::vector<std::string> path_;
-    bool has_naked_endline_;
-};
-
-inline std::ostream& operator<<(std::ostream& stream, const base& b)
-{
-    toml_writer writer{stream};
-    b.accept(writer);
-    return stream;
-}
-
-template <class T>
-std::ostream& operator<<(std::ostream& stream, const value<T>& v)
-{
-    toml_writer writer{stream};
-    v.accept(writer);
-    return stream;
-}
-
-inline std::ostream& operator<<(std::ostream& stream, const table& t)
-{
-    toml_writer writer{stream};
-    t.accept(writer);
-    return stream;
-}
-
-inline std::ostream& operator<<(std::ostream& stream, const table_array& t)
-{
-    toml_writer writer{stream};
-    t.accept(writer);
-    return stream;
-}
-
-inline std::ostream& operator<<(std::ostream& stream, const array& a)
-{
-    toml_writer writer{stream};
-    a.accept(writer);
-    return stream;
-}
-} // namespace cpptoml
-#endif // CPPTOML_H
diff --git a/src/toml11/LICENSE b/src/toml11/LICENSE
new file mode 100644
index 000000000..f55c511d6
--- /dev/null
+++ b/src/toml11/LICENSE
@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2017 Toru Niina
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
diff --git a/src/toml11/README.md b/src/toml11/README.md
new file mode 100644
index 000000000..62b586305
--- /dev/null
+++ b/src/toml11/README.md
@@ -0,0 +1,1966 @@
+toml11
+======
+
+[![Build Status on GitHub Actions](https://github.com/ToruNiina/toml11/workflows/build/badge.svg)](https://github.com/ToruNiina/toml11/actions)
+[![Build Status on TravisCI](https://travis-ci.org/ToruNiina/toml11.svg?branch=master)](https://travis-ci.org/ToruNiina/toml11)
+[![Build status on Appveyor](https://ci.appveyor.com/api/projects/status/m2n08a926asvg5mg/branch/master?svg=true)](https://ci.appveyor.com/project/ToruNiina/toml11/branch/master)
+[![Build status on CircleCI](https://circleci.com/gh/ToruNiina/toml11/tree/master.svg?style=svg)](https://circleci.com/gh/ToruNiina/toml11/tree/master)
+[![Version](https://img.shields.io/github/release/ToruNiina/toml11.svg?style=flat)](https://github.com/ToruNiina/toml11/releases)
+[![License](https://img.shields.io/github/license/ToruNiina/toml11.svg?style=flat)](LICENSE)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1209136.svg)](https://doi.org/10.5281/zenodo.1209136)
+
+toml11 is a C++11 (or later) header-only toml parser/encoder depending only on C++ standard library.
+
+- It is compatible to the latest version of [TOML v1.0.0](https://toml.io/en/v1.0.0).
+- It is one of the most TOML standard compliant libraries, tested with [the language agnostic test suite for TOML parsers by BurntSushi](https://github.com/BurntSushi/toml-test).
+- It shows highly informative error messages. You can see the error messages about invalid files at [CircleCI](https://circleci.com/gh/ToruNiina/toml11).
+- It has configurable container. You can use any random-access containers and key-value maps as backend containers.
+- It optionally preserves comments without any overhead.
+- It has configurable serializer that supports comments, inline tables, literal strings and multiline strings.
+- It supports user-defined type conversion from/into toml values.
+- It correctly handles UTF-8 sequences, with or without BOM, both on posix and Windows.
+
+## Example
+
+```cpp
+#include <toml.hpp>
+#include <iostream>
+
+int main()
+{
+    // ```toml
+    // title = "an example toml file"
+    // nums  = [3, 1, 4, 1, 5]
+    // ```
+    auto data = toml::parse("example.toml");
+
+    // find a value with the specified type from a table
+    std::string title = toml::find<std::string>(data, "title");
+
+    // convert the whole array into any container automatically
+    std::vector<int> nums = toml::find<std::vector<int>>(data, "nums");
+
+    // access with STL-like manner
+    if(!data.contains("foo"))
+    {
+        data["foo"] = "bar";
+    }
+
+    // pass a fallback
+    std::string name = toml::find_or<std::string>(data, "name", "not found");
+
+    // width-dependent formatting
+    std::cout << std::setw(80) << data << std::endl;
+
+    return 0;
+}
+```
+
+## Table of Contents
+
+- [Integration](#integration)
+- [Decoding a toml file](#decoding-a-toml-file)
+  - [In the case of syntax error](#in-the-case-of-syntax-error)
+  - [Invalid UTF-8 Codepoints](#invalid-utf-8-codepoints)
+- [Finding a toml value](#finding-a-toml-value)
+  - [Finding a value in a table](#finding-a-value-in-a-table)
+  - [In case of error](#in-case-of-error)
+  - [Dotted keys](#dotted-keys)
+- [Casting a toml value](#casting-a-toml-value)
+- [Checking value type](#checking-value-type)
+- [More about conversion](#more-about-conversion)
+  - [Converting an array](#converting-an-array)
+  - [Converting a table](#converting-a-table)
+  - [Getting an array of tables](#getting-an-array-of-tables)
+  - [Cost of conversion](#cost-of-conversion)
+  - [Converting datetime and its variants](#converting-datetime-and-its-variants)
+- [Getting with a fallback](#getting-with-a-fallback)
+- [Expecting conversion](#expecting-conversion)
+- [Visiting a toml::value](#visiting-a-tomlvalue)
+- [Constructing a toml::value](#constructing-a-tomlvalue)
+- [Preserving Comments](#preserving-comments)
+- [Customizing containers](#customizing-containers)
+- [TOML literal](#toml-literal)
+- [Conversion between toml value and arbitrary types](#conversion-between-toml-value-and-arbitrary-types)
+- [Formatting user-defined error messages](#formatting-user-defined-error-messages)
+- [Obtaining location information](#obtaining-location-information)
+- [Exceptions](#exceptions)
+- [Colorize Error Messages](#colorize-error-messages)
+- [Serializing TOML data](#serializing-toml-data)
+- [Underlying types](#underlying-types)
+- [Unreleased TOML features](#unreleased-toml-features)
+- [Breaking Changes from v2](#breaking-changes-from-v2)
+- [Running Tests](#running-tests)
+- [Contributors](#contributors)
+- [Licensing Terms](#licensing-terms)
+
+## Integration
+
+Just include the file after adding it to the include path.
+
+```cpp
+#include <toml.hpp> // that's all! now you can use it.
+#include <iostream>
+
+int main()
+{
+    const auto data  = toml::parse("example.toml");
+    const auto title = toml::find<std::string>(data, "title");
+    std::cout << "the title is " << title << std::endl;
+    return 0;
+}
+```
+
+The convenient way is to add this repository as a git-submodule or to install
+it in your system by CMake.
+
+Note for MSVC: We recommend to set `/Zc:__cplusplus` to detect C++ version correctly.
+
+## Decoding a toml file
+
+To parse a toml file, the only thing you have to do is
+to pass a filename to the `toml::parse` function.
+
+```cpp
+const std::string fname("sample.toml");
+const toml::value data = toml::parse(fname);
+```
+
+As required by the TOML specification, the top-level value is always a table.
+You can find a value inside it, cast it into a table explicitly, and insert it as a value into other `toml::value`.
+
+If it encounters an error while opening a file, it will throw `std::runtime_error`.
+
+You can also pass a `std::istream` to the  `toml::parse` function.
+To show a filename in an error message, however, it is recommended to pass the
+filename with the stream.
+
+```cpp
+std::ifstream ifs("sample.toml", std::ios_base::binary);
+assert(ifs.good());
+const auto data = toml::parse(ifs, /*optional -> */ "sample.toml");
+```
+
+**Note**: When you are **on Windows, open a file in binary mode**.
+If a file is opened in text-mode, CRLF ("\r\n") will automatically be
+converted to LF ("\n") and this causes inconsistency between file size
+and the contents that would be read. This causes weird error.
+
+### In the case of syntax error
+
+If there is a syntax error in a toml file, `toml::parse` will throw
+`toml::syntax_error` that inherits `std::exception`.
+
+toml11 has clean and informative error messages inspired by Rust and
+it looks like the following.
+
+```console
+terminate called after throwing an instance of 'toml::syntax_error'
+  what():  [error] toml::parse_table: invalid line format # error description
+ --> example.toml                                         # file name
+ 3 | a = 42 = true                                        # line num and content
+   |        ^------ expected newline, but got '='.        # error reason
+```
+
+If you (mistakenly) duplicate tables and got an error, it is helpful to see
+where they are. toml11 shows both at the same time like the following.
+
+```console
+terminate called after throwing an instance of 'toml::syntax_error'
+  what():  [error] toml::insert_value: table ("table") already exists.
+ --> duplicate-table.toml
+ 1 | [table]
+   | ~~~~~~~ table already exists here
+ ...
+ 3 | [table]
+   | ~~~~~~~ table defined twice
+```
+
+When toml11 encounters a malformed value, it tries to detect what type it is.
+Then it shows hints to fix the format. An error message while reading one of
+the malformed files in [the language agnostic test suite](https://github.com/BurntSushi/toml-test).
+is shown below.
+
+```console
+what(): [error] bad time: should be HH:MM:SS.subsec
+ --> ./datetime-malformed-no-secs.toml
+ 1 | no-secs = 1987-07-05T17:45Z
+   |                     ^------- HH:MM:SS.subsec
+   |
+Hint: pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999
+Hint: fail: 1979-05-27T7:32:00, 1979-05-27 17:32
+```
+
+You can find other examples in a job named `output_result` on
+[CircleCI](https://circleci.com/gh/ToruNiina/toml11).
+
+Since the error message generation is generally a difficult task, the current
+status is not ideal. If you encounter a weird error message, please let us know
+and contribute to improve the quality!
+
+### Invalid UTF-8 codepoints
+
+It throws `syntax_error` if a value of an escape sequence
+representing unicode character is not a valid UTF-8 codepoint.
+
+```console
+  what():  [error] toml::read_utf8_codepoint: input codepoint is too large.
+ --> utf8.toml
+ 1 | exceeds_unicode = "\U0011FFFF example"
+   |                              ^--------- should be in [0x00..0x10FFFF]
+```
+
+## Finding a toml value
+
+After parsing successfully, you can obtain the values from the result of
+`toml::parse` using `toml::find` function.
+
+```toml
+# sample.toml
+answer  = 42
+pi      = 3.14
+numbers = [1,2,3]
+time    = 1979-05-27T07:32:00Z
+```
+
+``` cpp
+const auto data      = toml::parse("sample.toml");
+const auto answer    = toml::find<std::int64_t    >(data, "answer");
+const auto pi        = toml::find<double          >(data, "pi");
+const auto numbers   = toml::find<std::vector<int>>(data, "numbers");
+const auto timepoint = toml::find<std::chrono::system_clock::time_point>(data, "time");
+```
+
+By default, `toml::find` returns a `toml::value`.
+
+```cpp
+const toml::value& answer = toml::find(data, "answer");
+```
+
+When you pass an exact TOML type that does not require type conversion,
+`toml::find` returns a reference without copying the value.
+
+```cpp
+const auto  data   = toml::parse("sample.toml");
+const auto& answer = toml::find<toml::integer>(data, "answer");
+```
+
+If the specified type requires conversion, you can't take a reference to the value.
+See also [underlying types](#underlying-types).
+
+**NOTE**: For some technical reason, automatic conversion between `integer` and
+`floating` is not supported. If you want to get a floating value even if a value
+has integer value, you need to convert it manually after obtaining a value,
+like the following.
+
+```cpp
+const auto vx = toml::find(data, "x");
+double x = vx.is_floating() ? vx.as_floating(std::nothrow) :
+           static_cast<double>(vx.as_integer()); // it throws if vx is neither
+                                                 // floating nor integer.
+```
+
+### Finding a value in a table
+
+There are several way to get a value defined in a table.
+First, you can get a table as a normal value and find a value from the table.
+
+```toml
+[fruit]
+name = "apple"
+[fruit.physical]
+color = "red"
+shape = "round"
+```
+
+``` cpp
+const auto  data  = toml::parse("fruit.toml");
+const auto& fruit = toml::find(data, "fruit");
+const auto  name  = toml::find<std::string>(fruit, "name");
+
+const auto& physical = toml::find(fruit, "physical");
+const auto  color    = toml::find<std::string>(physical, "color");
+const auto  shape    = toml::find<std::string>(physical, "shape");
+```
+
+Here, variable `fruit` is a `toml::value` and can be used as the first argument
+of `toml::find`.
+
+Second, you can pass as many arguments as the number of subtables to `toml::find`.
+
+```cpp
+const auto data  = toml::parse("fruit.toml");
+const auto color = toml::find<std::string>(data, "fruit", "physical", "color");
+const auto shape = toml::find<std::string>(data, "fruit", "physical", "shape");
+```
+
+### Finding a value in an array
+
+You can find n-th value in an array by `toml::find`.
+
+```toml
+values = ["foo", "bar", "baz"]
+```
+
+``` cpp
+const auto data   = toml::parse("sample.toml");
+const auto values = toml::find(data, "values");
+const auto bar    = toml::find<std::string>(values, 1);
+```
+
+`toml::find` can also search array recursively.
+
+```cpp
+const auto data = toml::parse("fruit.toml");
+const auto bar  = toml::find<std::string>(data, "values", 1);
+```
+
+Before calling `toml::find`, you can check if a value corresponding to a key
+exists. You can use both `bool toml::value::contains(const key&) const` and
+`std::size_t toml::value::count(const key&) const`. Those behaves like the
+`std::map::contains` and `std::map::count`.
+
+```cpp
+const auto data = toml::parse("fruit.toml");
+if(data.contains("fruit") && data.at("fruit").count("physical") != 0)
+{
+    // ...
+}
+```
+
+### In case of error
+
+If the value does not exist, `toml::find` throws `std::out_of_range` with the
+location of the table.
+
+```console
+terminate called after throwing an instance of 'std::out_of_range'
+  what():  [error] key "answer" not found
+ --> example.toml
+ 6 | [tab]
+   | ~~~~~ in this table
+```
+
+----
+
+If the specified type differs from the actual value contained, it throws
+`toml::type_error` that inherits `std::exception`.
+
+Similar to the case of syntax error, toml11 also displays clean error messages.
+The error message when you choose `int` to get `string` value would be like this.
+
+```console
+terminate called after throwing an instance of 'toml::type_error'
+  what():  [error] toml::value bad_cast to integer
+ --> example.toml
+ 3 | title = "TOML Example"
+   |         ~~~~~~~~~~~~~~ the actual type is string
+```
+
+**NOTE**: In order to show this kind of error message, all the toml values have
+a pointer to represent its range in a file. The entire contents of a file is
+shared by `toml::value`s and remains on the heap memory. It is recommended to
+destruct all the `toml::value` classes after configuring your application
+if you have a large TOML file compared to the memory resource.
+
+### Dotted keys
+
+TOML v0.5.0 has a new feature named "dotted keys".
+You can chain keys to represent the structure of the data.
+
+```toml
+physical.color = "orange"
+physical.shape = "round"
+```
+
+This is equivalent to the following.
+
+```toml
+[physical]
+color = "orange"
+shape = "round"
+```
+
+You can get both of the above tables with the same c++ code.
+
+```cpp
+const auto physical = toml::find(data, "physical");
+const auto color    = toml::find<std::string>(physical, "color");
+```
+
+The following code does not work for the above toml file.
+
+```cpp
+// XXX this does not work!
+const auto color = toml::find<std::string>(data, "physical.color");
+```
+
+The above code works with the following toml file.
+
+```toml
+"physical.color" = "orange"
+# equivalent to {"physical.color": "orange"},
+# NOT {"physical": {"color": "orange"}}.
+```
+
+
+## Casting a toml value
+
+### `toml::get`
+
+`toml::parse` returns `toml::value`. `toml::value` is a union type that can
+contain one of the following types.
+
+- `toml::boolean` (`bool`)
+- `toml::integer` (`std::int64_t`)
+- `toml::floating` (`double`)
+- `toml::string` (a type convertible to std::string)
+- `toml::local_date`
+- `toml::local_time`
+- `toml::local_datetime`
+- `toml::offset_datetime`
+- `toml::array` (by default, `std::vector<toml::value>`)
+  - It depends. See [customizing containers](#customizing-containers) for detail.
+- `toml::table` (by default, `std::unordered_map<toml::key, toml::value>`)
+  - It depends. See [customizing containers](#customizing-containers) for detail.
+
+To get a value inside, you can use `toml::get<T>()`. The usage is the same as
+`toml::find<T>` (actually, `toml::find` internally uses `toml::get` after casting
+a value to `toml::table`).
+
+``` cpp
+const toml::value  data    = toml::parse("sample.toml");
+const toml::value  answer_ = toml::get<toml::table >(data).at("answer");
+const std::int64_t answer  = toml::get<std::int64_t>(answer_);
+```
+
+When you pass an exact TOML type that does not require type conversion,
+`toml::get` returns a reference through which you can modify the content
+(if the `toml::value` is `const`, it returns `const` reference).
+
+```cpp
+toml::value   data    = toml::parse("sample.toml");
+toml::value   answer_ = toml::get<toml::table >(data).at("answer");
+toml::integer& answer = toml::get<toml::integer>(answer_);
+answer = 6 * 9; // write to data.answer. now `answer_` contains 54.
+```
+
+If the specified type requires conversion, you can't take a reference to the value.
+See also [underlying types](#underlying-types).
+
+It also throws a `toml::type_error` if the type differs.
+
+### `as_xxx`
+
+You can also use a member function to cast a value.
+
+```cpp
+const std::int64_t answer = data.as_table().at("answer").as_integer();
+```
+
+It also throws a `toml::type_error` if the type differs. If you are sure that
+the value `v` contains a value of the specified type, you can suppress checking
+by passing `std::nothrow`.
+
+```cpp
+const auto& answer = data.as_table().at("answer");
+if(answer.is_integer() && answer.as_integer(std::nothrow) == 42)
+{
+    std::cout << "value is 42" << std::endl;
+}
+```
+
+If `std::nothrow` is passed, the functions are marked as noexcept.
+
+By casting a `toml::value` into an array or a table, you can iterate over the
+elements.
+
+```cpp
+const auto data = toml::parse("example.toml");
+std::cout << "keys in the top-level table are the following: \n";
+for(const auto& [k, v] : data.as_table())
+{
+    std::cout << k << '\n';
+}
+
+const auto& fruits = toml::find(data, "fruits");
+for(const auto& v : fruits.as_array())
+{
+    std::cout << toml::find<std::string>(v, "name") << '\n';
+}
+```
+
+The full list of the functions is below.
+
+```cpp
+namespace toml {
+class value {
+    // ...
+    const boolean&         as_boolean()         const&;
+    const integer&         as_integer()         const&;
+    const floating&        as_floating()        const&;
+    const string&          as_string()          const&;
+    const offset_datetime& as_offset_datetime() const&;
+    const local_datetime&  as_local_datetime()  const&;
+    const local_date&      as_local_date()      const&;
+    const local_time&      as_local_time()      const&;
+    const array&           as_array()           const&;
+    const table&           as_table()           const&;
+    // --------------------------------------------------------
+    // non-const version
+    boolean&               as_boolean()         &;
+    // ditto...
+    // --------------------------------------------------------
+    // rvalue version
+    boolean&&              as_boolean()         &&;
+    // ditto...
+
+    // --------------------------------------------------------
+    // noexcept versions ...
+    const boolean&         as_boolean(const std::nothrow_t&) const& noexcept;
+    boolean&               as_boolean(const std::nothrow_t&) &      noexcept;
+    boolean&&              as_boolean(const std::nothrow_t&) &&     noexcept;
+    // ditto...
+};
+} // toml
+```
+
+### `at()`
+
+You can access to the element of a table and an array by `toml::basic_value::at`.
+
+```cpp
+const toml::value v{1,2,3,4,5};
+std::cout << v.at(2).as_integer() << std::endl; // 3
+
+const toml::value v{{"foo", 42}, {"bar", 3.14}};
+std::cout << v.at("foo").as_integer() << std::endl; // 42
+```
+
+If an invalid key (integer for a table, string for an array), it throws
+`toml::type_error` for the conversion. If the provided key is out-of-range,
+it throws `std::out_of_range`.
+
+Note that, although `std::string` has `at()` member function, `toml::value::at`
+throws if the contained type is a string. Because `std::string` does not
+contain `toml::value`.
+
+### `operator[]`
+
+You can also access to the element of a table and an array by
+`toml::basic_value::operator[]`.
+
+```cpp
+const toml::value v{1,2,3,4,5};
+std::cout << v[2].as_integer() << std::endl; // 3
+
+const toml::value v{{"foo", 42}, {"bar", 3.14}};
+std::cout << v["foo"].as_integer() << std::endl; // 42
+```
+
+When you access to a `toml::value` that is not initialized yet via
+`operator[](const std::string&)`, the `toml::value` will be a table,
+just like the `std::map`.
+
+```cpp
+toml::value v; // not initialized as a table.
+v["foo"] = 42; // OK. `v` will be a table.
+```
+
+Contrary, if you access to a `toml::value` that contains an array via `operator[]`,
+it does not check anything. It converts `toml::value` without type check and then
+access to the n-th element without boundary check, just like the `std::vector::operator[]`.
+
+```cpp
+toml::value v; // not initialized as an array
+v[2] = 42;     // error! UB
+```
+
+Please make sure that the `toml::value` has an array inside when you access to
+its element via `operator[]`.
+
+## Checking value type
+
+You can check the type of a value by `is_xxx` function.
+
+```cpp
+const toml::value v = /* ... */;
+if(v.is_integer())
+{
+    std::cout << "value is an integer" << std::endl;
+}
+```
+
+The complete list of the functions is below.
+
+```cpp
+namespace toml {
+class value {
+    // ...
+    bool is_boolean()         const noexcept;
+    bool is_integer()         const noexcept;
+    bool is_floating()        const noexcept;
+    bool is_string()          const noexcept;
+    bool is_offset_datetime() const noexcept;
+    bool is_local_datetime()  const noexcept;
+    bool is_local_date()      const noexcept;
+    bool is_local_time()      const noexcept;
+    bool is_array()           const noexcept;
+    bool is_table()           const noexcept;
+    bool is_uninitialized()   const noexcept;
+    // ...
+};
+} // toml
+```
+
+Also, you can get `enum class value_t` from `toml::value::type()`.
+
+```cpp
+switch(data.at("something").type())
+{
+    case toml::value_t::integer:  /*do some stuff*/ ; break;
+    case toml::value_t::floating: /*do some stuff*/ ; break;
+    case toml::value_t::string :  /*do some stuff*/ ; break;
+    default : throw std::runtime_error(
+        "unexpected type : " + toml::stringize(data.at("something").type()));
+}
+```
+
+The complete list of the `enum`s can be found in the section
+[underlying types](#underlying-types).
+
+The `enum`s can be used as a parameter of `toml::value::is` function like the following.
+
+```cpp
+toml::value v = /* ... */;
+if(v.is(toml::value_t::boolean)) // ...
+```
+
+## More about conversion
+
+Since `toml::find` internally uses `toml::get`, all the following examples work
+with both `toml::get` and `toml::find`.
+
+### Converting an array
+
+You can get any kind of `container` class from a `toml::array`
+except for `map`-like classes.
+
+``` cpp
+// # sample.toml
+// numbers = [1,2,3]
+
+const auto numbers = toml::find(data, "numbers");
+
+const auto vc  = toml::get<std::vector<int>  >(numbers);
+const auto ls  = toml::get<std::list<int>    >(numbers);
+const auto dq  = toml::get<std::deque<int>   >(numbers);
+const auto ar  = toml::get<std::array<int, 3>>(numbers);
+// if the size of data.at("numbers") is larger than that of std::array,
+// it will throw toml::type_error because std::array is not resizable.
+```
+
+Surprisingly, you can convert `toml::array` into `std::pair` and `std::tuple`.
+
+```cpp
+// numbers = [1,2,3]
+const auto tp = toml::get<std::tuple<short, int, unsigned int>>(numbers);
+```
+
+This functionality is helpful when you have a toml file like the following.
+
+```toml
+array_of_arrays = [[1, 2, 3], ["foo", "bar", "baz"]] # toml allows this
+```
+
+What is the corresponding C++ type?
+Obviously, it is a `std::pair` of `std::vector`s.
+
+```cpp
+const auto array_of_arrays = toml::find(data, "array_of_arrays");
+const auto aofa = toml::get<
+    std::pair<std::vector<int>, std::vector<std::string>>
+    >(array_of_arrays);
+```
+
+If you don't know the type of the elements, you can use `toml::array`,
+which is a `std::vector` of `toml::value`, instead.
+
+```cpp
+const auto a_of_a = toml::get<toml::array>(array_of_arrays);
+const auto first  = toml::get<std::vector<int>>(a_of_a.at(0));
+```
+
+You can change the implementation of `toml::array` with `std::deque` or some
+other array-like container. See [Customizing containers](#customizing-containers)
+for detail.
+
+### Converting a table
+
+When all the values of the table have the same type, toml11 allows you to
+convert a `toml::table` to a `map` that contains the convertible type.
+
+```toml
+[tab]
+key1 = "foo" # all the values are
+key2 = "bar" # toml String
+```
+
+```cpp
+const auto data = toml::parse("sample.toml");
+const auto tab = toml::find<std::map<std::string, std::string>>(data, "tab");
+std::cout << tab["key1"] << std::endl; // foo
+std::cout << tab["key2"] << std::endl; // bar
+```
+
+But since `toml::table` is just an alias of `std::unordered_map<toml::key, toml::value>`,
+normally you don't need to convert it because it has all the functionalities that
+`std::unordered_map` has (e.g. `operator[]`, `count`, and `find`). In most cases
+`toml::table` is sufficient.
+
+```cpp
+toml::table tab = toml::get<toml::table>(data);
+if(data.count("title") != 0)
+{
+    data["title"] = std::string("TOML example");
+}
+```
+
+You can change the implementation of `toml::table` with `std::map` or some
+other map-like container. See [Customizing containers](#customizing-containers)
+for detail.
+
+### Getting an array of tables
+
+An array of tables is just an array of tables.
+You can get it in completely the same way as the other arrays and tables.
+
+```toml
+# sample.toml
+array_of_inline_tables = [{key = "value1"}, {key = "value2"}, {key = "value3"}]
+
+[[array_of_tables]]
+key = "value4"
+[[array_of_tables]]
+key = "value5"
+[[array_of_tables]]
+key = "value6"
+```
+
+```cpp
+const auto data = toml::parse("sample.toml");
+const auto aot1 = toml::find<std::vector<toml::table>>(data, "array_of_inline_tables");
+const auto aot2 = toml::find<std::vector<toml::table>>(data, "array_of_tables");
+```
+
+### Cost of conversion
+
+Although conversion through `toml::(get|find)` is convenient, it has additional
+copy-cost because it copies data contained in `toml::value` to the
+user-specified type. Of course in some cases this overhead is not ignorable.
+
+```cpp
+// the following code constructs a std::vector.
+// it requires heap allocation for vector and element conversion.
+const auto array = toml::find<std::vector<int>>(data, "foo");
+```
+
+By passing the exact types, `toml::get` returns reference that has no overhead.
+
+``` cpp
+const auto& tab     = toml::find<toml::table>(data, "tab");
+const auto& numbers = toml::find<toml::array>(data, "numbers");
+```
+
+Also, `as_xxx` are zero-overhead because they always return a reference.
+
+``` cpp
+const auto& tab     = toml::find(data, "tab"    ).as_table();
+const auto& numbers = toml::find(data, "numbers").as_array();
+```
+
+In this case you need to call `toml::get` each time you access to
+the element of `toml::array` because `toml::array` is an array of `toml::value`.
+
+```cpp
+const auto& num0 = toml::get<toml::integer>(numbers.at(0));
+const auto& num1 = toml::get<toml::integer>(numbers.at(1));
+const auto& num2 = toml::get<toml::integer>(numbers.at(2));
+```
+
+### Converting datetime and its variants
+
+TOML v0.5.0 has 4 different datetime objects, `local_date`, `local_time`,
+`local_datetime`, and `offset_datetime`.
+
+Since `local_date`, `local_datetime`, and `offset_datetime` represent a time
+point, you can convert them to `std::chrono::system_clock::time_point`.
+
+Contrary, `local_time` does not represents a time point because they lack a
+date information, but it can be converted to `std::chrono::duration` that
+represents a duration from the beginning of the day, `00:00:00.000`.
+
+```toml
+# sample.toml
+date = 2018-12-23
+time = 12:30:00
+l_dt = 2018-12-23T12:30:00
+o_dt = 2018-12-23T12:30:00+09:30
+```
+
+```cpp
+const auto data = toml::parse("sample.toml");
+
+const auto date = toml::get<std::chrono::system_clock::time_point>(data.at("date"));
+const auto l_dt = toml::get<std::chrono::system_clock::time_point>(data.at("l_dt"));
+const auto o_dt = toml::get<std::chrono::system_clock::time_point>(data.at("o_dt"));
+
+const auto time = toml::get<std::chrono::minutes>(data.at("time")); // 12 * 60 + 30 min
+```
+
+`local_date` and `local_datetime` are assumed to be in the local timezone when
+they are converted into `time_point`. On the other hand, `offset_datetime` only
+uses the offset part of the data and it does not take local timezone into account.
+
+To contain datetime data, toml11 defines its own datetime types.
+For more detail, you can see the definitions in [toml/datetime.hpp](toml/datetime.hpp).
+
+## Getting with a fallback
+
+`toml::find_or` returns a default value if the value is not found or has a
+different type.
+
+```cpp
+const auto data = toml::parse("example.toml");
+const auto num  = toml::find_or(data, "num", 42);
+```
+
+It works recursively if you pass several keys for subtables.
+In that case, the last argument is considered to be the optional value.
+All other arguments between `toml::value` and the optinoal value are considered as keys.
+
+```cpp
+// [fruit.physical]
+// color = "red"
+auto data  = toml::parse("fruit.toml");
+auto color = toml::find_or(data, "fruit", "physical", "color", "red");
+//                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^  ^^^^^
+//                               arguments                     optional value
+```
+
+Also, `toml::get_or` returns a default value if `toml::get<T>` failed.
+
+```cpp
+toml::value v("foo"); // v contains String
+const int value = toml::get_or(v, 42); // conversion fails. it returns 42.
+```
+
+These functions automatically deduce what type you want to get
+from the default value you passed.
+
+To get a reference through this function, take care about the default value.
+
+```cpp
+toml::value v("foo"); // v contains String
+toml::integer& i = toml::get_or(v, 42); // does not work because binding `42`
+                                        // to `integer&` is invalid
+toml::integer opt = 42;
+toml::integer& i = toml::get_or(v, opt); // this works.
+```
+
+## Expecting conversion
+
+By using `toml::expect`, you will get your expected value or an error message
+without throwing `toml::type_error`.
+
+```cpp
+const auto value = toml::expect<std::string>(data.at("title"));
+if(value.is_ok()) {
+    std::cout << value.unwrap() << std::endl;
+} else {
+    std::cout << value.unwrap_err() << std::endl;
+}
+```
+
+Also, you can pass a function object to modify the expected value.
+
+```cpp
+const auto value = toml::expect<int>(data.at("number"))
+    .map(// function that receives expected type (here, int)
+    [](const int number) -> double {
+        return number * 1.5 + 1.0;
+    }).unwrap_or(/*default value =*/ 3.14);
+```
+
+## Visiting a toml::value
+
+toml11 provides `toml::visit` to apply a function to `toml::value` in the
+same way as `std::variant`.
+
+```cpp
+const toml::value v(3.14);
+toml::visit([](const auto& val) -> void {
+        std::cout << val << std::endl;
+    }, v);
+```
+
+The function object that would be passed to `toml::visit` must be able to
+receive all the possible TOML types. Also, the result types should be the same
+each other.
+
+## Constructing a toml::value
+
+`toml::value` can be constructed in various ways.
+
+```cpp
+toml::value v(true);     // boolean
+toml::value v(42);       // integer
+toml::value v(3.14);     // floating
+toml::value v("foobar"); // string
+toml::value v(toml::local_date(2019, toml::month_t::Apr, 1)); // date
+toml::value v{1, 2, 3, 4, 5};                                 // array
+toml::value v{{"foo", 42}, {"bar", 3.14}, {"baz", "qux"}};    // table
+```
+
+When constructing a string, you can choose to use either literal or basic string.
+By default, it will be a basic string.
+
+```cpp
+toml::value v("foobar", toml::string_t::basic  );
+toml::value v("foobar", toml::string_t::literal);
+```
+
+Datetime objects can be constructed from `std::tm` and
+`std::chrono::system_clock::time_point`. But you need to specify what type
+you use to avoid ambiguity.
+
+```cpp
+const auto now = std::chrono::system_clock::now();
+toml::value v(toml::local_date(now));
+toml::value v(toml::local_datetime(now));
+toml::value v(toml::offset_datetime(now));
+```
+
+Since local time is not equivalent to a time point, because it lacks date
+information, it will be constructed from `std::chrono::duration`.
+
+```cpp
+toml::value v(toml::local_time(std::chrono::hours(10)));
+```
+
+You can construct an array object not only from `initializer_list`, but also
+from STL containers. In that case, the element type must be convertible to
+`toml::value`.
+
+```cpp
+std::vector<int> vec{1,2,3,4,5};
+toml::value v(vec);
+```
+
+When you construct an array value, all the elements of `initializer_list`
+must be convertible into `toml::value`.
+
+If a `toml::value` has an array, you can `push_back` an element in it.
+
+```cpp
+toml::value v{1,2,3,4,5};
+v.push_back(6);
+```
+
+`emplace_back` also works.
+
+## Preserving comments
+
+toml11 v3 or later allows you yo choose whether comments are preserved or not via template parameter
+
+```cpp
+const auto data1 = toml::parse<toml::discard_comments >("example.toml");
+const auto data2 = toml::parse<toml::preserve_comments>("example.toml");
+```
+
+or macro definition.
+
+```cpp
+#define TOML11_PRESERVE_COMMENTS_BY_DEFAULT
+#include <toml11/toml.hpp>
+```
+
+This feature is controlled by template parameter in `toml::basic_value<...>`.
+`toml::value` is an alias of `toml::basic_value<...>`.
+
+If template parameter is explicitly specified, the return value of `toml::parse`
+will be `toml::basic_value<toml::preserve_comments>`.
+If the macro is defined, the alias `toml::value` will be
+`toml::basic_value<toml::preserve_comments>`.
+
+Comments related to a value can be obtained by `toml::value::comments()`.
+The return value has the same interface as `std::vector<std::string>`.
+
+```cpp
+const auto& com = v.comments();
+for(const auto& c : com)
+{
+    std::cout << c << std::endl;
+}
+```
+
+Comments just before and just after (within the same line) a value are kept in a value.
+
+```toml
+# this is a comment for v1.
+v1 = "foo"
+
+v2 = "bar" # this is a comment for v2.
+# Note that this comment is NOT a comment for v2.
+
+# this comment is not related to any value
+# because there are empty lines between v3.
+# this comment will be ignored even if you set `preserve_comments`.
+
+# this is a comment for v3
+# this is also a comment for v3.
+v3 = "baz" # ditto.
+```
+
+Each comment line becomes one element of a `std::vector`.
+
+Hash signs will be removed, but spaces after hash sign will not be removed.
+
+```cpp
+v1.comments().at(0) == " this is a comment for v1."s;
+
+v2.comments().at(1) == " this is a comment for v1."s;
+
+v3.comments().at(0) == " this is a comment for v3."s;
+v3.comments().at(1) == " this is also a comment for v3."s;
+v3.comments().at(2) == " ditto."s;
+```
+
+Note that a comment just after an opening brace of an array will not be a
+comment for the array.
+
+```toml
+# this is a comment for a.
+a = [ # this is not a comment for a. this will be ignored.
+  1, 2, 3,
+  # this is a comment for `42`.
+  42, # this is also a comment for `42`.
+  5
+] # this is a comment for a.
+```
+
+You can also append and modify comments.
+The interfaces are the same as `std::vector<std::string>`.
+
+```cpp
+toml::basic_value<toml::preserve_comments> v(42);
+v.comments().push_back(" add this comment.");
+// # add this comment.
+// i = 42
+```
+
+Also, you can pass a `std::vector<std::string>` when constructing a
+`toml::basic_value<toml::preserve_comments>`.
+
+```cpp
+std::vector<std::string> comments{"comment 1", "comment 2"};
+const toml::basic_value<toml::preserve_comments> v1(42, std::move(comments));
+const toml::basic_value<toml::preserve_comments> v2(42, {"comment 1", "comment 2"});
+```
+
+When `toml::discard_comments` is chosen, comments will not be contained in a value.
+`value::comments()` will always be kept empty.
+All the modification on comments would be ignored.
+All the element access in a `discard_comments` causes the same error as accessing
+an element of an empty `std::vector`.
+
+The comments will also be serialized. If comments exist, those comments will be
+added just before the values.
+
+__NOTE__: Result types from `toml::parse(...)` and
+`toml::parse<toml::preserve_comments>(...)` are different.
+
+## Customizing containers
+
+Actually, `toml::basic_value` has 3 template arguments.
+
+```cpp
+template<typename Comment, // discard/preserve_comment
+         template<typename ...> class Table = std::unordered_map,
+         template<typename ...> class Array = std::vector>
+class basic_value;
+```
+
+This enables you to change the containers used inside. E.g. you can use
+`std::map` to contain a table object instead of `std::unordered_map`.
+And also can use `std::deque` as a array object instead of `std::vector`.
+
+You can set these parameters while calling `toml::parse` function.
+
+```cpp
+const auto data = toml::parse<
+    toml::preserve_comments, std::map, std::deque
+    >("example.toml");
+```
+
+Needless to say, the result types from `toml::parse(...)` and
+`toml::parse<Com, Map, Cont>(...)` are different (unless you specify the same
+types as default).
+
+Note that, since `toml::table` and `toml::array` is an alias for a table and an
+array of a default `toml::value`, so it is different from the types actually
+contained in a `toml::basic_value` when you customize containers.
+To get the actual type in a generic way, use
+`typename toml::basic_type<C, T, A>::table_type` and
+`typename toml::basic_type<C, T, A>::array_type`.
+
+## TOML literal
+
+toml11 supports `"..."_toml` literal.
+It accept both a bare value and a file content.
+
+```cpp
+using namespace toml::literals::toml_literals;
+
+// `_toml` can convert a bare value without key
+const toml::value v = u8"0xDEADBEEF"_toml;
+// v is an Integer value containing 0xDEADBEEF.
+
+// raw string literal (`R"(...)"` is useful for this purpose)
+const toml::value t = u8R"(
+    title = "this is TOML literal"
+    [table]
+    key = "value"
+)"_toml;
+// the literal will be parsed and the result will be contained in t
+```
+
+The literal function is defined in the same way as the standard library literals
+such as `std::literals::string_literals::operator""s`.
+
+```cpp
+namespace toml
+{
+inline namespace literals
+{
+inline namespace toml_literals
+{
+toml::value operator"" _toml(const char* str, std::size_t len);
+} // toml_literals
+} // literals
+} // toml
+```
+
+Access to the operator can be gained with `using namespace toml::literals;`,
+`using namespace toml::toml_literals`, and `using namespace toml::literals::toml_literals`.
+
+Note that a key that is composed only of digits is allowed in TOML.
+And, unlike the file parser, toml-literal allows a bare value without a key.
+Thus it is difficult to distinguish arrays having integers and definitions of
+tables that are named as digits.
+Currently, literal `[1]` becomes a table named "1".
+To ensure a literal to be considered as an array with one element, you need to
+add a comma after the first element (like `[1,]`).
+
+```cpp
+"[1,2,3]"_toml;   // This is an array
+"[table]"_toml;   // This is a table that has an empty table named "table" inside.
+"[[1,2,3]]"_toml; // This is an array of arrays
+"[[table]]"_toml; // This is a table that has an array of tables inside.
+
+"[[1]]"_toml;     // This literal is ambiguous.
+                  // Currently, it becomes a table that has array of table "1".
+"1 = [{}]"_toml;  // This is a table that has an array of table named 1.
+"[[1,]]"_toml;    // This is an array of arrays.
+"[[1],]"_toml;    // ditto.
+```
+
+NOTE: `_toml` literal returns a `toml::value`  that does not have comments.
+
+## Conversion between toml value and arbitrary types
+
+You can also use `toml::get` and other related functions with the types
+you defined after you implement a way to convert it.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    int         a;
+    double      b;
+    std::string c;
+};
+} // ext
+
+const auto data = toml::parse("example.toml");
+
+// to do this
+const foo f = toml::find<ext::foo>(data, "foo");
+```
+
+There are 3 ways to use `toml::get` with the types that you defined.
+
+The first one is to implement `from_toml(const toml::value&)` member function.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    int         a;
+    double      b;
+    std::string c;
+
+    void from_toml(const toml::value& v)
+    {
+        this->a = toml::find<int        >(v, "a");
+        this->b = toml::find<double     >(v, "b");
+        this->c = toml::find<std::string>(v, "c");
+        return;
+    }
+};
+} // ext
+```
+
+In this way, because `toml::get` first constructs `foo` without arguments,
+the type should be default-constructible.
+
+The second is to implement `constructor(const toml::value&)`.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    explicit foo(const toml::value& v)
+        : a(toml::find<int>(v, "a")), b(toml::find<double>(v, "b")),
+          c(toml::find<std::string>(v, "c"))
+    {}
+
+    int         a;
+    double      b;
+    std::string c;
+};
+} // ext
+```
+
+Note that implicit default constructor declaration will be suppressed
+when a constructor is defined. If you want to use the struct (here, `foo`)
+in a container (e.g. `std::vector<foo>`), you may need to define default
+constructor explicitly.
+
+The third is to implement specialization of `toml::from` for your type.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    int         a;
+    double      b;
+    std::string c;
+};
+} // ext
+
+namespace toml
+{
+template<>
+struct from<ext::foo>
+{
+    static ext::foo from_toml(const value& v)
+    {
+        ext::foo f;
+        f.a = find<int        >(v, "a");
+        f.b = find<double     >(v, "b");
+        f.c = find<std::string>(v, "c");
+        return f;
+    }
+};
+} // toml
+```
+
+In this way, since the conversion function is defined outside of the class,
+you can add conversion between `toml::value` and classes defined in another library.
+
+In some cases, a class has a templatized constructor that takes a template, `T`.
+It confuses `toml::get/find<T>` because it makes the class "constructible" from
+`toml::value`. To avoid this problem, `toml::from` and `from_toml` always
+precede constructor. It makes easier to implement conversion between
+`toml::value` and types defined in other libraries because it skips constructor.
+
+But, importantly, you cannot define `toml::from<T>` and `T.from_toml` at the same
+time because it causes ambiguity in the overload resolution of `toml::get<T>` and `toml::find<T>`.
+
+So the precedence is `toml::from<T>` == `T.from_toml()` > `T(toml::value)`.
+
+If you want to convert any versions of `toml::basic_value`,
+you need to templatize the conversion function as follows.
+
+```cpp
+struct foo
+{
+    template<typename C, template<typename ...> class M, template<typename ...> class A>
+    void from_toml(const toml::basic_value<C, M, A>& v)
+    {
+        this->a = toml::find<int        >(v, "a");
+        this->b = toml::find<double     >(v, "b");
+        this->c = toml::find<std::string>(v, "c");
+        return;
+    }
+};
+// or
+namespace toml
+{
+template<>
+struct from<ext::foo>
+{
+    template<typename C, template<typename ...> class M, template<typename ...> class A>
+    static ext::foo from_toml(const basic_value<C, M, A>& v)
+    {
+        ext::foo f;
+        f.a = find<int        >(v, "a");
+        f.b = find<double     >(v, "b");
+        f.c = find<std::string>(v, "c");
+        return f;
+    }
+};
+} // toml
+```
+
+----
+
+The opposite direction is also supported in a similar way. You can directly
+pass your type to `toml::value`'s constructor by introducing `into_toml` or
+`toml::into<T>`.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    int         a;
+    double      b;
+    std::string c;
+
+    toml::value into_toml() const // you need to mark it const.
+    {
+        return toml::value{{"a", this->a}, {"b", this->b}, {"c", this->c}};
+    }
+};
+} // ext
+
+ext::foo    f{42, 3.14, "foobar"};
+toml::value v(f);
+```
+
+The definition of `toml::into<T>` is similar to `toml::from<T>`.
+
+```cpp
+namespace ext
+{
+struct foo
+{
+    int         a;
+    double      b;
+    std::string c;
+};
+} // ext
+
+namespace toml
+{
+template<>
+struct into<ext::foo>
+{
+    static toml::value into_toml(const ext::foo& f)
+    {
+        return toml::value{{"a", f.a}, {"b", f.b}, {"c", f.c}};
+    }
+};
+} // toml
+
+ext::foo    f{42, 3.14, "foobar"};
+toml::value v(f);
+```
+
+Any type that can be converted to `toml::value`, e.g. `int`, `toml::table` and
+`toml::array` are okay to return from `into_toml`.
+
+You can also return a custom `toml::basic_value` from `toml::into`.
+
+```cpp
+namespace toml
+{
+template<>
+struct into<ext::foo>
+{
+    static toml::basic_value<toml::preserve_comments> into_toml(const ext::foo& f)
+    {
+        toml::basic_value<toml::preserve_comments> v{{"a", f.a}, {"b", f.b}, {"c", f.c}};
+        v.comments().push_back(" comment");
+        return v;
+    }
+};
+} // toml
+```
+
+But note that, if this `basic_value` would be assigned into other `toml::value`
+that discards `comments`, the comments would be dropped.
+
+### Macro to automatically define conversion functions
+
+There is a helper macro that automatically generates conversion functions `from` and `into` for a simple struct.
+
+```cpp
+namespace foo
+{
+struct Foo
+{
+    std::string s;
+    double      d;
+    int         i;
+};
+} // foo
+
+TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(foo::Foo, s, d, i)
+
+int main()
+{
+    const auto file = toml::parse("example.toml");
+    auto f = toml::find<foo::Foo>(file, "foo");
+}
+```
+
+And then you can use `toml::find<foo::Foo>(file, "foo");`
+
+**Note** that, because of a slight difference in implementation of preprocessor between gcc/clang and MSVC, [you need to define `/Zc:preprocessor`](https://github.com/ToruNiina/toml11/issues/139#issuecomment-803683682) to use it in MSVC (Thank you @glebm !).
+
+## Formatting user-defined error messages
+
+When you encounter an error after you read the toml value, you may want to
+show the error with the value.
+
+toml11 provides you a function that formats user-defined error message with
+related values. With a code like the following,
+
+```cpp
+const auto value = toml::find<int>(data, "num");
+if(value < 0)
+{
+    std::cerr << toml::format_error("[error] value should be positive",
+                                    data.at("num"), "positive number required")
+              << std::endl;
+}
+```
+
+you will get an error message like this.
+
+```console
+[error] value should be positive
+ --> example.toml
+ 3 | num = -42
+   |       ~~~ positive number required
+```
+
+When you pass two values to `toml::format_error`,
+
+```cpp
+const auto min = toml::find<int>(range, "min");
+const auto max = toml::find<int>(range, "max");
+if(max < min)
+{
+    std::cerr << toml::format_error("[error] max should be larger than min",
+                                    data.at("min"), "minimum number here",
+                                    data.at("max"), "maximum number here");
+              << std::endl;
+}
+```
+
+you will get an error message like this.
+
+```console
+[error] max should be larger than min
+ --> example.toml
+ 3 | min = 54
+   |       ~~ minimum number here
+ ...
+ 4 | max = 42
+   |       ~~ maximum number here
+```
+
+You can print hints at the end of the message.
+
+```cpp
+std::vector<std::string> hints;
+hints.push_back("positive number means n >= 0.");
+hints.push_back("negative number is not positive.");
+std::cerr << toml::format_error("[error] value should be positive",
+                                data.at("num"), "positive number required", hints)
+          << std::endl;
+```
+
+```console
+[error] value should be positive
+ --> example.toml
+ 2 | num = 42
+   |       ~~ positive number required
+   |
+Hint: positive number means n >= 0.
+Hint: negative number is not positive.
+```
+
+## Obtaining location information
+
+You can also format error messages in your own way by using `source_location`.
+
+```cpp
+struct source_location
+{
+    std::uint_least32_t line()      const noexcept;
+    std::uint_least32_t column()    const noexcept;
+    std::uint_least32_t region()    const noexcept;
+    std::string const&  file_name() const noexcept;
+    std::string const&  line_str()  const noexcept;
+};
+// +-- line()       +--- length of the region (here, region() == 9)
+// v            .---+---.
+// 12 | value = "foo bar" <- line_str() returns the line itself.
+//              ^-------- column() points here
+```
+
+You can get this by
+```cpp
+const toml::value           v   = /*...*/;
+const toml::source_location loc = v.location();
+```
+
+## Exceptions
+
+The following `exception` classes inherits `toml::exception` that inherits
+`std::exception`.
+
+```cpp
+namespace toml {
+struct exception      : public std::exception  {/**/};
+struct syntax_error   : public toml::exception {/**/};
+struct type_error     : public toml::exception {/**/};
+struct internal_error : public toml::exception {/**/};
+} // toml
+```
+
+`toml::exception` has `toml::exception::location()` member function that returns
+`toml::source_location`, in addition to `what()`.
+
+```cpp
+namespace toml {
+struct exception : public std::exception
+{
+    // ...
+    source_location const& location() const noexcept;
+};
+} // toml
+```
+
+It represents where the error occurs.
+
+`syntax_error` will be thrown from `toml::parse` and `_toml` literal.
+`type_error` will be thrown from `toml::get/find`, `toml::value::as_xxx()`, and
+other functions that takes a content inside of `toml::value`.
+
+Note that, currently, from `toml::value::at()` and `toml::find(value, key)`
+may throw an `std::out_of_range` that does not inherits `toml::exception`.
+
+Also, in some cases, most likely in the file open error, it will throw an
+`std::runtime_error`.
+
+## Colorize Error Messages
+
+By defining `TOML11_COLORIZE_ERROR_MESSAGE`, the error messages from
+`toml::parse` and `toml::find|get` will be colorized. By default, this feature
+is turned off.
+
+With the following toml file taken from `toml-lang/toml/tests/hard_example.toml`,
+
+```toml
+[error]
+array = [
+         "This might most likely happen in multiline arrays",
+         Like here,
+         "or here,
+         and here"
+        ]     End of array comment, forgot the #
+```
+
+the error message would be like this.
+
+![error-message-1](https://github.com/ToruNiina/toml11/blob/misc/misc/toml11-err-msg-1.png)
+
+With the following,
+
+```toml
+[error]
+# array = [
+#          "This might most likely happen in multiline arrays",
+#          Like here,
+#          "or here,
+#          and here"
+#         ]     End of array comment, forgot the #
+number = 3.14  pi <--again forgot the #
+```
+
+the error message would be like this.
+
+![error-message-2](https://github.com/ToruNiina/toml11/blob/misc/misc/toml11-err-msg-2.png)
+
+The message would be messy when it is written to a file, not a terminal because
+it uses [ANSI escape code](https://en.wikipedia.org/wiki/ANSI_escape_code).
+
+Without `TOML11_COLORIZE_ERROR_MESSAGE`, you can still colorize user-defined
+error message by passing `true` to the `toml::format_error` function.
+If you define `TOML11_COLORIZE_ERROR_MESSAGE`, the value is `true` by default.
+If not, the default value would be `false`.
+
+```cpp
+std::cerr << toml::format_error("[error] value should be positive",
+                                data.at("num"), "positive number required",
+                                hints, /*colorize = */ true) << std::endl;
+```
+
+Note: It colorize `[error]` in red. That means that it detects `[error]` prefix
+at the front of the error message. If there is no `[error]` prefix,
+`format_error` adds it to the error message.
+
+## Serializing TOML data
+
+toml11 enables you to serialize data into toml format.
+
+```cpp
+const toml::value data{{"foo", 42}, {"bar", "baz"}};
+std::cout << data << std::endl;
+// bar = "baz"
+// foo = 42
+```
+
+toml11 automatically makes a small table and small array inline.
+You can specify the width to make them inline by `std::setw` for streams.
+
+```cpp
+const toml::value data{
+    {"qux",    {{"foo", 42}, {"bar", "baz"}}},
+    {"quux",   {"small", "array", "of", "strings"}},
+    {"foobar", {"this", "array", "of", "strings", "is", "too", "long",
+                "to", "print", "into", "single", "line", "isn't", "it?"}},
+};
+
+// the threshold becomes 80.
+std::cout << std::setw(80) << data << std::endl;
+// foobar = [
+// "this","array","of","strings","is","too","long","to","print","into",
+// "single","line","isn't","it?",
+// ]
+// quux = ["small","array","of","strings"]
+// qux = {bar="baz",foo=42}
+
+
+// the width is 0. nothing become inline.
+std::cout << std::setw(0) << data << std::endl;
+// foobar = [
+// "this",
+// ... (snip)
+// "it?",
+// ]
+// quux = [
+// "small",
+// "array",
+// "of",
+// "strings",
+// ]
+// [qux]
+// bar = "baz"
+// foo = 42
+```
+
+It is recommended to set width before printing data. Some I/O functions changes
+width to 0, and it makes all the stuff (including `toml::array`) multiline.
+The resulting files becomes too long.
+
+To control the precision of floating point numbers, you need to pass
+`std::setprecision` to stream.
+
+```cpp
+const toml::value data{
+    {"pi", 3.141592653589793},
+    {"e",  2.718281828459045}
+};
+std::cout << std::setprecision(17) << data << std::endl;
+// e = 2.7182818284590451
+// pi = 3.1415926535897931
+std::cout << std::setprecision( 7) << data << std::endl;
+// e = 2.718282
+// pi = 3.141593
+```
+
+There is another way to format toml values, `toml::format()`.
+It returns `std::string` that represents a value.
+
+```cpp
+const toml::value v{{"a", 42}};
+const std::string fmt = toml::format(v);
+// a = 42
+```
+
+Note that since `toml::format` formats a value, the resulting string may lack
+the key value.
+
+```cpp
+const toml::value v{3.14};
+const std::string fmt = toml::format(v);
+// 3.14
+```
+
+To control the width and precision, `toml::format` receives optional second and
+third arguments to set them. By default, the width is 80 and the precision is
+`std::numeric_limits<double>::max_digit10`.
+
+```cpp
+const auto serial = toml::format(data, /*width = */ 0, /*prec = */ 17);
+```
+
+When you pass a comment-preserving-value, the comment will also be serialized.
+An array or a table containing a value that has a comment would not be inlined.
+
+## Underlying types
+
+The toml types (can be used as `toml::*` in this library) and corresponding `enum` names are listed in the table below.
+
+| TOML type      | underlying c++ type                | enum class                       |
+| -------------- | ---------------------------------- | -------------------------------- |
+| Boolean        | `bool`                             | `toml::value_t::boolean`         |
+| Integer        | `std::int64_t`                     | `toml::value_t::integer`         |
+| Float          | `double`                           | `toml::value_t::floating`        |
+| String         | `toml::string`                     | `toml::value_t::string`          |
+| LocalDate      | `toml::local_date`                 | `toml::value_t::local_date`      |
+| LocalTime      | `toml::local_time`                 | `toml::value_t::local_time`      |
+| LocalDatetime  | `toml::local_datetime`             | `toml::value_t::local_datetime`  |
+| OffsetDatetime | `toml::offset_datetime`            | `toml::value_t::offset_datetime` |
+| Array          | `array-like<toml::value>`          | `toml::value_t::array`           |
+| Table          | `map-like<toml::key, toml::value>` | `toml::value_t::table`           |
+
+`array-like` and `map-like` are the STL containers that works like a `std::vector` and
+`std::unordered_map`, respectively. By default, `std::vector` and `std::unordered_map`
+are used. See [Customizing containers](#customizing-containers) for detail.
+
+`toml::string` is effectively the same as `std::string` but has an additional
+flag that represents a kind of a string, `string_t::basic` and `string_t::literal`.
+Although `std::string` is not an exact toml type, still you can get a reference
+that points to internal `std::string` by using `toml::get<std::string>()` for convenience.
+The most important difference between `std::string` and `toml::string` is that
+`toml::string` will be formatted as a TOML string when outputted with `ostream`.
+This feature is introduced to make it easy to write a custom serializer.
+
+`Datetime` variants are `struct` that are defined in this library.
+Because `std::chrono::system_clock::time_point` is a __time point__,
+not capable of representing a Local Time independent from a specific day.
+
+## Unreleased TOML features
+
+Since TOML v1.0.0-rc.1 has been released, those features are now activated by
+default. We no longer need to define `TOML11_USE_UNRELEASED_FEATURES`.
+
+- Leading zeroes in exponent parts of floats are permitted.
+  - e.g. `1.0e+01`, `5e+05`
+  - [toml-lang/toml/PR/656](https://github.com/toml-lang/toml/pull/656)
+- Allow raw tab characters in basic strings and multi-line basic strings.
+  - [toml-lang/toml/PR/627](https://github.com/toml-lang/toml/pull/627)
+- Allow heterogeneous arrays
+  - [toml-lang/toml/PR/676](https://github.com/toml-lang/toml/pull/676)
+
+## Note about heterogeneous arrays
+
+Although `toml::parse` allows heterogeneous arrays, constructor of `toml::value`
+does not. Here the reason is explained.
+
+```cpp
+// this won't be compiled
+toml::value v{
+    "foo", 3.14, 42, {1,2,3,4,5}, {{"key", "value"}}
+}
+```
+
+There is a workaround for this. By explicitly converting values into
+`toml::value`, you can initialize `toml::value` with a heterogeneous array.
+Also, you can first initialize a `toml::value` with an array and then
+`push_back` into it.
+
+```cpp
+// OK!
+toml::value v{
+    toml::value("foo"), toml::value(3.14), toml::value(42),
+    toml::value{1,2,3,4,5}, toml::value{{"key", "value"}}
+}
+
+// OK!
+toml::value v(toml::array{});
+v.push_back("foo");
+v.push_back(3.14);
+
+// OK!
+toml::array a;
+a.push_back("foo");
+a.push_back(3.14);
+toml::value v(std::move(a));
+```
+
+The reason why the first example is not allowed is the following.
+Let's assume that you are initializing a `toml::value` with a table.
+
+```cpp
+                    // # expecting TOML table.
+toml::value v{      // [v]
+    {"answer", 42}, // answer = 42
+    {"pi",   3.14}, // pi = 3.14
+    {"foo", "bar"}  // foo = "bar"
+};
+```
+
+This is indistinguishable from a (heterogeneous) TOML array definition.
+
+```toml
+v = [
+    ["answer", 42],
+    ["pi",   3.14],
+    ["foo", "bar"],
+]
+```
+
+This means that the above C++ code makes constructor's overload resolution
+ambiguous. So a constructor that allows both "table as an initializer-list" and
+"heterogeneous array as an initializer-list" cannot be implemented.
+
+Thus, although it is painful, we need to explicitly cast values into
+`toml::value` when you initialize heterogeneous array in a C++ code.
+
+```cpp
+toml::value v{
+    toml::value("foo"), toml::value(3.14), toml::value(42),
+    toml::value{1,2,3,4,5}, toml::value{{"key", "value"}}
+};
+```
+
+## Breaking Changes from v2
+
+Although toml11 is relatively new library (it's three years old now), it had
+some confusing and inconvenient user-interfaces because of historical reasons.
+
+Between v2 and v3, those interfaces are rearranged.
+
+- `toml::parse` now returns a `toml::value`, not `toml::table`.
+- `toml::value` is now an alias of `toml::basic_value<discard_comment, std::vector, std::unordered_map>`.
+  - See [Customizing containers](#customizing-containers) for detail.
+- The elements of `toml::value_t` are renamed as `snake_case`.
+  - See [Underlying types](#underlying-types) for detail.
+- Supports for the CamelCaseNames are dropped.
+  - See [Underlying types](#underlying-types) for detail.
+- `(is|as)_float` has been removed to make the function names consistent with others.
+  - Since `float` is a keyword, toml11 named a float type as `toml::floating`.
+  - Also a `value_t` corresponds to `toml::floating` is named `value_t::floating`.
+  - So `(is|as)_floating` is introduced and `is_float` has been removed.
+  - See [Casting a toml::value](#casting-a-tomlvalue) and [Checking value type](#checking-value-type) for detail.
+- An overload of `toml::find` for `toml::table` has been dropped. Use `toml::value` version instead.
+  - Because type conversion between a table and a value causes ambiguity while overload resolution
+  - Since `toml::parse` now returns a `toml::value`, this feature becomes less important.
+  - Also because `toml::table` is a normal STL container, implementing utility function is easy.
+  - See [Finding a toml::value](#finding-a-toml-value) for detail.
+- An overload of `operator<<` and `toml::format` for `toml::table`s are dropped.
+  - Use `toml::value` instead.
+  - See [Serializing TOML data](#serializing-toml-data) for detail.
+- Interface around comments.
+  - See [Preserving Comments](#preserving-comments) for detail.
+- An ancient `from_toml/into_toml` has been removed. Use arbitrary type conversion support.
+  - See [Conversion between toml value and arbitrary types](#conversion-between-toml-value-and-arbitrary-types) for detail.
+
+Such a big change will not happen in the coming years.
+
+## Running Tests
+
+After cloning this repository, run the following command (thank you @jwillikers
+for automating test set fetching!).
+
+```sh
+$ mkdir build
+$ cd build
+$ cmake .. -Dtoml11_BUILD_TEST=ON
+$ make
+$ make test
+```
+
+To run the language agnostic test suite, you need to compile
+`tests/check_toml_test.cpp` and pass it to the tester.
+
+## Contributors
+
+I appreciate the help of the contributors who introduced the great feature to this library.
+
+- Guillaume Fraux (@Luthaf)
+  - Windows support and CI on Appvayor
+  - Intel Compiler support
+- Quentin Khan (@xaxousis)
+  - Found & Fixed a bug around ODR
+  - Improved error messages for invalid keys to show the location where the parser fails
+- Petr Beneš (@wbenny)
+  - Fixed warnings on MSVC
+- Ivan Shynkarenka (@chronoxor)
+  - Fixed Visual Studio 2019 warnings
+- @khoitd1997
+  - Fixed warnings while type conversion
+- @KerstinKeller
+  - Added installation script to CMake
+- J.C. Moyer (@jcmoyer)
+  - Fixed an example code in the documentation
+- Jt Freeman (@blockparty-sh)
+  - Fixed feature test macro around `localtime_s`
+  - Suppress warnings in Debug mode
+- OGAWA Kenichi (@kenichiice)
+  - Suppress warnings on intel compiler
+- Jordan Williams (@jwillikers)
+  - Fixed clang range-loop-analysis warnings
+  - Fixed feature test macro to suppress -Wundef
+  - Use cache variables in CMakeLists.txt
+  - Automate test set fetching, update and refactor CMakeLists.txt
+- Scott McCaskill
+  - Parse 9 digits (nanoseconds) of fractional seconds in a `local_time`
+- Shu Wang (@halfelf)
+  - fix "Finding a value in an array" example in README
+- @maass-tv and @SeverinLeonhardt
+  - Fix MSVC warning C4866
+- OGAWA KenIchi (@kenichiice)
+  - Fix include path in README
+- Mohammed Alyousef (@MoAlyousef)
+  - Made testing optional in CMake
+- Ivan Shynkarenka (@chronoxor)
+  - Fix compilation error in `<filesystem>` with MinGW
+- Alex Merry (@amerry)
+  - Add missing include files
+- sneakypete81 (@sneakypete81)
+  - Fix typo in error message
+- Oliver Kahrmann (@founderio)
+  - Fix missing filename in error message if parsed file is empty
+- Karl Nilsson (@karl-nilsson)
+  - Fix many spelling errors
+- ohdarling88 (@ohdarling)
+  - Fix a bug in a constructor of serializer
+- estshorter (@estshorter)
+  - Fix MSVC warning C26478
+- Philip Top (@phlptp)
+  - Improve checking standard library feature availability check
+- Louis Marascio (@marascio)
+  - Fix free-nonheap-object warning
+
+
+## Licensing terms
+
+This product is licensed under the terms of the [MIT License](LICENSE).
+
+- Copyright (c) 2017-2021 Toru Niina
+
+All rights reserved.
diff --git a/src/toml11/toml.hpp b/src/toml11/toml.hpp
new file mode 100644
index 000000000..f34cfccca
--- /dev/null
+++ b/src/toml11/toml.hpp
@@ -0,0 +1,46 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2017 Toru Niina
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef TOML_FOR_MODERN_CPP
+#define TOML_FOR_MODERN_CPP
+
+#ifndef __cplusplus
+#    error "__cplusplus is not defined"
+#endif
+
+#if __cplusplus < 201103L && _MSC_VER < 1900
+#    error "toml11 requires C++11 or later."
+#endif
+
+#define TOML11_VERSION_MAJOR 3
+#define TOML11_VERSION_MINOR 7
+#define TOML11_VERSION_PATCH 0
+
+#include "toml/parser.hpp"
+#include "toml/literal.hpp"
+#include "toml/serializer.hpp"
+#include "toml/get.hpp"
+#include "toml/macros.hpp"
+
+#endif// TOML_FOR_MODERN_CPP
diff --git a/src/toml11/toml/color.hpp b/src/toml11/toml/color.hpp
new file mode 100644
index 000000000..4cb572cb0
--- /dev/null
+++ b/src/toml11/toml/color.hpp
@@ -0,0 +1,64 @@
+#ifndef TOML11_COLOR_HPP
+#define TOML11_COLOR_HPP
+#include <cstdint>
+#include <ostream>
+
+#ifdef TOML11_COLORIZE_ERROR_MESSAGE
+#define TOML11_ERROR_MESSAGE_COLORIZED true
+#else
+#define TOML11_ERROR_MESSAGE_COLORIZED false
+#endif
+
+namespace toml
+{
+
+// put ANSI escape sequence to ostream
+namespace color_ansi
+{
+namespace detail
+{
+inline int colorize_index()
+{
+    static const int index = std::ios_base::xalloc();
+    return index;
+}
+} // detail
+
+inline std::ostream& colorize(std::ostream& os)
+{
+    // by default, it is zero.
+    os.iword(detail::colorize_index()) = 1;
+    return os;
+}
+inline std::ostream& nocolorize(std::ostream& os)
+{
+    os.iword(detail::colorize_index()) = 0;
+    return os;
+}
+inline std::ostream& reset  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[00m";} return os;}
+inline std::ostream& bold   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[01m";} return os;}
+inline std::ostream& grey   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[30m";} return os;}
+inline std::ostream& red    (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[31m";} return os;}
+inline std::ostream& green  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[32m";} return os;}
+inline std::ostream& yellow (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[33m";} return os;}
+inline std::ostream& blue   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[34m";} return os;}
+inline std::ostream& magenta(std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[35m";} return os;}
+inline std::ostream& cyan   (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[36m";} return os;}
+inline std::ostream& white  (std::ostream& os)
+{if(os.iword(detail::colorize_index()) == 1) {os << "\033[37m";} return os;}
+} // color_ansi
+
+// ANSI escape sequence is the only and default colorization method currently
+namespace color = color_ansi;
+
+} // toml
+#endif// TOML11_COLOR_HPP
diff --git a/src/toml11/toml/combinator.hpp b/src/toml11/toml/combinator.hpp
new file mode 100644
index 000000000..33ecca1eb
--- /dev/null
+++ b/src/toml11/toml/combinator.hpp
@@ -0,0 +1,306 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_COMBINATOR_HPP
+#define TOML11_COMBINATOR_HPP
+#include <cassert>
+#include <cctype>
+#include <cstdio>
+
+#include <array>
+#include <iomanip>
+#include <iterator>
+#include <limits>
+#include <type_traits>
+
+#include "region.hpp"
+#include "result.hpp"
+#include "traits.hpp"
+#include "utility.hpp"
+
+// they scans characters and returns region if it matches to the condition.
+// when they fail, it does not change the location.
+// in lexer.hpp, these are used.
+
+namespace toml
+{
+namespace detail
+{
+
+// to output character as an error message.
+inline std::string show_char(const char c)
+{
+    // It suppresses an error that occurs only in Debug mode of MSVC++ on Windows.
+    // I'm not completely sure but they check the value of char to be in the
+    // range [0, 256) and some of the COMPLETELY VALID utf-8 character sometimes
+    // has negative value (if char has sign). So here it re-interprets c as
+    // unsigned char through pointer. In general, converting pointer to a
+    // pointer that has different type cause UB, but `(signed|unsigned)?char`
+    // are one of the exceptions. Converting pointer only to char and std::byte
+    // (c++17) are valid.
+    if(std::isgraph(*reinterpret_cast<unsigned char const*>(std::addressof(c))))
+    {
+        return std::string(1, c);
+    }
+    else
+    {
+        std::array<char, 5> buf;
+        buf.fill('\0');
+        const auto r = std::snprintf(
+                buf.data(), buf.size(), "0x%02x", static_cast<int>(c) & 0xFF);
+        (void) r; // Unused variable warning
+        assert(r == static_cast<int>(buf.size()) - 1);
+        return std::string(buf.data());
+    }
+}
+
+template<char C>
+struct character
+{
+    static constexpr char target = C;
+
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        if(loc.iter() == loc.end()) {return none();}
+        const auto first = loc.iter();
+
+        const char c = *(loc.iter());
+        if(c != target)
+        {
+            return none();
+        }
+        loc.advance(); // update location
+
+        return ok(region(loc, first, loc.iter()));
+    }
+};
+template<char C>
+constexpr char character<C>::target;
+
+// closed interval [Low, Up]. both Low and Up are included.
+template<char Low, char Up>
+struct in_range
+{
+    // assuming ascii part of UTF-8...
+    static_assert(Low <= Up, "lower bound should be less than upper bound.");
+
+    static constexpr char upper = Up;
+    static constexpr char lower = Low;
+
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        if(loc.iter() == loc.end()) {return none();}
+        const auto first = loc.iter();
+
+        const char c = *(loc.iter());
+        if(c < lower || upper < c)
+        {
+            return none();
+        }
+
+        loc.advance();
+        return ok(region(loc, first, loc.iter()));
+    }
+};
+template<char L, char U> constexpr char in_range<L, U>::upper;
+template<char L, char U> constexpr char in_range<L, U>::lower;
+
+// keep iterator if `Combinator` matches. otherwise, increment `iter` by 1 char.
+// for detecting invalid characters, like control sequences in toml string.
+template<typename Combinator>
+struct exclude
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        if(loc.iter() == loc.end()) {return none();}
+        auto first = loc.iter();
+
+        auto rslt = Combinator::invoke(loc);
+        if(rslt.is_ok())
+        {
+            loc.reset(first);
+            return none();
+        }
+        loc.reset(std::next(first)); // XXX maybe loc.advance() is okay but...
+        return ok(region(loc, first, loc.iter()));
+    }
+};
+
+// increment `iter`, if matches. otherwise, just return empty string.
+template<typename Combinator>
+struct maybe
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        const auto rslt = Combinator::invoke(loc);
+        if(rslt.is_ok())
+        {
+            return rslt;
+        }
+        return ok(region(loc));
+    }
+};
+
+template<typename ... Ts>
+struct sequence;
+
+template<typename Head, typename ... Tail>
+struct sequence<Head, Tail...>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        const auto first = loc.iter();
+        auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.reset(first);
+            return none();
+        }
+        return sequence<Tail...>::invoke(loc, std::move(rslt.unwrap()), first);
+    }
+
+    // called from the above function only, recursively.
+    template<typename Iterator>
+    static result<region, none_t>
+    invoke(location& loc, region reg, Iterator first)
+    {
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.reset(first);
+            return none();
+        }
+        reg += rslt.unwrap(); // concat regions
+        return sequence<Tail...>::invoke(loc, std::move(reg), first);
+    }
+};
+
+template<typename Head>
+struct sequence<Head>
+{
+    // would be called from sequence<T ...>::invoke only.
+    template<typename Iterator>
+    static result<region, none_t>
+    invoke(location& loc, region reg, Iterator first)
+    {
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_err())
+        {
+            loc.reset(first);
+            return none();
+        }
+        reg += rslt.unwrap(); // concat regions
+        return ok(reg);
+    }
+};
+
+template<typename ... Ts>
+struct either;
+
+template<typename Head, typename ... Tail>
+struct either<Head, Tail...>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        const auto rslt = Head::invoke(loc);
+        if(rslt.is_ok()) {return rslt;}
+        return either<Tail...>::invoke(loc);
+    }
+};
+template<typename Head>
+struct either<Head>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        return Head::invoke(loc);
+    }
+};
+
+template<typename T, typename N>
+struct repeat;
+
+template<std::size_t N> struct exactly{};
+template<std::size_t N> struct at_least{};
+struct unlimited{};
+
+template<typename T, std::size_t N>
+struct repeat<T, exactly<N>>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        region retval(loc);
+        const auto first = loc.iter();
+        for(std::size_t i=0; i<N; ++i)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                loc.reset(first);
+                return none();
+            }
+            retval += rslt.unwrap();
+        }
+        return ok(std::move(retval));
+    }
+};
+
+template<typename T, std::size_t N>
+struct repeat<T, at_least<N>>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        region retval(loc);
+
+        const auto first = loc.iter();
+        for(std::size_t i=0; i<N; ++i)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                loc.reset(first);
+                return none();
+            }
+            retval += rslt.unwrap();
+        }
+        while(true)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                return ok(std::move(retval));
+            }
+            retval += rslt.unwrap();
+        }
+    }
+};
+
+template<typename T>
+struct repeat<T, unlimited>
+{
+    static result<region, none_t>
+    invoke(location& loc)
+    {
+        region retval(loc);
+        while(true)
+        {
+            auto rslt = T::invoke(loc);
+            if(rslt.is_err())
+            {
+                return ok(std::move(retval));
+            }
+            retval += rslt.unwrap();
+        }
+    }
+};
+
+} // detail
+} // toml
+#endif// TOML11_COMBINATOR_HPP
diff --git a/src/toml11/toml/comments.hpp b/src/toml11/toml/comments.hpp
new file mode 100644
index 000000000..ec2504117
--- /dev/null
+++ b/src/toml11/toml/comments.hpp
@@ -0,0 +1,472 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_COMMENTS_HPP
+#define TOML11_COMMENTS_HPP
+#include <initializer_list>
+#include <iterator>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#ifdef TOML11_PRESERVE_COMMENTS_BY_DEFAULT
+#  define TOML11_DEFAULT_COMMENT_STRATEGY ::toml::preserve_comments
+#else
+#  define TOML11_DEFAULT_COMMENT_STRATEGY ::toml::discard_comments
+#endif
+
+// This file provides mainly two classes, `preserve_comments` and `discard_comments`.
+// Those two are a container that have the same interface as `std::vector<std::string>`
+// but bahaves in the opposite way. `preserve_comments` is just the same as
+// `std::vector<std::string>` and each `std::string` corresponds to a comment line.
+// Conversely, `discard_comments` discards all the strings and ignores everything
+// assigned in it. `discard_comments` is always empty and you will encounter an
+// error whenever you access to the element.
+namespace toml
+{
+struct discard_comments; // forward decl
+
+// use it in the following way
+//
+// const toml::basic_value<toml::preserve_comments> data =
+//     toml::parse<toml::preserve_comments>("example.toml");
+//
+// the interface is almost the same as std::vector<std::string>.
+struct preserve_comments
+{
+    // `container_type` is not provided in discard_comments.
+    // do not use this inner-type in a generic code.
+    using container_type         = std::vector<std::string>;
+
+    using size_type              = container_type::size_type;
+    using difference_type        = container_type::difference_type;
+    using value_type             = container_type::value_type;
+    using reference              = container_type::reference;
+    using const_reference        = container_type::const_reference;
+    using pointer                = container_type::pointer;
+    using const_pointer          = container_type::const_pointer;
+    using iterator               = container_type::iterator;
+    using const_iterator         = container_type::const_iterator;
+    using reverse_iterator       = container_type::reverse_iterator;
+    using const_reverse_iterator = container_type::const_reverse_iterator;
+
+    preserve_comments()  = default;
+    ~preserve_comments() = default;
+    preserve_comments(preserve_comments const&) = default;
+    preserve_comments(preserve_comments &&)     = default;
+    preserve_comments& operator=(preserve_comments const&) = default;
+    preserve_comments& operator=(preserve_comments &&)     = default;
+
+    explicit preserve_comments(const std::vector<std::string>& c): comments(c){}
+    explicit preserve_comments(std::vector<std::string>&& c)
+        : comments(std::move(c))
+    {}
+    preserve_comments& operator=(const std::vector<std::string>& c)
+    {
+        comments = c;
+        return *this;
+    }
+    preserve_comments& operator=(std::vector<std::string>&& c)
+    {
+        comments = std::move(c);
+        return *this;
+    }
+
+    explicit preserve_comments(const discard_comments&) {}
+
+    explicit preserve_comments(size_type n): comments(n) {}
+    preserve_comments(size_type n, const std::string& x): comments(n, x) {}
+    preserve_comments(std::initializer_list<std::string> x): comments(x) {}
+    template<typename InputIterator>
+    preserve_comments(InputIterator first, InputIterator last)
+        : comments(first, last)
+    {}
+
+    template<typename InputIterator>
+    void assign(InputIterator first, InputIterator last) {comments.assign(first, last);}
+    void assign(std::initializer_list<std::string> ini)  {comments.assign(ini);}
+    void assign(size_type n, const std::string& val)     {comments.assign(n, val);}
+
+    // Related to the issue #97.
+    //
+    // It is known that `std::vector::insert` and `std::vector::erase` in
+    // the standard library implementation included in GCC 4.8.5 takes
+    // `std::vector::iterator` instead of `std::vector::const_iterator`.
+    // Because of the const-correctness, we cannot convert a `const_iterator` to
+    // an `iterator`. It causes compilation error in GCC 4.8.5.
+#if defined(__GNUC__) && defined(__GNUC_MINOR__) && defined(__GNUC_PATCHLEVEL__) && !defined(__clang__)
+#  if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) <= 40805
+#    define TOML11_WORKAROUND_GCC_4_8_X_STANDARD_LIBRARY_IMPLEMENTATION
+#  endif
+#endif
+
+#ifdef TOML11_WORKAROUND_GCC_4_8_X_STANDARD_LIBRARY_IMPLEMENTATION
+    iterator insert(iterator p, const std::string& x)
+    {
+        return comments.insert(p, x);
+    }
+    iterator insert(iterator p, std::string&&      x)
+    {
+        return comments.insert(p, std::move(x));
+    }
+    void insert(iterator p, size_type n, const std::string& x)
+    {
+        return comments.insert(p, n, x);
+    }
+    template<typename InputIterator>
+    void insert(iterator p, InputIterator first, InputIterator last)
+    {
+        return comments.insert(p, first, last);
+    }
+    void insert(iterator p, std::initializer_list<std::string> ini)
+    {
+        return comments.insert(p, ini);
+    }
+
+    template<typename ... Ts>
+    iterator emplace(iterator p, Ts&& ... args)
+    {
+        return comments.emplace(p, std::forward<Ts>(args)...);
+    }
+
+    iterator erase(iterator pos) {return comments.erase(pos);}
+    iterator erase(iterator first, iterator last)
+    {
+        return comments.erase(first, last);
+    }
+#else
+    iterator insert(const_iterator p, const std::string& x)
+    {
+        return comments.insert(p, x);
+    }
+    iterator insert(const_iterator p, std::string&&      x)
+    {
+        return comments.insert(p, std::move(x));
+    }
+    iterator insert(const_iterator p, size_type n, const std::string& x)
+    {
+        return comments.insert(p, n, x);
+    }
+    template<typename InputIterator>
+    iterator insert(const_iterator p, InputIterator first, InputIterator last)
+    {
+        return comments.insert(p, first, last);
+    }
+    iterator insert(const_iterator p, std::initializer_list<std::string> ini)
+    {
+        return comments.insert(p, ini);
+    }
+
+    template<typename ... Ts>
+    iterator emplace(const_iterator p, Ts&& ... args)
+    {
+        return comments.emplace(p, std::forward<Ts>(args)...);
+    }
+
+    iterator erase(const_iterator pos) {return comments.erase(pos);}
+    iterator erase(const_iterator first, const_iterator last)
+    {
+        return comments.erase(first, last);
+    }
+#endif
+
+    void swap(preserve_comments& other) {comments.swap(other.comments);}
+
+    void push_back(const std::string& v) {comments.push_back(v);}
+    void push_back(std::string&&      v) {comments.push_back(std::move(v));}
+    void pop_back()                      {comments.pop_back();}
+
+    template<typename ... Ts>
+    void emplace_back(Ts&& ... args) {comments.emplace_back(std::forward<Ts>(args)...);}
+
+    void clear() {comments.clear();}
+
+    size_type size()     const noexcept {return comments.size();}
+    size_type max_size() const noexcept {return comments.max_size();}
+    size_type capacity() const noexcept {return comments.capacity();}
+    bool      empty()    const noexcept {return comments.empty();}
+
+    void reserve(size_type n)                      {comments.reserve(n);}
+    void resize(size_type n)                       {comments.resize(n);}
+    void resize(size_type n, const std::string& c) {comments.resize(n, c);}
+    void shrink_to_fit()                           {comments.shrink_to_fit();}
+
+    reference       operator[](const size_type n)       noexcept {return comments[n];}
+    const_reference operator[](const size_type n) const noexcept {return comments[n];}
+    reference       at(const size_type n)       {return comments.at(n);}
+    const_reference at(const size_type n) const {return comments.at(n);}
+    reference       front()       noexcept {return comments.front();}
+    const_reference front() const noexcept {return comments.front();}
+    reference       back()        noexcept {return comments.back();}
+    const_reference back()  const noexcept {return comments.back();}
+
+    pointer         data()        noexcept {return comments.data();}
+    const_pointer   data()  const noexcept {return comments.data();}
+
+    iterator       begin()        noexcept {return comments.begin();}
+    iterator       end()          noexcept {return comments.end();}
+    const_iterator begin()  const noexcept {return comments.begin();}
+    const_iterator end()    const noexcept {return comments.end();}
+    const_iterator cbegin() const noexcept {return comments.cbegin();}
+    const_iterator cend()   const noexcept {return comments.cend();}
+
+    reverse_iterator       rbegin()        noexcept {return comments.rbegin();}
+    reverse_iterator       rend()          noexcept {return comments.rend();}
+    const_reverse_iterator rbegin()  const noexcept {return comments.rbegin();}
+    const_reverse_iterator rend()    const noexcept {return comments.rend();}
+    const_reverse_iterator crbegin() const noexcept {return comments.crbegin();}
+    const_reverse_iterator crend()   const noexcept {return comments.crend();}
+
+    friend bool operator==(const preserve_comments&, const preserve_comments&);
+    friend bool operator!=(const preserve_comments&, const preserve_comments&);
+    friend bool operator< (const preserve_comments&, const preserve_comments&);
+    friend bool operator<=(const preserve_comments&, const preserve_comments&);
+    friend bool operator> (const preserve_comments&, const preserve_comments&);
+    friend bool operator>=(const preserve_comments&, const preserve_comments&);
+
+    friend void swap(preserve_comments&, std::vector<std::string>&);
+    friend void swap(std::vector<std::string>&, preserve_comments&);
+
+  private:
+
+    container_type comments;
+};
+
+inline bool operator==(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments == rhs.comments;}
+inline bool operator!=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments != rhs.comments;}
+inline bool operator< (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <  rhs.comments;}
+inline bool operator<=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments <= rhs.comments;}
+inline bool operator> (const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >  rhs.comments;}
+inline bool operator>=(const preserve_comments& lhs, const preserve_comments& rhs) {return lhs.comments >= rhs.comments;}
+
+inline void swap(preserve_comments& lhs, preserve_comments& rhs)
+{
+    lhs.swap(rhs);
+    return;
+}
+inline void swap(preserve_comments& lhs, std::vector<std::string>& rhs)
+{
+    lhs.comments.swap(rhs);
+    return;
+}
+inline void swap(std::vector<std::string>& lhs, preserve_comments& rhs)
+{
+    lhs.swap(rhs.comments);
+    return;
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const preserve_comments& com)
+{
+    for(const auto& c : com)
+    {
+        os << '#' << c << '\n';
+    }
+    return os;
+}
+
+namespace detail
+{
+
+// To provide the same interface with `preserve_comments`, `discard_comments`
+// should have an iterator. But it does not contain anything, so we need to
+// add an iterator that points nothing.
+//
+// It always points null, so DO NOT unwrap this iterator. It always crashes
+// your program.
+template<typename T, bool is_const>
+struct empty_iterator
+{
+    using value_type        = T;
+    using reference_type    = typename std::conditional<is_const, T const&, T&>::type;
+    using pointer_type      = typename std::conditional<is_const, T const*, T*>::type;
+    using difference_type   = std::ptrdiff_t;
+    using iterator_category = std::random_access_iterator_tag;
+
+    empty_iterator()  = default;
+    ~empty_iterator() = default;
+    empty_iterator(empty_iterator const&) = default;
+    empty_iterator(empty_iterator &&)     = default;
+    empty_iterator& operator=(empty_iterator const&) = default;
+    empty_iterator& operator=(empty_iterator &&)     = default;
+
+    // DO NOT call these operators.
+    reference_type operator*()  const noexcept {std::terminate();}
+    pointer_type   operator->() const noexcept {return nullptr;}
+    reference_type operator[](difference_type) const noexcept {return this->operator*();}
+
+    // These operators do nothing.
+    empty_iterator& operator++()    noexcept {return *this;}
+    empty_iterator  operator++(int) noexcept {return *this;}
+    empty_iterator& operator--()    noexcept {return *this;}
+    empty_iterator  operator--(int) noexcept {return *this;}
+
+    empty_iterator& operator+=(difference_type) noexcept {return *this;}
+    empty_iterator& operator-=(difference_type) noexcept {return *this;}
+
+    empty_iterator  operator+(difference_type) const noexcept {return *this;}
+    empty_iterator  operator-(difference_type) const noexcept {return *this;}
+};
+
+template<typename T, bool C>
+bool operator==(const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return true;}
+template<typename T, bool C>
+bool operator!=(const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return false;}
+template<typename T, bool C>
+bool operator< (const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return false;}
+template<typename T, bool C>
+bool operator<=(const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return true;}
+template<typename T, bool C>
+bool operator> (const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return false;}
+template<typename T, bool C>
+bool operator>=(const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return true;}
+
+template<typename T, bool C>
+typename empty_iterator<T, C>::difference_type
+operator-(const empty_iterator<T, C>&, const empty_iterator<T, C>&) noexcept {return 0;}
+
+template<typename T, bool C>
+empty_iterator<T, C>
+operator+(typename empty_iterator<T, C>::difference_type, const empty_iterator<T, C>& rhs) noexcept {return rhs;}
+template<typename T, bool C>
+empty_iterator<T, C>
+operator+(const empty_iterator<T, C>& lhs, typename empty_iterator<T, C>::difference_type) noexcept {return lhs;}
+
+} // detail
+
+// The default comment type. It discards all the comments. It requires only one
+// byte to contain, so the memory footprint is smaller than preserve_comments.
+//
+// It just ignores `push_back`, `insert`, `erase`, and any other modifications.
+// IT always returns size() == 0, the iterator taken by `begin()` is always the
+// same as that of `end()`, and accessing through `operator[]` or iterators
+// always causes a segmentation fault. DO NOT access to the element of this.
+//
+// Why this is chose as the default type is because the last version (2.x.y)
+// does not contain any comments in a value. To minimize the impact on the
+// efficiency, this is chosen as a default.
+//
+// To reduce the memory footprint, later we can try empty base optimization (EBO).
+struct discard_comments
+{
+    using size_type              = std::size_t;
+    using difference_type        = std::ptrdiff_t;
+    using value_type             = std::string;
+    using reference              = std::string&;
+    using const_reference        = std::string const&;
+    using pointer                = std::string*;
+    using const_pointer          = std::string const*;
+    using iterator               = detail::empty_iterator<std::string, false>;
+    using const_iterator         = detail::empty_iterator<std::string, true>;
+    using reverse_iterator       = detail::empty_iterator<std::string, false>;
+    using const_reverse_iterator = detail::empty_iterator<std::string, true>;
+
+    discard_comments() = default;
+    ~discard_comments() = default;
+    discard_comments(discard_comments const&) = default;
+    discard_comments(discard_comments &&)     = default;
+    discard_comments& operator=(discard_comments const&) = default;
+    discard_comments& operator=(discard_comments &&)     = default;
+
+    explicit discard_comments(const std::vector<std::string>&) noexcept {}
+    explicit discard_comments(std::vector<std::string>&&)      noexcept {}
+    discard_comments& operator=(const std::vector<std::string>&) noexcept {return *this;}
+    discard_comments& operator=(std::vector<std::string>&&)      noexcept {return *this;}
+
+    explicit discard_comments(const preserve_comments&)        noexcept {}
+
+    explicit discard_comments(size_type) noexcept {}
+    discard_comments(size_type, const std::string&) noexcept {}
+    discard_comments(std::initializer_list<std::string>) noexcept {}
+    template<typename InputIterator>
+    discard_comments(InputIterator, InputIterator) noexcept {}
+
+    template<typename InputIterator>
+    void assign(InputIterator, InputIterator)       noexcept {}
+    void assign(std::initializer_list<std::string>) noexcept {}
+    void assign(size_type, const std::string&)      noexcept {}
+
+    iterator insert(const_iterator, const std::string&)                 {return iterator{};}
+    iterator insert(const_iterator, std::string&&)                      {return iterator{};}
+    iterator insert(const_iterator, size_type, const std::string&)      {return iterator{};}
+    template<typename InputIterator>
+    iterator insert(const_iterator, InputIterator, InputIterator)       {return iterator{};}
+    iterator insert(const_iterator, std::initializer_list<std::string>) {return iterator{};}
+
+    template<typename ... Ts>
+    iterator emplace(const_iterator, Ts&& ...)     {return iterator{};}
+    iterator erase(const_iterator)                 {return iterator{};}
+    iterator erase(const_iterator, const_iterator) {return iterator{};}
+
+    void swap(discard_comments&) {return;}
+
+    void push_back(const std::string&) {return;}
+    void push_back(std::string&&     ) {return;}
+    void pop_back()                    {return;}
+
+    template<typename ... Ts>
+    void emplace_back(Ts&& ...) {return;}
+
+    void clear() {return;}
+
+    size_type size()     const noexcept {return 0;}
+    size_type max_size() const noexcept {return 0;}
+    size_type capacity() const noexcept {return 0;}
+    bool      empty()    const noexcept {return true;}
+
+    void reserve(size_type)                    {return;}
+    void resize(size_type)                     {return;}
+    void resize(size_type, const std::string&) {return;}
+    void shrink_to_fit()                       {return;}
+
+    // DO NOT access to the element of this container. This container is always
+    // empty, so accessing through operator[], front/back, data causes address
+    // error.
+
+    reference       operator[](const size_type)       noexcept {return *data();}
+    const_reference operator[](const size_type) const noexcept {return *data();}
+    reference       at(const size_type)       {throw std::out_of_range("toml::discard_comment is always empty.");}
+    const_reference at(const size_type) const {throw std::out_of_range("toml::discard_comment is always empty.");}
+    reference       front()       noexcept {return *data();}
+    const_reference front() const noexcept {return *data();}
+    reference       back()        noexcept {return *data();}
+    const_reference back()  const noexcept {return *data();}
+
+    pointer         data()        noexcept {return nullptr;}
+    const_pointer   data()  const noexcept {return nullptr;}
+
+    iterator       begin()        noexcept {return iterator{};}
+    iterator       end()          noexcept {return iterator{};}
+    const_iterator begin()  const noexcept {return const_iterator{};}
+    const_iterator end()    const noexcept {return const_iterator{};}
+    const_iterator cbegin() const noexcept {return const_iterator{};}
+    const_iterator cend()   const noexcept {return const_iterator{};}
+
+    reverse_iterator       rbegin()        noexcept {return iterator{};}
+    reverse_iterator       rend()          noexcept {return iterator{};}
+    const_reverse_iterator rbegin()  const noexcept {return const_iterator{};}
+    const_reverse_iterator rend()    const noexcept {return const_iterator{};}
+    const_reverse_iterator crbegin() const noexcept {return const_iterator{};}
+    const_reverse_iterator crend()   const noexcept {return const_iterator{};}
+};
+
+inline bool operator==(const discard_comments&, const discard_comments&) noexcept {return true;}
+inline bool operator!=(const discard_comments&, const discard_comments&) noexcept {return false;}
+inline bool operator< (const discard_comments&, const discard_comments&) noexcept {return false;}
+inline bool operator<=(const discard_comments&, const discard_comments&) noexcept {return true;}
+inline bool operator> (const discard_comments&, const discard_comments&) noexcept {return false;}
+inline bool operator>=(const discard_comments&, const discard_comments&) noexcept {return true;}
+
+inline void swap(const discard_comments&, const discard_comments&) noexcept {return;}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const discard_comments&)
+{
+    return os;
+}
+
+} // toml11
+#endif// TOML11_COMMENTS_HPP
diff --git a/src/toml11/toml/datetime.hpp b/src/toml11/toml/datetime.hpp
new file mode 100644
index 000000000..d8127c150
--- /dev/null
+++ b/src/toml11/toml/datetime.hpp
@@ -0,0 +1,631 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_DATETIME_HPP
+#define TOML11_DATETIME_HPP
+#include <cstdint>
+#include <cstdlib>
+#include <ctime>
+
+#include <array>
+#include <chrono>
+#include <iomanip>
+#include <ostream>
+#include <tuple>
+
+namespace toml
+{
+
+// To avoid non-threadsafe std::localtime. In C11 (not C++11!), localtime_s is
+// provided in the absolutely same purpose, but C++11 is actually not compatible
+// with C11. We need to dispatch the function depending on the OS.
+namespace detail
+{
+// TODO: find more sophisticated way to handle this
+#if (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 1) || defined(_XOPEN_SOURCE) || defined(_BSD_SOURCE) || defined(_SVID_SOURCE) || defined(_POSIX_SOURCE)
+inline std::tm localtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::localtime_r(src, &dst);
+    if (!result) { throw std::runtime_error("localtime_r failed."); }
+    return dst;
+}
+inline std::tm gmtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::gmtime_r(src, &dst);
+    if (!result) { throw std::runtime_error("gmtime_r failed."); }
+    return dst;
+}
+#elif defined(_MSC_VER)
+inline std::tm localtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::localtime_s(&dst, src);
+    if (result) { throw std::runtime_error("localtime_s failed."); }
+    return dst;
+}
+inline std::tm gmtime_s(const std::time_t* src)
+{
+    std::tm dst;
+    const auto result = ::gmtime_s(&dst, src);
+    if (result) { throw std::runtime_error("gmtime_s failed."); }
+    return dst;
+}
+#else // fallback. not threadsafe
+inline std::tm localtime_s(const std::time_t* src)
+{
+    const auto result = std::localtime(src);
+    if (!result) { throw std::runtime_error("localtime failed."); }
+    return *result;
+}
+inline std::tm gmtime_s(const std::time_t* src)
+{
+    const auto result = std::gmtime(src);
+    if (!result) { throw std::runtime_error("gmtime failed."); }
+    return *result;
+}
+#endif
+} // detail
+
+enum class month_t : std::uint8_t
+{
+    Jan =  0,
+    Feb =  1,
+    Mar =  2,
+    Apr =  3,
+    May =  4,
+    Jun =  5,
+    Jul =  6,
+    Aug =  7,
+    Sep =  8,
+    Oct =  9,
+    Nov = 10,
+    Dec = 11
+};
+
+struct local_date
+{
+    std::int16_t year;   // A.D. (like, 2018)
+    std::uint8_t month;  // [0, 11]
+    std::uint8_t day;    // [1, 31]
+
+    local_date(int y, month_t m, int d)
+        : year (static_cast<std::int16_t>(y)),
+          month(static_cast<std::uint8_t>(m)),
+          day  (static_cast<std::uint8_t>(d))
+    {}
+
+    explicit local_date(const std::tm& t)
+        : year (static_cast<std::int16_t>(t.tm_year + 1900)),
+          month(static_cast<std::uint8_t>(t.tm_mon)),
+          day  (static_cast<std::uint8_t>(t.tm_mday))
+    {}
+
+    explicit local_date(const std::chrono::system_clock::time_point& tp)
+    {
+        const auto t    = std::chrono::system_clock::to_time_t(tp);
+        const auto time = detail::localtime_s(&t);
+        *this = local_date(time);
+    }
+
+    explicit local_date(const std::time_t t)
+        : local_date(std::chrono::system_clock::from_time_t(t))
+    {}
+
+    operator std::chrono::system_clock::time_point() const
+    {
+        // std::mktime returns date as local time zone. no conversion needed
+        std::tm t;
+        t.tm_sec   = 0;
+        t.tm_min   = 0;
+        t.tm_hour  = 0;
+        t.tm_mday  = static_cast<int>(this->day);
+        t.tm_mon   = static_cast<int>(this->month);
+        t.tm_year  = static_cast<int>(this->year) - 1900;
+        t.tm_wday  = 0; // the value will be ignored
+        t.tm_yday  = 0; // the value will be ignored
+        t.tm_isdst = -1;
+        return std::chrono::system_clock::from_time_t(std::mktime(&t));
+    }
+
+    operator std::time_t() const
+    {
+        return std::chrono::system_clock::to_time_t(
+                std::chrono::system_clock::time_point(*this));
+    }
+
+    local_date() = default;
+    ~local_date() = default;
+    local_date(local_date const&) = default;
+    local_date(local_date&&)      = default;
+    local_date& operator=(local_date const&) = default;
+    local_date& operator=(local_date&&)      = default;
+};
+
+inline bool operator==(const local_date& lhs, const local_date& rhs)
+{
+    return std::make_tuple(lhs.year, lhs.month, lhs.day) ==
+           std::make_tuple(rhs.year, rhs.month, rhs.day);
+}
+inline bool operator!=(const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator< (const local_date& lhs, const local_date& rhs)
+{
+    return std::make_tuple(lhs.year, lhs.month, lhs.day) <
+           std::make_tuple(rhs.year, rhs.month, rhs.day);
+}
+inline bool operator<=(const local_date& lhs, const local_date& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+inline bool operator> (const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs <= rhs);
+}
+inline bool operator>=(const local_date& lhs, const local_date& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const local_date& date)
+{
+    os << std::setfill('0') << std::setw(4) << static_cast<int>(date.year )     << '-';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.month) + 1 << '-';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(date.day  )    ;
+    return os;
+}
+
+struct local_time
+{
+    std::uint8_t  hour;        // [0, 23]
+    std::uint8_t  minute;      // [0, 59]
+    std::uint8_t  second;      // [0, 60]
+    std::uint16_t millisecond; // [0, 999]
+    std::uint16_t microsecond; // [0, 999]
+    std::uint16_t nanosecond;  // [0, 999]
+
+    local_time(int h, int m, int s,
+               int ms = 0, int us = 0, int ns = 0)
+        : hour  (static_cast<std::uint8_t>(h)),
+          minute(static_cast<std::uint8_t>(m)),
+          second(static_cast<std::uint8_t>(s)),
+          millisecond(static_cast<std::uint16_t>(ms)),
+          microsecond(static_cast<std::uint16_t>(us)),
+          nanosecond (static_cast<std::uint16_t>(ns))
+    {}
+
+    explicit local_time(const std::tm& t)
+        : hour  (static_cast<std::uint8_t>(t.tm_hour)),
+          minute(static_cast<std::uint8_t>(t.tm_min)),
+          second(static_cast<std::uint8_t>(t.tm_sec)),
+          millisecond(0), microsecond(0), nanosecond(0)
+    {}
+
+    template<typename Rep, typename Period>
+    explicit local_time(const std::chrono::duration<Rep, Period>& t)
+    {
+        const auto h = std::chrono::duration_cast<std::chrono::hours>(t);
+        this->hour = static_cast<std::uint8_t>(h.count());
+        const auto t2 = t - h;
+        const auto m = std::chrono::duration_cast<std::chrono::minutes>(t2);
+        this->minute = static_cast<std::uint8_t>(m.count());
+        const auto t3 = t2 - m;
+        const auto s = std::chrono::duration_cast<std::chrono::seconds>(t3);
+        this->second = static_cast<std::uint8_t>(s.count());
+        const auto t4 = t3 - s;
+        const auto ms = std::chrono::duration_cast<std::chrono::milliseconds>(t4);
+        this->millisecond = static_cast<std::uint16_t>(ms.count());
+        const auto t5 = t4 - ms;
+        const auto us = std::chrono::duration_cast<std::chrono::microseconds>(t5);
+        this->microsecond = static_cast<std::uint16_t>(us.count());
+        const auto t6 = t5 - us;
+        const auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(t6);
+        this->nanosecond = static_cast<std::uint16_t>(ns.count());
+    }
+
+    operator std::chrono::nanoseconds() const
+    {
+        return std::chrono::nanoseconds (this->nanosecond)  +
+               std::chrono::microseconds(this->microsecond) +
+               std::chrono::milliseconds(this->millisecond) +
+               std::chrono::seconds(this->second) +
+               std::chrono::minutes(this->minute) +
+               std::chrono::hours(this->hour);
+    }
+
+    local_time() = default;
+    ~local_time() = default;
+    local_time(local_time const&) = default;
+    local_time(local_time&&)      = default;
+    local_time& operator=(local_time const&) = default;
+    local_time& operator=(local_time&&)      = default;
+};
+
+inline bool operator==(const local_time& lhs, const local_time& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) ==
+           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
+}
+inline bool operator!=(const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator< (const local_time& lhs, const local_time& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute, lhs.second, lhs.millisecond, lhs.microsecond, lhs.nanosecond) <
+           std::make_tuple(rhs.hour, rhs.minute, rhs.second, rhs.millisecond, rhs.microsecond, rhs.nanosecond);
+}
+inline bool operator<=(const local_time& lhs, const local_time& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+inline bool operator> (const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs <= rhs);
+}
+inline bool operator>=(const local_time& lhs, const local_time& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const local_time& time)
+{
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.hour  ) << ':';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.minute) << ':';
+    os << std::setfill('0') << std::setw(2) << static_cast<int>(time.second);
+    if(time.millisecond != 0 || time.microsecond != 0 || time.nanosecond != 0)
+    {
+        os << '.';
+        os << std::setfill('0') << std::setw(3) << static_cast<int>(time.millisecond);
+        if(time.microsecond != 0 || time.nanosecond != 0)
+        {
+            os << std::setfill('0') << std::setw(3) << static_cast<int>(time.microsecond);
+            if(time.nanosecond != 0)
+            {
+                os << std::setfill('0') << std::setw(3) << static_cast<int>(time.nanosecond);
+            }
+        }
+    }
+    return os;
+}
+
+struct time_offset
+{
+    std::int8_t hour;   // [-12, 12]
+    std::int8_t minute; // [-59, 59]
+
+    time_offset(int h, int m)
+        : hour  (static_cast<std::int8_t>(h)),
+          minute(static_cast<std::int8_t>(m))
+    {}
+
+    operator std::chrono::minutes() const
+    {
+        return std::chrono::minutes(this->minute) +
+               std::chrono::hours(this->hour);
+    }
+
+    time_offset() = default;
+    ~time_offset() = default;
+    time_offset(time_offset const&) = default;
+    time_offset(time_offset&&)      = default;
+    time_offset& operator=(time_offset const&) = default;
+    time_offset& operator=(time_offset&&)      = default;
+};
+
+inline bool operator==(const time_offset& lhs, const time_offset& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute) ==
+           std::make_tuple(rhs.hour, rhs.minute);
+}
+inline bool operator!=(const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator< (const time_offset& lhs, const time_offset& rhs)
+{
+    return std::make_tuple(lhs.hour, lhs.minute) <
+           std::make_tuple(rhs.hour, rhs.minute);
+}
+inline bool operator<=(const time_offset& lhs, const time_offset& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+inline bool operator> (const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs <= rhs);
+}
+inline bool operator>=(const time_offset& lhs, const time_offset& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const time_offset& offset)
+{
+    if(offset.hour == 0 && offset.minute == 0)
+    {
+        os << 'Z';
+        return os;
+    }
+    int minute = static_cast<int>(offset.hour) * 60 + offset.minute;
+    if(minute < 0){os << '-'; minute = std::abs(minute);} else {os << '+';}
+    os << std::setfill('0') << std::setw(2) << minute / 60 << ':';
+    os << std::setfill('0') << std::setw(2) << minute % 60;
+    return os;
+}
+
+struct local_datetime
+{
+    local_date date;
+    local_time time;
+
+    local_datetime(local_date d, local_time t): date(d), time(t) {}
+
+    explicit local_datetime(const std::tm& t): date(t), time(t){}
+
+    explicit local_datetime(const std::chrono::system_clock::time_point& tp)
+    {
+        const auto t = std::chrono::system_clock::to_time_t(tp);
+        std::tm ltime = detail::localtime_s(&t);
+
+        this->date = local_date(ltime);
+        this->time = local_time(ltime);
+
+        // std::tm lacks subsecond information, so diff between tp and tm
+        // can be used to get millisecond & microsecond information.
+        const auto t_diff = tp -
+            std::chrono::system_clock::from_time_t(std::mktime(&ltime));
+        this->time.millisecond = static_cast<std::uint16_t>(
+          std::chrono::duration_cast<std::chrono::milliseconds>(t_diff).count());
+        this->time.microsecond = static_cast<std::uint16_t>(
+          std::chrono::duration_cast<std::chrono::microseconds>(t_diff).count());
+        this->time.nanosecond = static_cast<std::uint16_t>(
+          std::chrono::duration_cast<std::chrono::nanoseconds >(t_diff).count());
+    }
+
+    explicit local_datetime(const std::time_t t)
+        : local_datetime(std::chrono::system_clock::from_time_t(t))
+    {}
+
+    operator std::chrono::system_clock::time_point() const
+    {
+        using internal_duration =
+            typename std::chrono::system_clock::time_point::duration;
+
+        // Normally DST begins at A.M. 3 or 4. If we re-use conversion operator
+        // of local_date and local_time independently, the conversion fails if
+        // it is the day when DST begins or ends. Since local_date considers the
+        // time is 00:00 A.M. and local_time does not consider DST because it
+        // does not have any date information. We need to consider both date and
+        // time information at the same time to convert it correctly.
+
+        std::tm t;
+        t.tm_sec   = static_cast<int>(this->time.second);
+        t.tm_min   = static_cast<int>(this->time.minute);
+        t.tm_hour  = static_cast<int>(this->time.hour);
+        t.tm_mday  = static_cast<int>(this->date.day);
+        t.tm_mon   = static_cast<int>(this->date.month);
+        t.tm_year  = static_cast<int>(this->date.year) - 1900;
+        t.tm_wday  = 0; // the value will be ignored
+        t.tm_yday  = 0; // the value will be ignored
+        t.tm_isdst = -1;
+
+        // std::mktime returns date as local time zone. no conversion needed
+        auto dt = std::chrono::system_clock::from_time_t(std::mktime(&t));
+        dt += std::chrono::duration_cast<internal_duration>(
+                std::chrono::milliseconds(this->time.millisecond) +
+                std::chrono::microseconds(this->time.microsecond) +
+                std::chrono::nanoseconds (this->time.nanosecond));
+        return dt;
+    }
+
+    operator std::time_t() const
+    {
+        return std::chrono::system_clock::to_time_t(
+                std::chrono::system_clock::time_point(*this));
+    }
+
+    local_datetime() = default;
+    ~local_datetime() = default;
+    local_datetime(local_datetime const&) = default;
+    local_datetime(local_datetime&&)      = default;
+    local_datetime& operator=(local_datetime const&) = default;
+    local_datetime& operator=(local_datetime&&)      = default;
+};
+
+inline bool operator==(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time) ==
+           std::make_tuple(rhs.date, rhs.time);
+}
+inline bool operator!=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator< (const local_datetime& lhs, const local_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time) <
+           std::make_tuple(rhs.date, rhs.time);
+}
+inline bool operator<=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+inline bool operator> (const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs <= rhs);
+}
+inline bool operator>=(const local_datetime& lhs, const local_datetime& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const local_datetime& dt)
+{
+    os << dt.date << 'T' << dt.time;
+    return os;
+}
+
+struct offset_datetime
+{
+    local_date  date;
+    local_time  time;
+    time_offset offset;
+
+    offset_datetime(local_date d, local_time t, time_offset o)
+        : date(d), time(t), offset(o)
+    {}
+    offset_datetime(const local_datetime& dt, time_offset o)
+        : date(dt.date), time(dt.time), offset(o)
+    {}
+    explicit offset_datetime(const local_datetime& ld)
+        : date(ld.date), time(ld.time), offset(get_local_offset(nullptr))
+          // use the current local timezone offset
+    {}
+    explicit offset_datetime(const std::chrono::system_clock::time_point& tp)
+        : offset(0, 0) // use gmtime
+    {
+        const auto timet = std::chrono::system_clock::to_time_t(tp);
+        const auto tm    = detail::gmtime_s(&timet);
+        this->date = local_date(tm);
+        this->time = local_time(tm);
+    }
+    explicit offset_datetime(const std::time_t& t)
+        : offset(0, 0) // use gmtime
+    {
+        const auto tm    = detail::gmtime_s(&t);
+        this->date = local_date(tm);
+        this->time = local_time(tm);
+    }
+    explicit offset_datetime(const std::tm& t)
+        : offset(0, 0) // assume gmtime
+    {
+        this->date = local_date(t);
+        this->time = local_time(t);
+    }
+
+    operator std::chrono::system_clock::time_point() const
+    {
+        // get date-time
+        using internal_duration =
+            typename std::chrono::system_clock::time_point::duration;
+
+        // first, convert it to local date-time information in the same way as
+        // local_datetime does. later we will use time_t to adjust time offset.
+        std::tm t;
+        t.tm_sec   = static_cast<int>(this->time.second);
+        t.tm_min   = static_cast<int>(this->time.minute);
+        t.tm_hour  = static_cast<int>(this->time.hour);
+        t.tm_mday  = static_cast<int>(this->date.day);
+        t.tm_mon   = static_cast<int>(this->date.month);
+        t.tm_year  = static_cast<int>(this->date.year) - 1900;
+        t.tm_wday  = 0; // the value will be ignored
+        t.tm_yday  = 0; // the value will be ignored
+        t.tm_isdst = -1;
+        const std::time_t tp_loc = std::mktime(std::addressof(t));
+
+        auto tp = std::chrono::system_clock::from_time_t(tp_loc);
+        tp += std::chrono::duration_cast<internal_duration>(
+                std::chrono::milliseconds(this->time.millisecond) +
+                std::chrono::microseconds(this->time.microsecond) +
+                std::chrono::nanoseconds (this->time.nanosecond));
+
+        // Since mktime uses local time zone, it should be corrected.
+        // `12:00:00+09:00` means `03:00:00Z`. So mktime returns `03:00:00Z` if
+        // we are in `+09:00` timezone. To represent `12:00:00Z` there, we need
+        // to add `+09:00` to `03:00:00Z`.
+        //    Here, it uses the time_t converted from date-time info to handle
+        // daylight saving time.
+        const auto ofs = get_local_offset(std::addressof(tp_loc));
+        tp += std::chrono::hours  (ofs.hour);
+        tp += std::chrono::minutes(ofs.minute);
+
+        // We got `12:00:00Z` by correcting local timezone applied by mktime.
+        // Then we will apply the offset. Let's say `12:00:00-08:00` is given.
+        // And now, we have `12:00:00Z`. `12:00:00-08:00` means `20:00:00Z`.
+        // So we need to subtract the offset.
+        tp -= std::chrono::minutes(this->offset);
+        return tp;
+    }
+
+    operator std::time_t() const
+    {
+        return std::chrono::system_clock::to_time_t(
+                std::chrono::system_clock::time_point(*this));
+    }
+
+    offset_datetime() = default;
+    ~offset_datetime() = default;
+    offset_datetime(offset_datetime const&) = default;
+    offset_datetime(offset_datetime&&)      = default;
+    offset_datetime& operator=(offset_datetime const&) = default;
+    offset_datetime& operator=(offset_datetime&&)      = default;
+
+  private:
+
+    static time_offset get_local_offset(const std::time_t* tp)
+    {
+        // get local timezone with the same date-time information as mktime
+        const auto t = detail::localtime_s(tp);
+
+        std::array<char, 6> buf;
+        const auto result = std::strftime(buf.data(), 6, "%z", &t); // +hhmm\0
+        if(result != 5)
+        {
+            throw std::runtime_error("toml::offset_datetime: cannot obtain "
+                                     "timezone information of current env");
+        }
+        const int ofs = std::atoi(buf.data());
+        const int ofs_h = ofs / 100;
+        const int ofs_m = ofs - (ofs_h * 100);
+        return time_offset(ofs_h, ofs_m);
+    }
+};
+
+inline bool operator==(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time, lhs.offset) ==
+           std::make_tuple(rhs.date, rhs.time, rhs.offset);
+}
+inline bool operator!=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator< (const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return std::make_tuple(lhs.date, lhs.time, lhs.offset) <
+           std::make_tuple(rhs.date, rhs.time, rhs.offset);
+}
+inline bool operator<=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+inline bool operator> (const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs <= rhs);
+}
+inline bool operator>=(const offset_datetime& lhs, const offset_datetime& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const offset_datetime& dt)
+{
+    os << dt.date << 'T' << dt.time << dt.offset;
+    return os;
+}
+
+}//toml
+#endif// TOML11_DATETIME
diff --git a/src/toml11/toml/exception.hpp b/src/toml11/toml/exception.hpp
new file mode 100644
index 000000000..c64651d0a
--- /dev/null
+++ b/src/toml11/toml/exception.hpp
@@ -0,0 +1,65 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_EXCEPTION_HPP
+#define TOML11_EXCEPTION_HPP
+#include <stdexcept>
+#include <string>
+
+#include "source_location.hpp"
+
+namespace toml
+{
+
+struct exception : public std::exception
+{
+  public:
+    explicit exception(const source_location& loc): loc_(loc) {}
+    virtual ~exception() noexcept override = default;
+    virtual const char* what() const noexcept override {return "";}
+    virtual source_location const& location() const noexcept {return loc_;}
+
+  protected:
+    source_location loc_;
+};
+
+struct syntax_error : public toml::exception
+{
+  public:
+    explicit syntax_error(const std::string& what_arg, const source_location& loc)
+        : exception(loc), what_(what_arg)
+    {}
+    virtual ~syntax_error() noexcept override = default;
+    virtual const char* what() const noexcept override {return what_.c_str();}
+
+  protected:
+    std::string what_;
+};
+
+struct type_error : public toml::exception
+{
+  public:
+    explicit type_error(const std::string& what_arg, const source_location& loc)
+        : exception(loc), what_(what_arg)
+    {}
+    virtual ~type_error() noexcept override = default;
+    virtual const char* what() const noexcept override {return what_.c_str();}
+
+  protected:
+    std::string what_;
+};
+
+struct internal_error : public toml::exception
+{
+  public:
+    explicit internal_error(const std::string& what_arg, const source_location& loc)
+        : exception(loc), what_(what_arg)
+    {}
+    virtual ~internal_error() noexcept override = default;
+    virtual const char* what() const noexcept override {return what_.c_str();}
+
+  protected:
+    std::string what_;
+};
+
+} // toml
+#endif // TOML_EXCEPTION
diff --git a/src/toml11/toml/from.hpp b/src/toml11/toml/from.hpp
new file mode 100644
index 000000000..10815caf5
--- /dev/null
+++ b/src/toml11/toml/from.hpp
@@ -0,0 +1,19 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_FROM_HPP
+#define TOML11_FROM_HPP
+
+namespace toml
+{
+
+template<typename T>
+struct from;
+// {
+//     static T from_toml(const toml::value& v)
+//     {
+//         // User-defined conversions ...
+//     }
+// };
+
+} // toml
+#endif // TOML11_FROM_HPP
diff --git a/src/toml11/toml/get.hpp b/src/toml11/toml/get.hpp
new file mode 100644
index 000000000..d7fdf553b
--- /dev/null
+++ b/src/toml11/toml/get.hpp
@@ -0,0 +1,1117 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_GET_HPP
+#define TOML11_GET_HPP
+#include <algorithm>
+
+#include "from.hpp"
+#include "result.hpp"
+#include "value.hpp"
+
+namespace toml
+{
+
+// ============================================================================
+// exact toml::* type
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> &
+get(basic_value<C, M, V>& v)
+{
+    return v.template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>();
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
+get(const basic_value<C, M, V>& v)
+{
+    return v.template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>();
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>
+get(basic_value<C, M, V>&& v)
+{
+    return T(std::move(v).template cast<detail::type_to_enum<T, basic_value<C, M, V>>::value>());
+}
+
+// ============================================================================
+// T == toml::value; identity transformation.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T>&
+get(basic_value<C, M, V>& v)
+{
+    return v;
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T> const&
+get(const basic_value<C, M, V>& v)
+{
+    return v;
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, basic_value<C, M, V>>::value, T>
+get(basic_value<C, M, V>&& v)
+{
+    return basic_value<C, M, V>(std::move(v));
+}
+
+// ============================================================================
+// T == toml::basic_value<C2, M2, V2>; basic_value -> basic_value
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<detail::conjunction<detail::is_basic_value<T>,
+    detail::negation<std::is_same<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return T(v);
+}
+
+// ============================================================================
+// integer convertible from toml::Integer
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<detail::conjunction<
+    std::is_integral<T>,                            // T is integral
+    detail::negation<std::is_same<T, bool>>,        // but not bool
+    detail::negation<                               // but not toml::integer
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return static_cast<T>(v.as_integer());
+}
+
+// ============================================================================
+// floating point convertible from toml::Float
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<detail::conjunction<
+    std::is_floating_point<T>,                      // T is floating_point
+    detail::negation<                               // but not toml::floating
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return static_cast<T>(v.as_floating());
+}
+
+// ============================================================================
+// std::string; toml uses its own toml::string, but it should be convertible to
+// std::string seamlessly
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
+get(basic_value<C, M, V>& v)
+{
+    return v.as_string().str;
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string> const&
+get(const basic_value<C, M, V>& v)
+{
+    return v.as_string().str;
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, std::string>::value, std::string>
+get(basic_value<C, M, V>&& v)
+{
+    return std::string(std::move(v.as_string().str));
+}
+
+// ============================================================================
+// std::string_view
+
+#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<std::is_same<T, std::string_view>::value, std::string_view>
+get(const basic_value<C, M, V>& v)
+{
+    return std::string_view(v.as_string().str);
+}
+#endif
+
+// ============================================================================
+// std::chrono::duration from toml::local_time.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<detail::is_chrono_duration<T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return std::chrono::duration_cast<T>(
+            std::chrono::nanoseconds(v.as_local_time()));
+}
+
+// ============================================================================
+// std::chrono::system_clock::time_point from toml::datetime variants
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+inline detail::enable_if_t<
+    std::is_same<std::chrono::system_clock::time_point, T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    switch(v.type())
+    {
+        case value_t::local_date:
+        {
+            return std::chrono::system_clock::time_point(v.as_local_date());
+        }
+        case value_t::local_datetime:
+        {
+            return std::chrono::system_clock::time_point(v.as_local_datetime());
+        }
+        case value_t::offset_datetime:
+        {
+            return std::chrono::system_clock::time_point(v.as_offset_datetime());
+        }
+        default:
+        {
+            throw type_error(detail::format_underline("toml::value: "
+                "bad_cast to std::chrono::system_clock::time_point", {
+                    {v.location(), concat_to_string("the actual type is ", v.type())}
+                }), v.location());
+        }
+    }
+}
+
+// ============================================================================
+// forward declaration to use this recursively. ignore this and go ahead.
+
+// array-like type with push_back(value) method
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_container<T>,         // T is a container
+    detail::has_push_back_method<T>, // T::push_back(value) works
+    detail::negation<                // but not toml::array
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>&);
+
+// array-like type without push_back(value) method
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_container<T>,                           // T is a container
+    detail::negation<detail::has_push_back_method<T>>, // w/o push_back(...)
+    detail::negation<                                  // not toml::array
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>&);
+
+// std::pair<T1, T2>
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_std_pair<T>::value, T>
+get(const basic_value<C, M, V>&);
+
+// std::tuple<T1, T2, ...>
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_std_tuple<T>::value, T>
+get(const basic_value<C, M, V>&);
+
+// map-like classes
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_map<T>, // T is map
+    detail::negation<  // but not toml::table
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>&);
+
+// T.from_toml(v)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::negation<                         // not a toml::* type
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>,
+    detail::has_from_toml_method<T, C, M, V>, // but has from_toml(toml::value)
+    std::is_default_constructible<T>          // and default constructible
+    >::value, T>
+get(const basic_value<C, M, V>&);
+
+// toml::from<T>::from_toml(v)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::has_specialized_from<T>::value, T>
+get(const basic_value<C, M, V>&);
+
+// T(const toml::value&) and T is not toml::basic_value,
+// and it does not have `from<T>` nor `from_toml`.
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::negation<detail::is_basic_value<T>>,
+    std::is_constructible<T, const basic_value<C, M, V>&>,
+    detail::negation<detail::has_from_toml_method<T, C, M, V>>,
+    detail::negation<detail::has_specialized_from<T>>
+    >::value, T>
+get(const basic_value<C, M, V>&);
+
+// ============================================================================
+// array-like types; most likely STL container, like std::vector, etc.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_container<T>,         // T is a container
+    detail::has_push_back_method<T>, // container.push_back(elem) works
+    detail::negation<                // but not toml::array
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    using value_type = typename T::value_type;
+    const auto& ary = v.as_array();
+
+    T container;
+    try_reserve(container, ary.size());
+
+    for(const auto& elem : ary)
+    {
+        container.push_back(get<value_type>(elem));
+    }
+    return container;
+}
+
+// ============================================================================
+// std::forward_list does not have push_back, insert, or emplace.
+// It has insert_after, emplace_after, push_front.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_std_forward_list<T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    using value_type = typename T::value_type;
+    T container;
+    for(const auto& elem : v.as_array())
+    {
+        container.push_front(get<value_type>(elem));
+    }
+    container.reverse();
+    return container;
+}
+
+// ============================================================================
+// array-like types, without push_back(). most likely [std|boost]::array.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_container<T>,                           // T is a container
+    detail::negation<detail::has_push_back_method<T>>, // w/o push_back
+    detail::negation<                                  // T is not toml::array
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    using value_type = typename T::value_type;
+    const auto& ar = v.as_array();
+
+    T container;
+    if(ar.size() != container.size())
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "toml::get: specified container size is ", container.size(),
+            " but there are ", ar.size(), " elements in toml array."), {
+                {v.location(), "here"}
+            }));
+    }
+    for(std::size_t i=0; i<ar.size(); ++i)
+    {
+        container[i] = ::toml::get<value_type>(ar[i]);
+    }
+    return container;
+}
+
+// ============================================================================
+// std::pair.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_std_pair<T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    using first_type  = typename T::first_type;
+    using second_type = typename T::second_type;
+
+    const auto& ar = v.as_array();
+    if(ar.size() != 2)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "toml::get: specified std::pair but there are ", ar.size(),
+            " elements in toml array."), {{v.location(), "here"}}));
+    }
+    return std::make_pair(::toml::get<first_type >(ar.at(0)),
+                          ::toml::get<second_type>(ar.at(1)));
+}
+
+// ============================================================================
+// std::tuple.
+
+namespace detail
+{
+template<typename T, typename Array, std::size_t ... I>
+T get_tuple_impl(const Array& a, index_sequence<I...>)
+{
+    return std::make_tuple(
+        ::toml::get<typename std::tuple_element<I, T>::type>(a.at(I))...);
+}
+} // detail
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_std_tuple<T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    const auto& ar = v.as_array();
+    if(ar.size() != std::tuple_size<T>::value)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "toml::get: specified std::tuple with ",
+            std::tuple_size<T>::value, " elements, but there are ", ar.size(),
+            " elements in toml array."), {{v.location(), "here"}}));
+    }
+    return detail::get_tuple_impl<T>(ar,
+            detail::make_index_sequence<std::tuple_size<T>::value>{});
+}
+
+// ============================================================================
+// map-like types; most likely STL map, like std::map or std::unordered_map.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::is_map<T>, // T is map
+    detail::negation<  // but not toml::array
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    using key_type    = typename T::key_type;
+    using mapped_type = typename T::mapped_type;
+    static_assert(std::is_convertible<std::string, key_type>::value,
+                  "toml::get only supports map type of which key_type is "
+                  "convertible from std::string.");
+    T map;
+    for(const auto& kv : v.as_table())
+    {
+        map.emplace(key_type(kv.first), get<mapped_type>(kv.second));
+    }
+    return map;
+}
+
+// ============================================================================
+// user-defined, but compatible types.
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::negation<                         // not a toml::* type
+        detail::is_exact_toml_type<T, basic_value<C, M, V>>>,
+    detail::has_from_toml_method<T, C, M, V>, // but has from_toml(toml::value) memfn
+    std::is_default_constructible<T>          // and default constructible
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    T ud;
+    ud.from_toml(v);
+    return ud;
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::has_specialized_from<T>::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return ::toml::from<T>::from_toml(v);
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::negation<detail::is_basic_value<T>>,                // T is not a toml::value
+    std::is_constructible<T, const basic_value<C, M, V>&>,      // T is constructible from toml::value
+    detail::negation<detail::has_from_toml_method<T, C, M, V>>, // and T does not have T.from_toml(v);
+    detail::negation<detail::has_specialized_from<T>>           // and T does not have toml::from<T>{};
+    >::value, T>
+get(const basic_value<C, M, V>& v)
+{
+    return T(v);
+}
+
+// ============================================================================
+// find
+
+// ----------------------------------------------------------------------------
+// these overloads do not require to set T. and returns value itself.
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> const& find(const basic_value<C, M, V>& v, const key& ky)
+{
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return tab.at(ky);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>& find(basic_value<C, M, V>& v, const key& ky)
+{
+    auto& tab = v.as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return tab.at(ky);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> find(basic_value<C, M, V>&& v, const key& ky)
+{
+    typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return basic_value<C, M, V>(std::move(tab.at(ky)));
+}
+
+// ----------------------------------------------------------------------------
+// find(value, idx)
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> const&
+find(const basic_value<C, M, V>& v, const std::size_t idx)
+{
+    const auto& ary = v.as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return ary.at(idx);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>& find(basic_value<C, M, V>& v, const std::size_t idx)
+{
+    auto& ary = v.as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return ary.at(idx);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> find(basic_value<C, M, V>&& v, const std::size_t idx)
+{
+    auto& ary = v.as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return basic_value<C, M, V>(std::move(ary.at(idx)));
+}
+
+// ----------------------------------------------------------------------------
+// find<T>(value, key);
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V> const&>()))
+find(const basic_value<C, M, V>& v, const key& ky)
+{
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return ::toml::get<T>(tab.at(ky));
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
+find(basic_value<C, M, V>& v, const key& ky)
+{
+    auto& tab = v.as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return ::toml::get<T>(tab.at(ky));
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
+find(basic_value<C, M, V>&& v, const key& ky)
+{
+    typename basic_value<C, M, V>::table_type tab = std::move(v).as_table();
+    if(tab.count(ky) == 0)
+    {
+        detail::throw_key_not_found_error(v, ky);
+    }
+    return ::toml::get<T>(std::move(tab.at(ky)));
+}
+
+// ----------------------------------------------------------------------------
+// find<T>(value, idx)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V> const&>()))
+find(const basic_value<C, M, V>& v, const std::size_t idx)
+{
+    const auto& ary = v.as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return ::toml::get<T>(ary.at(idx));
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
+find(basic_value<C, M, V>& v, const std::size_t idx)
+{
+    auto& ary = v.as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return ::toml::get<T>(ary.at(idx));
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
+find(basic_value<C, M, V>&& v, const std::size_t idx)
+{
+    typename basic_value<C, M, V>::array_type ary = std::move(v).as_array();
+    if(ary.size() <= idx)
+    {
+        throw std::out_of_range(detail::format_underline(concat_to_string(
+            "index ", idx, " is out of range"), {{v.location(), "in this array"}}));
+    }
+    return ::toml::get<T>(std::move(ary.at(idx)));
+}
+
+// --------------------------------------------------------------------------
+// toml::find(toml::value, toml::key, Ts&& ... keys)
+
+namespace detail
+{
+// It suppresses warnings by -Wsign-conversion. Let's say we have the following
+// code.
+// ```cpp
+// const auto x = toml::find<std::string>(data, "array", 0);
+// ```
+// Here, the type of literal number `0` is `int`. `int` is a signed integer.
+// `toml::find` takes `std::size_t` as an index. So it causes implicit sign
+// conversion and `-Wsign-conversion` warns about it. Using `0u` instead of `0`
+// suppresses the warning, but it makes user code messy.
+//     To suppress this warning, we need to be aware of type conversion caused
+// by `toml::find(v, key1, key2, ... keys)`. But the thing is that the types of
+// keys can be any combination of {string-like, size_t-like}. Of course we can't
+// write down all the combinations. Thus we need to use some function that
+// recognize the type of argument and cast it into `std::string` or
+// `std::size_t` depending on the context.
+//     `key_cast` does the job. It has 2 overloads. One is invoked when the
+// argument type is an integer and cast the argument into `std::size_t`. The
+// other is invoked when the argument type is not an integer, possibly one of
+// std::string, const char[N] or const char*, and construct std::string from
+// the argument.
+//     `toml::find(v, k1, k2, ... ks)` uses `key_cast` before passing `ks` to
+// `toml::find(v, k)` to suppress -Wsign-conversion.
+
+template<typename T>
+enable_if_t<conjunction<std::is_integral<remove_cvref_t<T>>,
+            negation<std::is_same<remove_cvref_t<T>, bool>>>::value, std::size_t>
+key_cast(T&& v) noexcept
+{
+    return std::size_t(v);
+}
+template<typename T>
+enable_if_t<negation<conjunction<std::is_integral<remove_cvref_t<T>>,
+            negation<std::is_same<remove_cvref_t<T>, bool>>>>::value, std::string>
+key_cast(T&& v) noexcept
+{
+    return std::string(std::forward<T>(v));
+}
+} // detail
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+const basic_value<C, M, V>&
+find(const basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find(::toml::find(v, detail::key_cast(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+basic_value<C, M, V>&
+find(basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find(::toml::find(v, detail::key_cast(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+basic_value<C, M, V>
+find(basic_value<C, M, V>&& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find(::toml::find(std::move(v), std::forward<Key1>(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+decltype(::toml::get<T>(std::declval<const basic_value<C, M, V>&>()))
+find(const basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find<T>(::toml::find(v, detail::key_cast(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&>()))
+find(basic_value<C, M, V>& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find<T>(::toml::find(v, detail::key_cast(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V,
+         typename Key1, typename Key2, typename ... Keys>
+decltype(::toml::get<T>(std::declval<basic_value<C, M, V>&&>()))
+find(basic_value<C, M, V>&& v, Key1&& k1, Key2&& k2, Keys&& ... keys)
+{
+    return ::toml::find<T>(::toml::find(std::move(v), detail::key_cast(k1)),
+            detail::key_cast(k2), std::forward<Keys>(keys)...);
+}
+
+// ============================================================================
+// get_or(value, fallback)
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> const&
+get_or(const basic_value<C, M, V>& v, const basic_value<C, M, V>&)
+{
+    return v;
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>&
+get_or(basic_value<C, M, V>& v, basic_value<C, M, V>&)
+{
+    return v;
+}
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>
+get_or(basic_value<C, M, V>&& v, basic_value<C, M, V>&&)
+{
+    return v;
+}
+
+// ----------------------------------------------------------------------------
+// specialization for the exact toml types (return type becomes lvalue ref)
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
+get_or(const basic_value<C, M, V>& v, const T& opt)
+{
+    try
+    {
+        return get<detail::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>&
+get_or(basic_value<C, M, V>& v, T& opt)
+{
+    try
+    {
+        return get<detail::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_exact_toml_type<detail::remove_cvref_t<T>,
+    basic_value<C, M, V>>::value, detail::remove_cvref_t<T>>
+get_or(basic_value<C, M, V>&& v, T&& opt)
+{
+    try
+    {
+        return get<detail::remove_cvref_t<T>>(std::move(v));
+    }
+    catch(...)
+    {
+        return detail::remove_cvref_t<T>(std::forward<T>(opt));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// specialization for std::string (return type becomes lvalue ref)
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<std::is_same<detail::remove_cvref_t<T>, std::string>::value,
+    std::string> const&
+get_or(const basic_value<C, M, V>& v, const T& opt)
+{
+    try
+    {
+        return v.as_string().str;
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
+get_or(basic_value<C, M, V>& v, T& opt)
+{
+    try
+    {
+        return v.as_string().str;
+    }
+    catch(...)
+    {
+        return opt;
+    }
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    std::is_same<detail::remove_cvref_t<T>, std::string>::value, std::string>
+get_or(basic_value<C, M, V>&& v, T&& opt)
+{
+    try
+    {
+        return std::move(v.as_string().str);
+    }
+    catch(...)
+    {
+        return std::string(std::forward<T>(opt));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// specialization for string literal
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::is_string_literal<
+    typename std::remove_reference<T>::type>::value, std::string>
+get_or(const basic_value<C, M, V>& v, T&& opt)
+{
+    try
+    {
+        return std::move(v.as_string().str);
+    }
+    catch(...)
+    {
+        return std::string(std::forward<T>(opt));
+    }
+}
+
+// ----------------------------------------------------------------------------
+// others (require type conversion and return type cannot be lvalue reference)
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    detail::negation<detail::is_exact_toml_type<detail::remove_cvref_t<T>,
+        basic_value<C, M, V>>>,
+    detail::negation<std::is_same<std::string, detail::remove_cvref_t<T>>>,
+    detail::negation<detail::is_string_literal<
+        typename std::remove_reference<T>::type>>
+    >::value, detail::remove_cvref_t<T>>
+get_or(const basic_value<C, M, V>& v, T&& opt)
+{
+    try
+    {
+        return get<detail::remove_cvref_t<T>>(v);
+    }
+    catch(...)
+    {
+        return detail::remove_cvref_t<T>(std::forward<T>(opt));
+    }
+}
+
+// ===========================================================================
+// find_or(value, key, fallback)
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V> const&
+find_or(const basic_value<C, M, V>& v, const key& ky,
+        const basic_value<C, M, V>& opt)
+{
+    if(!v.is_table()) {return opt;}
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return tab.at(ky);
+}
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>&
+find_or(basic_value<C, M, V>& v, const toml::key& ky, basic_value<C, M, V>& opt)
+{
+    if(!v.is_table()) {return opt;}
+    auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return tab.at(ky);
+}
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+basic_value<C, M, V>
+find_or(basic_value<C, M, V>&& v, const toml::key& ky, basic_value<C, M, V>&& opt)
+{
+    if(!v.is_table()) {return opt;}
+    auto tab = std::move(v).as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return basic_value<C, M, V>(std::move(tab.at(ky)));
+}
+
+// ---------------------------------------------------------------------------
+// exact types (return type can be a reference)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T> const&
+find_or(const basic_value<C, M, V>& v, const key& ky, const T& opt)
+{
+    if(!v.is_table()) {return opt;}
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return get_or(tab.at(ky), opt);
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value, T>&
+find_or(basic_value<C, M, V>& v, const toml::key& ky, T& opt)
+{
+    if(!v.is_table()) {return opt;}
+    auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return get_or(tab.at(ky), opt);
+}
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_exact_toml_type<T, basic_value<C, M, V>>::value,
+    detail::remove_cvref_t<T>>
+find_or(basic_value<C, M, V>&& v, const toml::key& ky, T&& opt)
+{
+    if(!v.is_table()) {return std::forward<T>(opt);}
+    auto tab = std::move(v).as_table();
+    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
+    return get_or(std::move(tab.at(ky)), std::forward<T>(opt));
+}
+
+// ---------------------------------------------------------------------------
+// std::string (return type can be a reference)
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<std::is_same<T, std::string>::value, std::string> const&
+find_or(const basic_value<C, M, V>& v, const key& ky, const T& opt)
+{
+    if(!v.is_table()) {return opt;}
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return get_or(tab.at(ky), opt);
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<std::is_same<T, std::string>::value, std::string>&
+find_or(basic_value<C, M, V>& v, const toml::key& ky, T& opt)
+{
+    if(!v.is_table()) {return opt;}
+    auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return opt;}
+    return get_or(tab.at(ky), opt);
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<std::is_same<T, std::string>::value, std::string>
+find_or(basic_value<C, M, V>&& v, const toml::key& ky, T&& opt)
+{
+    if(!v.is_table()) {return std::forward<T>(opt);}
+    auto tab = std::move(v).as_table();
+    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
+    return get_or(std::move(tab.at(ky)), std::forward<T>(opt));
+}
+
+// ---------------------------------------------------------------------------
+// string literal (deduced as std::string)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<
+    detail::is_string_literal<typename std::remove_reference<T>::type>::value,
+    std::string>
+find_or(const basic_value<C, M, V>& v, const toml::key& ky, T&& opt)
+{
+    if(!v.is_table()) {return std::string(opt);}
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return std::string(opt);}
+    return get_or(tab.at(ky), std::forward<T>(opt));
+}
+
+// ---------------------------------------------------------------------------
+// others (require type conversion and return type cannot be lvalue reference)
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+detail::enable_if_t<detail::conjunction<
+    // T is not an exact toml type
+    detail::negation<detail::is_exact_toml_type<
+        detail::remove_cvref_t<T>, basic_value<C, M, V>>>,
+    // T is not std::string
+    detail::negation<std::is_same<std::string, detail::remove_cvref_t<T>>>,
+    // T is not a string literal
+    detail::negation<detail::is_string_literal<
+        typename std::remove_reference<T>::type>>
+    >::value, detail::remove_cvref_t<T>>
+find_or(const basic_value<C, M, V>& v, const toml::key& ky, T&& opt)
+{
+    if(!v.is_table()) {return std::forward<T>(opt);}
+    const auto& tab = v.as_table();
+    if(tab.count(ky) == 0) {return std::forward<T>(opt);}
+    return get_or(tab.at(ky), std::forward<T>(opt));
+}
+
+// ---------------------------------------------------------------------------
+// recursive find-or with type deduction (find_or(value, keys, opt))
+
+template<typename Value, typename ... Ks,
+         typename detail::enable_if_t<(sizeof...(Ks) > 1), std::nullptr_t> = nullptr>
+         // here we need to add SFINAE in the template parameter to avoid
+         // infinite recursion in type deduction on gcc
+auto find_or(Value&& v, const toml::key& ky, Ks&& ... keys)
+    -> decltype(find_or(std::forward<Value>(v), ky, detail::last_one(std::forward<Ks>(keys)...)))
+{
+    if(!v.is_table())
+    {
+        return detail::last_one(std::forward<Ks>(keys)...);
+    }
+    auto&& tab = std::forward<Value>(v).as_table();
+    if(tab.count(ky) == 0)
+    {
+        return detail::last_one(std::forward<Ks>(keys)...);
+    }
+    return find_or(std::forward<decltype(tab)>(tab).at(ky), std::forward<Ks>(keys)...);
+}
+
+// ---------------------------------------------------------------------------
+// recursive find_or with explicit type specialization, find_or<int>(value, keys...)
+
+template<typename T, typename Value, typename ... Ks,
+         typename detail::enable_if_t<(sizeof...(Ks) > 1), std::nullptr_t> = nullptr>
+         // here we need to add SFINAE in the template parameter to avoid
+         // infinite recursion in type deduction on gcc
+auto find_or(Value&& v, const toml::key& ky, Ks&& ... keys)
+    -> decltype(find_or<T>(std::forward<Value>(v), ky, detail::last_one(std::forward<Ks>(keys)...)))
+{
+    if(!v.is_table())
+    {
+        return detail::last_one(std::forward<Ks>(keys)...);
+    }
+    auto&& tab = std::forward<Value>(v).as_table();
+    if(tab.count(ky) == 0)
+    {
+        return detail::last_one(std::forward<Ks>(keys)...);
+    }
+    return find_or(std::forward<decltype(tab)>(tab).at(ky), std::forward<Ks>(keys)...);
+}
+
+// ============================================================================
+// expect
+
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+result<T, std::string> expect(const basic_value<C, M, V>& v) noexcept
+{
+    try
+    {
+        return ok(get<T>(v));
+    }
+    catch(const std::exception& e)
+    {
+        return err(e.what());
+    }
+}
+template<typename T, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+result<T, std::string>
+expect(const basic_value<C, M, V>& v, const toml::key& k) noexcept
+{
+    try
+    {
+        return ok(find<T>(v, k));
+    }
+    catch(const std::exception& e)
+    {
+        return err(e.what());
+    }
+}
+
+} // toml
+#endif// TOML11_GET
diff --git a/src/toml11/toml/into.hpp b/src/toml11/toml/into.hpp
new file mode 100644
index 000000000..74495560e
--- /dev/null
+++ b/src/toml11/toml/into.hpp
@@ -0,0 +1,19 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_INTO_HPP
+#define TOML11_INTO_HPP
+
+namespace toml
+{
+
+template<typename T>
+struct into;
+// {
+//     static toml::value into_toml(const T& user_defined_type)
+//     {
+//         // User-defined conversions ...
+//     }
+// };
+
+} // toml
+#endif // TOML11_INTO_HPP
diff --git a/src/toml11/toml/lexer.hpp b/src/toml11/toml/lexer.hpp
new file mode 100644
index 000000000..ea5050b8d
--- /dev/null
+++ b/src/toml11/toml/lexer.hpp
@@ -0,0 +1,293 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_LEXER_HPP
+#define TOML11_LEXER_HPP
+#include <istream>
+#include <sstream>
+#include <stdexcept>
+#include <fstream>
+
+#include "combinator.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+// these scans contents from current location in a container of char
+// and extract a region that matches their own pattern.
+// to see the implementation of each component, see combinator.hpp.
+
+using lex_wschar  = either<character<' '>, character<'\t'>>;
+using lex_ws      = repeat<lex_wschar, at_least<1>>;
+using lex_newline = either<character<'\n'>,
+                           sequence<character<'\r'>, character<'\n'>>>;
+using lex_lower   = in_range<'a', 'z'>;
+using lex_upper   = in_range<'A', 'Z'>;
+using lex_alpha   = either<lex_lower, lex_upper>;
+using lex_digit   = in_range<'0', '9'>;
+using lex_nonzero = in_range<'1', '9'>;
+using lex_oct_dig = in_range<'0', '7'>;
+using lex_bin_dig = in_range<'0', '1'>;
+using lex_hex_dig = either<lex_digit, in_range<'A', 'F'>, in_range<'a', 'f'>>;
+
+using lex_hex_prefix = sequence<character<'0'>, character<'x'>>;
+using lex_oct_prefix = sequence<character<'0'>, character<'o'>>;
+using lex_bin_prefix = sequence<character<'0'>, character<'b'>>;
+using lex_underscore = character<'_'>;
+using lex_plus       = character<'+'>;
+using lex_minus      = character<'-'>;
+using lex_sign       = either<lex_plus, lex_minus>;
+
+// digit | nonzero 1*(digit | _ digit)
+using lex_unsigned_dec_int = either<sequence<lex_nonzero, repeat<
+    either<lex_digit, sequence<lex_underscore, lex_digit>>, at_least<1>>>,
+    lex_digit>;
+// (+|-)? unsigned_dec_int
+using lex_dec_int = sequence<maybe<lex_sign>, lex_unsigned_dec_int>;
+
+// hex_prefix hex_dig *(hex_dig | _ hex_dig)
+using lex_hex_int = sequence<lex_hex_prefix, sequence<lex_hex_dig, repeat<
+    either<lex_hex_dig, sequence<lex_underscore, lex_hex_dig>>, unlimited>>>;
+// oct_prefix oct_dig *(oct_dig | _ oct_dig)
+using lex_oct_int = sequence<lex_oct_prefix, sequence<lex_oct_dig, repeat<
+    either<lex_oct_dig, sequence<lex_underscore, lex_oct_dig>>, unlimited>>>;
+// bin_prefix bin_dig *(bin_dig | _ bin_dig)
+using lex_bin_int = sequence<lex_bin_prefix, sequence<lex_bin_dig, repeat<
+    either<lex_bin_dig, sequence<lex_underscore, lex_bin_dig>>, unlimited>>>;
+
+// (dec_int | hex_int | oct_int | bin_int)
+using lex_integer = either<lex_bin_int, lex_oct_int, lex_hex_int, lex_dec_int>;
+
+// ===========================================================================
+
+using lex_inf = sequence<character<'i'>, character<'n'>, character<'f'>>;
+using lex_nan = sequence<character<'n'>, character<'a'>, character<'n'>>;
+using lex_special_float = sequence<maybe<lex_sign>, either<lex_inf, lex_nan>>;
+
+using lex_zero_prefixable_int = sequence<lex_digit, repeat<either<lex_digit,
+    sequence<lex_underscore, lex_digit>>, unlimited>>;
+
+using lex_fractional_part = sequence<character<'.'>, lex_zero_prefixable_int>;
+
+using lex_exponent_part   = sequence<either<character<'e'>, character<'E'>>,
+        maybe<lex_sign>, lex_zero_prefixable_int>;
+
+using lex_float = either<lex_special_float,
+      sequence<lex_dec_int, either<lex_exponent_part,
+      sequence<lex_fractional_part, maybe<lex_exponent_part>>>>>;
+
+// ===========================================================================
+
+using lex_true = sequence<character<'t'>, character<'r'>,
+                          character<'u'>, character<'e'>>;
+using lex_false = sequence<character<'f'>, character<'a'>, character<'l'>,
+                           character<'s'>, character<'e'>>;
+using lex_boolean = either<lex_true, lex_false>;
+
+// ===========================================================================
+
+using lex_date_fullyear = repeat<lex_digit, exactly<4>>;
+using lex_date_month    = repeat<lex_digit, exactly<2>>;
+using lex_date_mday     = repeat<lex_digit, exactly<2>>;
+using lex_time_delim    = either<character<'T'>, character<'t'>, character<' '>>;
+using lex_time_hour     = repeat<lex_digit, exactly<2>>;
+using lex_time_minute   = repeat<lex_digit, exactly<2>>;
+using lex_time_second   = repeat<lex_digit, exactly<2>>;
+using lex_time_secfrac  = sequence<character<'.'>,
+                                   repeat<lex_digit, at_least<1>>>;
+
+using lex_time_numoffset = sequence<either<character<'+'>, character<'-'>>,
+                                    sequence<lex_time_hour, character<':'>,
+                                             lex_time_minute>>;
+using lex_time_offset = either<character<'Z'>, character<'z'>,
+                               lex_time_numoffset>;
+
+using lex_partial_time = sequence<lex_time_hour,   character<':'>,
+                                  lex_time_minute, character<':'>,
+                                  lex_time_second, maybe<lex_time_secfrac>>;
+using lex_full_date    = sequence<lex_date_fullyear, character<'-'>,
+                                  lex_date_month,    character<'-'>,
+                                  lex_date_mday>;
+using lex_full_time    = sequence<lex_partial_time, lex_time_offset>;
+
+using lex_offset_date_time = sequence<lex_full_date, lex_time_delim, lex_full_time>;
+using lex_local_date_time  = sequence<lex_full_date, lex_time_delim, lex_partial_time>;
+using lex_local_date       = lex_full_date;
+using lex_local_time       = lex_partial_time;
+
+// ===========================================================================
+
+using lex_quotation_mark  = character<'"'>;
+using lex_basic_unescaped = exclude<either<in_range<0x00, 0x08>, // 0x09 (tab) is allowed
+                                           in_range<0x0A, 0x1F>,
+                                           character<0x22>, character<0x5C>,
+                                           character<0x7F>>>;
+
+using lex_escape          = character<'\\'>;
+using lex_escape_unicode_short = sequence<character<'u'>,
+                                          repeat<lex_hex_dig, exactly<4>>>;
+using lex_escape_unicode_long  = sequence<character<'U'>,
+                                          repeat<lex_hex_dig, exactly<8>>>;
+using lex_escape_seq_char = either<character<'"'>, character<'\\'>,
+                                   character<'b'>, character<'f'>,
+                                   character<'n'>, character<'r'>,
+                                   character<'t'>,
+                                   lex_escape_unicode_short,
+                                   lex_escape_unicode_long
+                                   >;
+using lex_escaped      = sequence<lex_escape, lex_escape_seq_char>;
+using lex_basic_char   = either<lex_basic_unescaped, lex_escaped>;
+using lex_basic_string = sequence<lex_quotation_mark,
+                                  repeat<lex_basic_char, unlimited>,
+                                  lex_quotation_mark>;
+
+// After toml post-v0.5.0, it is explicitly clarified how quotes in ml-strings
+// are allowed to be used.
+// After this, the following strings are *explicitly* allowed.
+// - One or two `"`s in a multi-line basic string is allowed wherever it is.
+// - Three consecutive `"`s in a multi-line basic string is considered as a delimiter.
+// - One or two `"`s can appear just before or after the delimiter.
+// ```toml
+// str4 = """Here are two quotation marks: "". Simple enough."""
+// str5 = """Here are three quotation marks: ""\"."""
+// str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
+// str7 = """"This," she said, "is just a pointless statement.""""
+// ```
+// In the current implementation (v3.3.0), it is difficult to parse `str7` in
+// the above example. It is difficult to recognize `"` at the end of string body
+// collectly. It will be misunderstood as a `"""` delimiter and an additional,
+// invalid `"`. Like this:
+// ```console
+//   what():  [error] toml::parse_table: invalid line format
+//  --> hoge.toml
+//     |
+//  13 | str7 = """"This," she said, "is just a pointless statement.""""
+//     |                                                               ^- expected newline, but got '"'.
+// ```
+// As a quick workaround for this problem, `lex_ml_basic_string_delim` was
+// split into two, `lex_ml_basic_string_open` and `lex_ml_basic_string_close`.
+// `lex_ml_basic_string_open` allows only `"""`. `_close` allows 3-5 `"`s.
+// In parse_ml_basic_string() function, the trailing `"`s will be attached to
+// the string body.
+//
+using lex_ml_basic_string_delim = repeat<lex_quotation_mark, exactly<3>>;
+using lex_ml_basic_string_open  = lex_ml_basic_string_delim;
+using lex_ml_basic_string_close = sequence<
+        repeat<lex_quotation_mark, exactly<3>>,
+        maybe<lex_quotation_mark>, maybe<lex_quotation_mark>
+    >;
+
+using lex_ml_basic_unescaped    = exclude<either<in_range<0x00, 0x08>, // 0x09 is tab
+                                                 in_range<0x0A, 0x1F>,
+                                                 character<0x5C>, // backslash
+                                                 character<0x7F>, // DEL
+                                                 lex_ml_basic_string_delim>>;
+
+using lex_ml_basic_escaped_newline = sequence<
+        lex_escape, maybe<lex_ws>, lex_newline,
+        repeat<either<lex_ws, lex_newline>, unlimited>>;
+
+using lex_ml_basic_char = either<lex_ml_basic_unescaped, lex_escaped>;
+using lex_ml_basic_body = repeat<either<lex_ml_basic_char, lex_newline,
+                                        lex_ml_basic_escaped_newline>,
+                                 unlimited>;
+using lex_ml_basic_string = sequence<lex_ml_basic_string_open,
+                                     lex_ml_basic_body,
+                                     lex_ml_basic_string_close>;
+
+using lex_literal_char = exclude<either<in_range<0x00, 0x08>, in_range<0x0A, 0x1F>,
+                                        character<0x7F>, character<0x27>>>;
+using lex_apostrophe = character<'\''>;
+using lex_literal_string = sequence<lex_apostrophe,
+                                    repeat<lex_literal_char, unlimited>,
+                                    lex_apostrophe>;
+
+// the same reason as above.
+using lex_ml_literal_string_delim = repeat<lex_apostrophe, exactly<3>>;
+using lex_ml_literal_string_open  = lex_ml_literal_string_delim;
+using lex_ml_literal_string_close = sequence<
+        repeat<lex_apostrophe, exactly<3>>,
+        maybe<lex_apostrophe>, maybe<lex_apostrophe>
+    >;
+
+using lex_ml_literal_char = exclude<either<in_range<0x00, 0x08>,
+                                           in_range<0x0A, 0x1F>,
+                                           character<0x7F>,
+                                           lex_ml_literal_string_delim>>;
+using lex_ml_literal_body = repeat<either<lex_ml_literal_char, lex_newline>,
+                                   unlimited>;
+using lex_ml_literal_string = sequence<lex_ml_literal_string_open,
+                                       lex_ml_literal_body,
+                                       lex_ml_literal_string_close>;
+
+using lex_string = either<lex_ml_basic_string,   lex_basic_string,
+                          lex_ml_literal_string, lex_literal_string>;
+
+// ===========================================================================
+using lex_dot_sep = sequence<maybe<lex_ws>, character<'.'>, maybe<lex_ws>>;
+
+using lex_unquoted_key = repeat<either<lex_alpha, lex_digit,
+                                       character<'-'>, character<'_'>>,
+                                at_least<1>>;
+using lex_quoted_key = either<lex_basic_string, lex_literal_string>;
+using lex_simple_key = either<lex_unquoted_key, lex_quoted_key>;
+using lex_dotted_key = sequence<lex_simple_key,
+                                repeat<sequence<lex_dot_sep, lex_simple_key>,
+                                       at_least<1>
+                                       >
+                                >;
+using lex_key = either<lex_dotted_key, lex_simple_key>;
+
+using lex_keyval_sep = sequence<maybe<lex_ws>,
+                                character<'='>,
+                                maybe<lex_ws>>;
+
+using lex_std_table_open  = character<'['>;
+using lex_std_table_close = character<']'>;
+using lex_std_table       = sequence<lex_std_table_open,
+                                     maybe<lex_ws>,
+                                     lex_key,
+                                     maybe<lex_ws>,
+                                     lex_std_table_close>;
+
+using lex_array_table_open  = sequence<lex_std_table_open,  lex_std_table_open>;
+using lex_array_table_close = sequence<lex_std_table_close, lex_std_table_close>;
+using lex_array_table       = sequence<lex_array_table_open,
+                                       maybe<lex_ws>,
+                                       lex_key,
+                                       maybe<lex_ws>,
+                                       lex_array_table_close>;
+
+using lex_utf8_1byte = in_range<0x00, 0x7F>;
+using lex_utf8_2byte = sequence<
+        in_range<static_cast<char>(0xC2), static_cast<char>(0xDF)>,
+        in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>
+    >;
+using lex_utf8_3byte = sequence<either<
+        sequence<character<static_cast<char>(0xE0)>,                          in_range<static_cast<char>(0xA0), static_cast<char>(0xBF)>>,
+        sequence<in_range <static_cast<char>(0xE1), static_cast<char>(0xEC)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>,
+        sequence<character<static_cast<char>(0xED)>,                          in_range<static_cast<char>(0x80), static_cast<char>(0x9F)>>,
+        sequence<in_range <static_cast<char>(0xEE), static_cast<char>(0xEF)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>
+    >, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>;
+using lex_utf8_4byte = sequence<either<
+        sequence<character<static_cast<char>(0xF0)>,                          in_range<static_cast<char>(0x90), static_cast<char>(0xBF)>>,
+        sequence<in_range <static_cast<char>(0xF1), static_cast<char>(0xF3)>, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>,
+        sequence<character<static_cast<char>(0xF4)>,                          in_range<static_cast<char>(0x80), static_cast<char>(0x8F)>>
+    >, in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>,
+       in_range<static_cast<char>(0x80), static_cast<char>(0xBF)>>;
+using lex_utf8_code = either<
+        lex_utf8_1byte,
+        lex_utf8_2byte,
+        lex_utf8_3byte,
+        lex_utf8_4byte
+    >;
+
+using lex_comment_start_symbol = character<'#'>;
+using lex_non_eol_ascii = either<character<0x09>, in_range<0x20, 0x7E>>;
+using lex_comment = sequence<lex_comment_start_symbol, repeat<either<
+    lex_non_eol_ascii, lex_utf8_2byte, lex_utf8_3byte, lex_utf8_4byte>, unlimited>>;
+
+} // detail
+} // toml
+#endif // TOML_LEXER_HPP
diff --git a/src/toml11/toml/literal.hpp b/src/toml11/toml/literal.hpp
new file mode 100644
index 000000000..04fbbc13e
--- /dev/null
+++ b/src/toml11/toml/literal.hpp
@@ -0,0 +1,113 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_LITERAL_HPP
+#define TOML11_LITERAL_HPP
+#include "parser.hpp"
+
+namespace toml
+{
+inline namespace literals
+{
+inline namespace toml_literals
+{
+
+// implementation
+inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
+literal_internal_impl(::toml::detail::location loc)
+{
+    using value_type = ::toml::basic_value<
+        TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>;
+    // if there are some comments or empty lines, skip them.
+    using skip_line = ::toml::detail::repeat<toml::detail::sequence<
+            ::toml::detail::maybe<::toml::detail::lex_ws>,
+            ::toml::detail::maybe<::toml::detail::lex_comment>,
+            ::toml::detail::lex_newline
+        >, ::toml::detail::at_least<1>>;
+    skip_line::invoke(loc);
+
+    // if there are some whitespaces before a value, skip them.
+    using skip_ws = ::toml::detail::repeat<
+        ::toml::detail::lex_ws, ::toml::detail::at_least<1>>;
+    skip_ws::invoke(loc);
+
+    // to distinguish arrays and tables, first check it is a table or not.
+    //
+    // "[1,2,3]"_toml;   // this is an array
+    // "[table]"_toml;   // a table that has an empty table named "table" inside.
+    // "[[1,2,3]]"_toml; // this is an array of arrays
+    // "[[table]]"_toml; // this is a table that has an array of tables inside.
+    //
+    // "[[1]]"_toml;     // this can be both... (currently it becomes a table)
+    // "1 = [{}]"_toml;  // this is a table that has an array of table named 1.
+    // "[[1,]]"_toml;    // this is an array of arrays.
+    // "[[1],]"_toml;    // this also.
+
+    const auto the_front = loc.iter();
+
+    const bool is_table_key = ::toml::detail::lex_std_table::invoke(loc);
+    loc.reset(the_front);
+
+    const bool is_aots_key  = ::toml::detail::lex_array_table::invoke(loc);
+    loc.reset(the_front);
+
+    // If it is neither a table-key or a array-of-table-key, it may be a value.
+    if(!is_table_key && !is_aots_key)
+    {
+        if(auto data = ::toml::detail::parse_value<value_type>(loc))
+        {
+            return data.unwrap();
+        }
+    }
+
+    // Note that still it can be a table, because the literal might be something
+    // like the following.
+    // ```cpp
+    // R"( // c++11 raw string literals
+    //   key = "value"
+    //   int = 42
+    // )"_toml;
+    // ```
+    // It is a valid toml file.
+    // It should be parsed as if we parse a file with this content.
+
+    if(auto data = ::toml::detail::parse_toml_file<value_type>(loc))
+    {
+        return data.unwrap();
+    }
+    else // none of them.
+    {
+        throw ::toml::syntax_error(data.unwrap_err(), source_location(loc));
+    }
+
+}
+
+inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
+operator"" _toml(const char* str, std::size_t len)
+{
+    ::toml::detail::location loc(
+            std::string("TOML literal encoded in a C++ code"),
+            std::vector<char>(str, str + len));
+    // literal length does not include the null character at the end.
+    return literal_internal_impl(std::move(loc));
+}
+
+// value of __cplusplus in C++2a/20 mode is not fixed yet along compilers.
+// So here we use the feature test macro for `char8_t` itself.
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+// value of u8"" literal has been changed from char to char8_t and char8_t is
+// NOT compatible to char
+inline ::toml::basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>
+operator"" _toml(const char8_t* str, std::size_t len)
+{
+    ::toml::detail::location loc(
+            std::string("TOML literal encoded in a C++ code"),
+            std::vector<char>(reinterpret_cast<const char*>(str),
+                              reinterpret_cast<const char*>(str) + len));
+    return literal_internal_impl(std::move(loc));
+}
+#endif
+
+} // toml_literals
+} // literals
+} // toml
+#endif//TOML11_LITERAL_HPP
diff --git a/src/toml11/toml/macros.hpp b/src/toml11/toml/macros.hpp
new file mode 100644
index 000000000..e8f91aecd
--- /dev/null
+++ b/src/toml11/toml/macros.hpp
@@ -0,0 +1,121 @@
+#ifndef TOML11_MACROS_HPP
+#define TOML11_MACROS_HPP
+
+#define TOML11_STRINGIZE_AUX(x) #x
+#define TOML11_STRINGIZE(x)     TOML11_STRINGIZE_AUX(x)
+
+#define TOML11_CONCATENATE_AUX(x, y) x##y
+#define TOML11_CONCATENATE(x, y)     TOML11_CONCATENATE_AUX(x, y)
+
+// ============================================================================
+// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE
+
+#ifndef TOML11_WITHOUT_DEFINE_NON_INTRUSIVE
+
+// ----------------------------------------------------------------------------
+// TOML11_ARGS_SIZE
+
+#define TOML11_INDEX_RSEQ() \
+    32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, \
+    16, 15, 14, 13, 12, 11, 10,  9,  8,  7,  6,  5,  4,  3,  2,  1, 0
+#define TOML11_ARGS_SIZE_IMPL(\
+    ARG1,  ARG2,  ARG3,  ARG4,  ARG5,  ARG6,  ARG7,  ARG8,  ARG9,  ARG10, \
+    ARG11, ARG12, ARG13, ARG14, ARG15, ARG16, ARG17, ARG18, ARG19, ARG20, \
+    ARG21, ARG22, ARG23, ARG24, ARG25, ARG26, ARG27, ARG28, ARG29, ARG30, \
+    ARG31, ARG32, N, ...) N
+#define TOML11_ARGS_SIZE_AUX(...) TOML11_ARGS_SIZE_IMPL(__VA_ARGS__)
+#define TOML11_ARGS_SIZE(...) TOML11_ARGS_SIZE_AUX(__VA_ARGS__, TOML11_INDEX_RSEQ())
+
+// ----------------------------------------------------------------------------
+// TOML11_FOR_EACH_VA_ARGS
+
+#define TOML11_FOR_EACH_VA_ARGS_AUX_1( FUNCTOR, ARG1     ) FUNCTOR(ARG1)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_2( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_1( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_3( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_2( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_4( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_3( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_5( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_4( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_6( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_5( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_7( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_6( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_8( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_7( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_9( FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_8( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_10(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_9( FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_11(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_10(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_12(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_11(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_13(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_12(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_14(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_13(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_15(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_14(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_16(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_15(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_17(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_16(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_18(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_17(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_19(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_18(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_20(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_19(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_21(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_20(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_22(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_21(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_23(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_22(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_24(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_23(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_25(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_24(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_26(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_25(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_27(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_26(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_28(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_27(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_29(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_28(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_30(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_29(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_31(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_30(FUNCTOR, __VA_ARGS__)
+#define TOML11_FOR_EACH_VA_ARGS_AUX_32(FUNCTOR, ARG1, ...) FUNCTOR(ARG1) TOML11_FOR_EACH_VA_ARGS_AUX_31(FUNCTOR, __VA_ARGS__)
+
+#define TOML11_FOR_EACH_VA_ARGS(FUNCTOR, ...)\
+    TOML11_CONCATENATE(TOML11_FOR_EACH_VA_ARGS_AUX_, TOML11_ARGS_SIZE(__VA_ARGS__))(FUNCTOR, __VA_ARGS__)
+
+// ----------------------------------------------------------------------------
+// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE
+
+// use it in the following way.
+// ```cpp
+// namespace foo
+// {
+// struct Foo
+// {
+//     std::string s;
+//     double      d;
+//     int         i;
+// };
+// } // foo
+//
+// TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(foo::Foo, s, d, i)
+// ```
+// And then you can use `toml::find<foo::Foo>(file, "foo");`
+//
+#define TOML11_FIND_MEMBER_VARIABLE_FROM_VALUE(VAR_NAME)\
+    obj.VAR_NAME = toml::find<decltype(obj.VAR_NAME)>(v, TOML11_STRINGIZE(VAR_NAME));
+
+#define TOML11_ASSIGN_MEMBER_VARIABLE_TO_VALUE(VAR_NAME)\
+    v[TOML11_STRINGIZE(VAR_NAME)] = obj.VAR_NAME;
+
+#define TOML11_DEFINE_CONVERSION_NON_INTRUSIVE(NAME, ...)\
+    namespace toml {                                                                     \
+    template<>                                                                           \
+    struct from<NAME>                                                                    \
+    {                                                                                    \
+        template<typename C, template<typename ...> class T,                             \
+                 template<typename ...> class A>                                         \
+        static NAME from_toml(const basic_value<C, T, A>& v)                             \
+        {                                                                                \
+            NAME obj;                                                                    \
+            TOML11_FOR_EACH_VA_ARGS(TOML11_FIND_MEMBER_VARIABLE_FROM_VALUE, __VA_ARGS__) \
+            return obj;                                                                  \
+        }                                                                                \
+    };                                                                                   \
+    template<>                                                                           \
+    struct into<NAME>                                                                    \
+    {                                                                                    \
+        static value into_toml(const NAME& obj)                                          \
+        {                                                                                \
+            ::toml::value v = ::toml::table{};                                           \
+            TOML11_FOR_EACH_VA_ARGS(TOML11_ASSIGN_MEMBER_VARIABLE_TO_VALUE, __VA_ARGS__) \
+            return v;                                                                    \
+        }                                                                                \
+    };                                                                                   \
+    } /* toml */
+
+#endif// TOML11_WITHOUT_DEFINE_NON_INTRUSIVE
+
+#endif// TOML11_MACROS_HPP
diff --git a/src/toml11/toml/parser.hpp b/src/toml11/toml/parser.hpp
new file mode 100644
index 000000000..e31179918
--- /dev/null
+++ b/src/toml11/toml/parser.hpp
@@ -0,0 +1,2364 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_PARSER_HPP
+#define TOML11_PARSER_HPP
+#include <cstring>
+#include <fstream>
+#include <sstream>
+
+#include "combinator.hpp"
+#include "lexer.hpp"
+#include "region.hpp"
+#include "result.hpp"
+#include "types.hpp"
+#include "value.hpp"
+
+#ifndef TOML11_DISABLE_STD_FILESYSTEM
+#ifdef __cpp_lib_filesystem
+#if __has_include(<filesystem>)
+#define TOML11_HAS_STD_FILESYSTEM
+#include <filesystem>
+#endif // has_include(<string_view>)
+#endif // __cpp_lib_filesystem
+#endif // TOML11_DISABLE_STD_FILESYSTEM
+
+namespace toml
+{
+namespace detail
+{
+
+inline result<std::pair<boolean, region>, std::string>
+parse_boolean(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_boolean::invoke(loc))
+    {
+        const auto reg = token.unwrap();
+        if     (reg.str() == "true")  {return ok(std::make_pair(true,  reg));}
+        else if(reg.str() == "false") {return ok(std::make_pair(false, reg));}
+        else // internal error.
+        {
+            throw internal_error(format_underline(
+                "toml::parse_boolean: internal error",
+                {{source_location(reg), "invalid token"}}),
+                source_location(reg));
+        }
+    }
+    loc.reset(first); //rollback
+    return err(format_underline("toml::parse_boolean: ",
+               {{source_location(loc), "the next token is not a boolean"}}));
+}
+
+inline result<std::pair<integer, region>, std::string>
+parse_binary_integer(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_bin_int::invoke(loc))
+    {
+        auto str = token.unwrap().str();
+        assert(str.size() > 2); // minimum -> 0b1
+        integer retval(0), base(1);
+        for(auto i(str.rbegin()), e(str.rend() - 2); i!=e; ++i)
+        {
+            if     (*i == '1'){retval += base; base *= 2;}
+            else if(*i == '0'){base *= 2;}
+            else if(*i == '_'){/* do nothing. */}
+            else // internal error.
+            {
+                throw internal_error(format_underline(
+                    "toml::parse_integer: internal error",
+                    {{source_location(token.unwrap()), "invalid token"}}),
+                    source_location(loc));
+            }
+        }
+        return ok(std::make_pair(retval, token.unwrap()));
+    }
+    loc.reset(first);
+    return err(format_underline("toml::parse_binary_integer:",
+               {{source_location(loc), "the next token is not an integer"}}));
+}
+
+inline result<std::pair<integer, region>, std::string>
+parse_octal_integer(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_oct_int::invoke(loc))
+    {
+        auto str = token.unwrap().str();
+        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+        str.erase(str.begin()); str.erase(str.begin()); // remove `0o` prefix
+
+        std::istringstream iss(str);
+        integer retval(0);
+        iss >> std::oct >> retval;
+        return ok(std::make_pair(retval, token.unwrap()));
+    }
+    loc.reset(first);
+    return err(format_underline("toml::parse_octal_integer:",
+               {{source_location(loc), "the next token is not an integer"}}));
+}
+
+inline result<std::pair<integer, region>, std::string>
+parse_hexadecimal_integer(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_hex_int::invoke(loc))
+    {
+        auto str = token.unwrap().str();
+        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+        str.erase(str.begin()); str.erase(str.begin()); // remove `0x` prefix
+
+        std::istringstream iss(str);
+        integer retval(0);
+        iss >> std::hex >> retval;
+        return ok(std::make_pair(retval, token.unwrap()));
+    }
+    loc.reset(first);
+    return err(format_underline("toml::parse_hexadecimal_integer",
+               {{source_location(loc), "the next token is not an integer"}}));
+}
+
+inline result<std::pair<integer, region>, std::string>
+parse_integer(location& loc)
+{
+    const auto first = loc.iter();
+    if(first != loc.end() && *first == '0')
+    {
+        const auto second = std::next(first);
+        if(second == loc.end()) // the token is just zero.
+        {
+            loc.advance();
+            return ok(std::make_pair(0, region(loc, first, second)));
+        }
+
+        if(*second == 'b') {return parse_binary_integer     (loc);} // 0b1100
+        if(*second == 'o') {return parse_octal_integer      (loc);} // 0o775
+        if(*second == 'x') {return parse_hexadecimal_integer(loc);} // 0xC0FFEE
+
+        if(std::isdigit(*second))
+        {
+            return err(format_underline("toml::parse_integer: "
+                "leading zero in an Integer is not allowed.",
+                {{source_location(loc), "leading zero"}}));
+        }
+        else if(std::isalpha(*second))
+        {
+             return err(format_underline("toml::parse_integer: "
+                "unknown integer prefix appeared.",
+                {{source_location(loc), "none of 0x, 0o, 0b"}}));
+        }
+    }
+
+    if(const auto token = lex_dec_int::invoke(loc))
+    {
+        auto str = token.unwrap().str();
+        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+
+        std::istringstream iss(str);
+        integer retval(0);
+        iss >> retval;
+        return ok(std::make_pair(retval, token.unwrap()));
+    }
+    loc.reset(first);
+    return err(format_underline("toml::parse_integer: ",
+               {{source_location(loc), "the next token is not an integer"}}));
+}
+
+inline result<std::pair<floating, region>, std::string>
+parse_floating(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_float::invoke(loc))
+    {
+        auto str = token.unwrap().str();
+        if(str == "inf" || str == "+inf")
+        {
+            if(std::numeric_limits<floating>::has_infinity)
+            {
+                return ok(std::make_pair(
+                    std::numeric_limits<floating>::infinity(), token.unwrap()));
+            }
+            else
+            {
+                throw std::domain_error("toml::parse_floating: inf value found"
+                    " but the current environment does not support inf. Please"
+                    " make sure that the floating-point implementation conforms"
+                    " IEEE 754/ISO 60559 international standard.");
+            }
+        }
+        else if(str == "-inf")
+        {
+            if(std::numeric_limits<floating>::has_infinity)
+            {
+                return ok(std::make_pair(
+                    -std::numeric_limits<floating>::infinity(), token.unwrap()));
+            }
+            else
+            {
+                throw std::domain_error("toml::parse_floating: inf value found"
+                    " but the current environment does not support inf. Please"
+                    " make sure that the floating-point implementation conforms"
+                    " IEEE 754/ISO 60559 international standard.");
+            }
+        }
+        else if(str == "nan" || str == "+nan")
+        {
+            if(std::numeric_limits<floating>::has_quiet_NaN)
+            {
+                return ok(std::make_pair(
+                    std::numeric_limits<floating>::quiet_NaN(), token.unwrap()));
+            }
+            else if(std::numeric_limits<floating>::has_signaling_NaN)
+            {
+                return ok(std::make_pair(
+                    std::numeric_limits<floating>::signaling_NaN(), token.unwrap()));
+            }
+            else
+            {
+                throw std::domain_error("toml::parse_floating: NaN value found"
+                    " but the current environment does not support NaN. Please"
+                    " make sure that the floating-point implementation conforms"
+                    " IEEE 754/ISO 60559 international standard.");
+            }
+        }
+        else if(str == "-nan")
+        {
+            if(std::numeric_limits<floating>::has_quiet_NaN)
+            {
+                return ok(std::make_pair(
+                    -std::numeric_limits<floating>::quiet_NaN(), token.unwrap()));
+            }
+            else if(std::numeric_limits<floating>::has_signaling_NaN)
+            {
+                return ok(std::make_pair(
+                    -std::numeric_limits<floating>::signaling_NaN(), token.unwrap()));
+            }
+            else
+            {
+                throw std::domain_error("toml::parse_floating: NaN value found"
+                    " but the current environment does not support NaN. Please"
+                    " make sure that the floating-point implementation conforms"
+                    " IEEE 754/ISO 60559 international standard.");
+            }
+        }
+        str.erase(std::remove(str.begin(), str.end(), '_'), str.end());
+        std::istringstream iss(str);
+        floating v(0.0);
+        iss >> v;
+        return ok(std::make_pair(v, token.unwrap()));
+    }
+    loc.reset(first);
+    return err(format_underline("toml::parse_floating: ",
+               {{source_location(loc), "the next token is not a float"}}));
+}
+
+inline std::string read_utf8_codepoint(const region& reg, const location& loc)
+{
+    const auto str = reg.str().substr(1);
+    std::uint_least32_t codepoint;
+    std::istringstream iss(str);
+    iss >> std::hex >> codepoint;
+
+    const auto to_char = [](const std::uint_least32_t i) noexcept -> char {
+        const auto uc = static_cast<unsigned char>(i);
+        return *reinterpret_cast<const char*>(std::addressof(uc));
+    };
+
+    std::string character;
+    if(codepoint < 0x80) // U+0000 ... U+0079 ; just an ASCII.
+    {
+        character += static_cast<char>(codepoint);
+    }
+    else if(codepoint < 0x800) //U+0080 ... U+07FF
+    {
+        // 110yyyyx 10xxxxxx; 0x3f == 0b0011'1111
+        character += to_char(0xC0| codepoint >> 6);
+        character += to_char(0x80|(codepoint & 0x3F));
+    }
+    else if(codepoint < 0x10000) // U+0800...U+FFFF
+    {
+        if(0xD800 <= codepoint && codepoint <= 0xDFFF)
+        {
+            throw syntax_error(format_underline(
+                "toml::read_utf8_codepoint: codepoints in the range "
+                "[0xD800, 0xDFFF] are not valid UTF-8.", {{
+                    source_location(loc), "not a valid UTF-8 codepoint"
+                }}), source_location(loc));
+        }
+        assert(codepoint < 0xD800 || 0xDFFF < codepoint);
+        // 1110yyyy 10yxxxxx 10xxxxxx
+        character += to_char(0xE0| codepoint >> 12);
+        character += to_char(0x80|(codepoint >> 6 & 0x3F));
+        character += to_char(0x80|(codepoint      & 0x3F));
+    }
+    else if(codepoint < 0x110000) // U+010000 ... U+10FFFF
+    {
+        // 11110yyy 10yyxxxx 10xxxxxx 10xxxxxx
+        character += to_char(0xF0| codepoint >> 18);
+        character += to_char(0x80|(codepoint >> 12 & 0x3F));
+        character += to_char(0x80|(codepoint >> 6  & 0x3F));
+        character += to_char(0x80|(codepoint       & 0x3F));
+    }
+    else // out of UTF-8 region
+    {
+        throw syntax_error(format_underline("toml::read_utf8_codepoint:"
+            " input codepoint is too large.",
+            {{source_location(loc), "should be in [0x00..0x10FFFF]"}}),
+            source_location(loc));
+    }
+    return character;
+}
+
+inline result<std::string, std::string> parse_escape_sequence(location& loc)
+{
+    const auto first = loc.iter();
+    if(first == loc.end() || *first != '\\')
+    {
+        return err(format_underline("toml::parse_escape_sequence: ", {{
+            source_location(loc), "the next token is not a backslash \"\\\""}}));
+    }
+    loc.advance();
+    switch(*loc.iter())
+    {
+        case '\\':{loc.advance(); return ok(std::string("\\"));}
+        case '"' :{loc.advance(); return ok(std::string("\""));}
+        case 'b' :{loc.advance(); return ok(std::string("\b"));}
+        case 't' :{loc.advance(); return ok(std::string("\t"));}
+        case 'n' :{loc.advance(); return ok(std::string("\n"));}
+        case 'f' :{loc.advance(); return ok(std::string("\f"));}
+        case 'r' :{loc.advance(); return ok(std::string("\r"));}
+        case 'u' :
+        {
+            if(const auto token = lex_escape_unicode_short::invoke(loc))
+            {
+                return ok(read_utf8_codepoint(token.unwrap(), loc));
+            }
+            else
+            {
+                return err(format_underline("parse_escape_sequence: "
+                           "invalid token found in UTF-8 codepoint uXXXX.",
+                           {{source_location(loc), "here"}}));
+            }
+        }
+        case 'U':
+        {
+            if(const auto token = lex_escape_unicode_long::invoke(loc))
+            {
+                return ok(read_utf8_codepoint(token.unwrap(), loc));
+            }
+            else
+            {
+                return err(format_underline("parse_escape_sequence: "
+                           "invalid token found in UTF-8 codepoint Uxxxxxxxx",
+                           {{source_location(loc), "here"}}));
+            }
+        }
+    }
+
+    const auto msg = format_underline("parse_escape_sequence: "
+           "unknown escape sequence appeared.", {{source_location(loc),
+           "escape sequence is one of \\, \", b, t, n, f, r, uxxxx, Uxxxxxxxx"}},
+           /* Hints = */{"if you want to write backslash as just one backslash, "
+           "use literal string like: regex    = '<\\i\\c*\\s*>'"});
+    loc.reset(first);
+    return err(msg);
+}
+
+inline std::ptrdiff_t check_utf8_validity(const std::string& reg)
+{
+    location loc("tmp", reg);
+    const auto u8 = repeat<lex_utf8_code, unlimited>::invoke(loc);
+    if(!u8 || loc.iter() != loc.end())
+    {
+        const auto error_location = std::distance(loc.begin(), loc.iter());
+        assert(0 <= error_location);
+        return error_location;
+    }
+    return -1;
+}
+
+inline result<std::pair<toml::string, region>, std::string>
+parse_ml_basic_string(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_ml_basic_string::invoke(loc))
+    {
+        auto inner_loc = loc;
+        inner_loc.reset(first);
+
+        std::string retval;
+        retval.reserve(token.unwrap().size());
+
+        auto delim = lex_ml_basic_string_open::invoke(inner_loc);
+        if(!delim)
+        {
+            throw internal_error(format_underline(
+                "parse_ml_basic_string: invalid token",
+                {{source_location(inner_loc), "should be \"\"\""}}),
+                source_location(inner_loc));
+        }
+        // immediate newline is ignored (if exists)
+        /* discard return value */ lex_newline::invoke(inner_loc);
+
+        delim = none();
+        while(!delim)
+        {
+            using lex_unescaped_seq = repeat<
+                either<lex_ml_basic_unescaped, lex_newline>, unlimited>;
+            if(auto unescaped = lex_unescaped_seq::invoke(inner_loc))
+            {
+                retval += unescaped.unwrap().str();
+            }
+            if(auto escaped = parse_escape_sequence(inner_loc))
+            {
+                retval += escaped.unwrap();
+            }
+            if(auto esc_nl = lex_ml_basic_escaped_newline::invoke(inner_loc))
+            {
+                // ignore newline after escape until next non-ws char
+            }
+            if(inner_loc.iter() == inner_loc.end())
+            {
+                throw internal_error(format_underline(
+                    "parse_ml_basic_string: unexpected end of region",
+                    {{source_location(inner_loc), "not sufficient token"}}),
+                    source_location(inner_loc));
+            }
+            delim = lex_ml_basic_string_close::invoke(inner_loc);
+        }
+        // `lex_ml_basic_string_close` allows 3 to 5 `"`s to allow 1 or 2 `"`s
+        // at just before the delimiter. Here, we need to attach `"`s at the
+        // end of the string body, if it exists.
+        // For detail, see the definition of `lex_ml_basic_string_close`.
+        assert(std::all_of(delim.unwrap().first(), delim.unwrap().last(),
+                           [](const char c) noexcept {return c == '\"';}));
+        switch(delim.unwrap().size())
+        {
+            case 3: {break;}
+            case 4: {retval += "\"";  break;}
+            case 5: {retval += "\"\""; break;}
+            default:
+            {
+                throw internal_error(format_underline(
+                    "parse_ml_basic_string: closing delimiter has invalid length",
+                    {{source_location(inner_loc), "end of this"}}),
+                    source_location(inner_loc));
+            }
+        }
+
+        const auto err_loc = check_utf8_validity(token.unwrap().str());
+        if(err_loc == -1)
+        {
+            return ok(std::make_pair(toml::string(retval), token.unwrap()));
+        }
+        else
+        {
+            inner_loc.reset(first);
+            inner_loc.advance(err_loc);
+            throw syntax_error(format_underline(
+                "parse_ml_basic_string: invalid utf8 sequence found",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+    }
+    else
+    {
+        loc.reset(first);
+        return err(format_underline("toml::parse_ml_basic_string: "
+                   "the next token is not a valid multiline string",
+                   {{source_location(loc), "here"}}));
+    }
+}
+
+inline result<std::pair<toml::string, region>, std::string>
+parse_basic_string(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_basic_string::invoke(loc))
+    {
+        auto inner_loc = loc;
+        inner_loc.reset(first);
+
+        auto quot = lex_quotation_mark::invoke(inner_loc);
+        if(!quot)
+        {
+            throw internal_error(format_underline("parse_basic_string: "
+                "invalid token", {{source_location(inner_loc), "should be \""}}),
+                source_location(inner_loc));
+        }
+
+        std::string retval;
+        retval.reserve(token.unwrap().size());
+
+        quot = none();
+        while(!quot)
+        {
+            using lex_unescaped_seq = repeat<lex_basic_unescaped, unlimited>;
+            if(auto unescaped = lex_unescaped_seq::invoke(inner_loc))
+            {
+                retval += unescaped.unwrap().str();
+            }
+            if(auto escaped = parse_escape_sequence(inner_loc))
+            {
+                retval += escaped.unwrap();
+            }
+            if(inner_loc.iter() == inner_loc.end())
+            {
+                throw internal_error(format_underline(
+                    "parse_basic_string: unexpected end of region",
+                    {{source_location(inner_loc), "not sufficient token"}}),
+                    source_location(inner_loc));
+            }
+            quot = lex_quotation_mark::invoke(inner_loc);
+        }
+
+        const auto err_loc = check_utf8_validity(token.unwrap().str());
+        if(err_loc == -1)
+        {
+            return ok(std::make_pair(toml::string(retval), token.unwrap()));
+        }
+        else
+        {
+            inner_loc.reset(first);
+            inner_loc.advance(err_loc);
+            throw syntax_error(format_underline(
+                "parse_ml_basic_string: invalid utf8 sequence found",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+    }
+    else
+    {
+        loc.reset(first); // rollback
+        return err(format_underline("toml::parse_basic_string: "
+                   "the next token is not a valid string",
+                   {{source_location(loc), "here"}}));
+    }
+}
+
+inline result<std::pair<toml::string, region>, std::string>
+parse_ml_literal_string(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_ml_literal_string::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto open = lex_ml_literal_string_open::invoke(inner_loc);
+        if(!open)
+        {
+            throw internal_error(format_underline(
+                "parse_ml_literal_string: invalid token",
+                {{source_location(inner_loc), "should be '''"}}),
+                source_location(inner_loc));
+        }
+        // immediate newline is ignored (if exists)
+        /* discard return value */ lex_newline::invoke(inner_loc);
+
+        const auto body = lex_ml_literal_body::invoke(inner_loc);
+
+        const auto close = lex_ml_literal_string_close::invoke(inner_loc);
+        if(!close)
+        {
+            throw internal_error(format_underline(
+                "parse_ml_literal_string: invalid token",
+                {{source_location(inner_loc), "should be '''"}}),
+                source_location(inner_loc));
+        }
+        // `lex_ml_literal_string_close` allows 3 to 5 `'`s to allow 1 or 2 `'`s
+        // at just before the delimiter. Here, we need to attach `'`s at the
+        // end of the string body, if it exists.
+        // For detail, see the definition of `lex_ml_basic_string_close`.
+
+        std::string retval = body.unwrap().str();
+        assert(std::all_of(close.unwrap().first(), close.unwrap().last(),
+                           [](const char c) noexcept {return c == '\'';}));
+        switch(close.unwrap().size())
+        {
+            case 3: {break;}
+            case 4: {retval += "'";  break;}
+            case 5: {retval += "''"; break;}
+            default:
+            {
+                throw internal_error(format_underline(
+                    "parse_ml_literal_string: closing delimiter has invalid length",
+                    {{source_location(inner_loc), "end of this"}}),
+                    source_location(inner_loc));
+            }
+        }
+
+        const auto err_loc = check_utf8_validity(token.unwrap().str());
+        if(err_loc == -1)
+        {
+            return ok(std::make_pair(toml::string(retval, toml::string_t::literal),
+                                     token.unwrap()));
+        }
+        else
+        {
+            inner_loc.reset(first);
+            inner_loc.advance(err_loc);
+            throw syntax_error(format_underline(
+                "parse_ml_basic_string: invalid utf8 sequence found",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+    }
+    else
+    {
+        loc.reset(first); // rollback
+        return err(format_underline("toml::parse_ml_literal_string: "
+                   "the next token is not a valid multiline literal string",
+                   {{source_location(loc), "here"}}));
+    }
+}
+
+inline result<std::pair<toml::string, region>, std::string>
+parse_literal_string(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_literal_string::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto open = lex_apostrophe::invoke(inner_loc);
+        if(!open)
+        {
+            throw internal_error(format_underline(
+                "parse_literal_string: invalid token",
+                {{source_location(inner_loc), "should be '"}}),
+                source_location(inner_loc));
+        }
+
+        const auto body = repeat<lex_literal_char, unlimited>::invoke(inner_loc);
+
+        const auto close = lex_apostrophe::invoke(inner_loc);
+        if(!close)
+        {
+            throw internal_error(format_underline(
+                "parse_literal_string: invalid token",
+                {{source_location(inner_loc), "should be '"}}),
+                source_location(inner_loc));
+        }
+
+        const auto err_loc = check_utf8_validity(token.unwrap().str());
+        if(err_loc == -1)
+        {
+            return ok(std::make_pair(
+                      toml::string(body.unwrap().str(), toml::string_t::literal),
+                      token.unwrap()));
+        }
+        else
+        {
+            inner_loc.reset(first);
+            inner_loc.advance(err_loc);
+            throw syntax_error(format_underline(
+                "parse_ml_basic_string: invalid utf8 sequence found",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+    }
+    else
+    {
+        loc.reset(first); // rollback
+        return err(format_underline("toml::parse_literal_string: "
+                   "the next token is not a valid literal string",
+                   {{source_location(loc), "here"}}));
+    }
+}
+
+inline result<std::pair<toml::string, region>, std::string>
+parse_string(location& loc)
+{
+    if(loc.iter() != loc.end() && *(loc.iter()) == '"')
+    {
+        if(loc.iter() + 1 != loc.end() && *(loc.iter() + 1) == '"' &&
+           loc.iter() + 2 != loc.end() && *(loc.iter() + 2) == '"')
+        {
+            return parse_ml_basic_string(loc);
+        }
+        else
+        {
+            return parse_basic_string(loc);
+        }
+    }
+    else if(loc.iter() != loc.end() && *(loc.iter()) == '\'')
+    {
+        if(loc.iter() + 1 != loc.end() && *(loc.iter() + 1) == '\'' &&
+           loc.iter() + 2 != loc.end() && *(loc.iter() + 2) == '\'')
+        {
+            return parse_ml_literal_string(loc);
+        }
+        else
+        {
+            return parse_literal_string(loc);
+        }
+    }
+    return err(format_underline("toml::parse_string: ",
+                {{source_location(loc), "the next token is not a string"}}));
+}
+
+inline result<std::pair<local_date, region>, std::string>
+parse_local_date(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_local_date::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto y = lex_date_fullyear::invoke(inner_loc);
+        if(!y || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_inner_local_date: invalid year format",
+                {{source_location(inner_loc), "should be `-`"}}),
+                source_location(inner_loc));
+        }
+        inner_loc.advance();
+        const auto m = lex_date_month::invoke(inner_loc);
+        if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != '-')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_date: invalid month format",
+                {{source_location(inner_loc), "should be `-`"}}),
+                source_location(inner_loc));
+        }
+        inner_loc.advance();
+        const auto d = lex_date_mday::invoke(inner_loc);
+        if(!d)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_date: invalid day format",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+
+        const auto year  = static_cast<std::int16_t>(from_string<int>(y.unwrap().str(), 0));
+        const auto month = static_cast<std::int8_t >(from_string<int>(m.unwrap().str(), 0));
+        const auto day   = static_cast<std::int8_t >(from_string<int>(d.unwrap().str(), 0));
+
+        // We briefly check whether the input date is valid or not. But here, we
+        // only check if the RFC3339 compliance.
+        //     Actually there are several special date that does not exist,
+        // because of historical reasons, such as 1582/10/5-1582/10/14 (only in
+        // several countries). But here, we do not care about such a complicated
+        // rule. It makes the code complicated and there is only low probability
+        // that such a specific date is needed in practice. If someone need to
+        // validate date accurately, that means that the one need a specialized
+        // library for their purpose in a different layer.
+        {
+            const bool is_leap = (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0));
+            const auto max_day = (month == 2) ? (is_leap ? 29 : 28) :
+                ((month == 4 || month == 6 || month == 9 || month == 11) ? 30 : 31);
+
+            if((month < 1 || 12 < month) || (day < 1 || max_day < day))
+            {
+                throw syntax_error(format_underline("toml::parse_date: "
+                    "invalid date: it does not conform RFC3339.", {{
+                    source_location(loc), "month should be 01-12, day should be"
+                    " 01-28,29,30,31, depending on month/year."
+                    }}), source_location(inner_loc));
+            }
+        }
+        return ok(std::make_pair(local_date(year, static_cast<month_t>(month - 1), day),
+                                 token.unwrap()));
+    }
+    else
+    {
+        loc.reset(first);
+        return err(format_underline("toml::parse_local_date: ",
+            {{source_location(loc), "the next token is not a local_date"}}));
+    }
+}
+
+inline result<std::pair<local_time, region>, std::string>
+parse_local_time(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_local_time::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto h = lex_time_hour::invoke(inner_loc);
+        if(!h || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_time: invalid year format",
+                {{source_location(inner_loc), "should be `:`"}}),
+                source_location(inner_loc));
+        }
+        inner_loc.advance();
+        const auto m = lex_time_minute::invoke(inner_loc);
+        if(!m || inner_loc.iter() == inner_loc.end() || *inner_loc.iter() != ':')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_time: invalid month format",
+                {{source_location(inner_loc), "should be `:`"}}),
+                source_location(inner_loc));
+        }
+        inner_loc.advance();
+        const auto s = lex_time_second::invoke(inner_loc);
+        if(!s)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_time: invalid second format",
+                {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+        }
+
+        const int hour   = from_string<int>(h.unwrap().str(), 0);
+        const int minute = from_string<int>(m.unwrap().str(), 0);
+        const int second = from_string<int>(s.unwrap().str(), 0);
+
+        if((hour   < 0 || 23 < hour) || (minute < 0 || 59 < minute) ||
+           (second < 0 || 60 < second)) // it may be leap second
+        {
+            throw syntax_error(format_underline("toml::parse_time: "
+                "invalid time: it does not conform RFC3339.", {{
+                source_location(loc), "hour should be 00-23, minute should be"
+                " 00-59, second should be 00-60 (depending on the leap"
+                " second rules.)"}}), source_location(inner_loc));
+        }
+
+        local_time time(hour, minute, second, 0, 0);
+
+        const auto before_secfrac = inner_loc.iter();
+        if(const auto secfrac = lex_time_secfrac::invoke(inner_loc))
+        {
+            auto sf = secfrac.unwrap().str();
+            sf.erase(sf.begin()); // sf.front() == '.'
+            switch(sf.size() % 3)
+            {
+                case 2:  sf += '0';  break;
+                case 1:  sf += "00"; break;
+                case 0:  break;
+                default: break;
+            }
+            if(sf.size() >= 9)
+            {
+                time.millisecond = from_string<std::uint16_t>(sf.substr(0, 3), 0u);
+                time.microsecond = from_string<std::uint16_t>(sf.substr(3, 3), 0u);
+                time.nanosecond  = from_string<std::uint16_t>(sf.substr(6, 3), 0u);
+            }
+            else if(sf.size() >= 6)
+            {
+                time.millisecond = from_string<std::uint16_t>(sf.substr(0, 3), 0u);
+                time.microsecond = from_string<std::uint16_t>(sf.substr(3, 3), 0u);
+            }
+            else if(sf.size() >= 3)
+            {
+                time.millisecond = from_string<std::uint16_t>(sf, 0u);
+                time.microsecond = 0u;
+            }
+        }
+        else
+        {
+            if(before_secfrac != inner_loc.iter())
+            {
+                throw internal_error(format_underline(
+                    "toml::parse_local_time: invalid subsecond format",
+                    {{source_location(inner_loc), "here"}}),
+                source_location(inner_loc));
+            }
+        }
+        return ok(std::make_pair(time, token.unwrap()));
+    }
+    else
+    {
+        loc.reset(first);
+        return err(format_underline("toml::parse_local_time: ",
+            {{source_location(loc), "the next token is not a local_time"}}));
+    }
+}
+
+inline result<std::pair<local_datetime, region>, std::string>
+parse_local_datetime(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_local_date_time::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+        const auto date = parse_local_date(inner_loc);
+        if(!date || inner_loc.iter() == inner_loc.end())
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_datetime: invalid datetime format",
+                {{source_location(inner_loc), "date, not datetime"}}),
+                source_location(inner_loc));
+        }
+        const char delim = *(inner_loc.iter());
+        if(delim != 'T' && delim != 't' && delim != ' ')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_datetime: invalid datetime format",
+                {{source_location(inner_loc), "should be `T` or ` ` (space)"}}),
+                source_location(inner_loc));
+        }
+        inner_loc.advance();
+        const auto time = parse_local_time(inner_loc);
+        if(!time)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_local_datetime: invalid datetime format",
+                {{source_location(inner_loc), "invalid time format"}}),
+                source_location(inner_loc));
+        }
+        return ok(std::make_pair(
+            local_datetime(date.unwrap().first, time.unwrap().first),
+            token.unwrap()));
+    }
+    else
+    {
+        loc.reset(first);
+        return err(format_underline("toml::parse_local_datetime: ",
+            {{source_location(loc), "the next token is not a local_datetime"}}));
+    }
+}
+
+inline result<std::pair<offset_datetime, region>, std::string>
+parse_offset_datetime(location& loc)
+{
+    const auto first = loc.iter();
+    if(const auto token = lex_offset_date_time::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+        const auto datetime = parse_local_datetime(inner_loc);
+        if(!datetime || inner_loc.iter() == inner_loc.end())
+        {
+            throw internal_error(format_underline(
+                "toml::parse_offset_datetime: invalid datetime format",
+                {{source_location(inner_loc), "date, not datetime"}}),
+                source_location(inner_loc));
+        }
+        time_offset offset(0, 0);
+        if(const auto ofs = lex_time_numoffset::invoke(inner_loc))
+        {
+            const auto str = ofs.unwrap().str();
+
+            const auto hour   = from_string<int>(str.substr(1,2), 0);
+            const auto minute = from_string<int>(str.substr(4,2), 0);
+
+            if((hour < 0 || 23 < hour) || (minute < 0 || 59 < minute))
+            {
+                throw syntax_error(format_underline("toml::parse_offset_datetime: "
+                    "invalid offset: it does not conform RFC3339.", {{
+                    source_location(loc), "month should be 01-12, day should be"
+                    " 01-28,29,30,31, depending on month/year."
+                    }}), source_location(inner_loc));
+            }
+
+            if(str.front() == '+')
+            {
+                offset = time_offset(hour, minute);
+            }
+            else
+            {
+                offset = time_offset(-hour, -minute);
+            }
+        }
+        else if(*inner_loc.iter() != 'Z' && *inner_loc.iter() != 'z')
+        {
+            throw internal_error(format_underline(
+                "toml::parse_offset_datetime: invalid datetime format",
+                {{source_location(inner_loc), "should be `Z` or `+HH:MM`"}}),
+                source_location(inner_loc));
+        }
+        return ok(std::make_pair(offset_datetime(datetime.unwrap().first, offset),
+                                 token.unwrap()));
+    }
+    else
+    {
+        loc.reset(first);
+        return err(format_underline("toml::parse_offset_datetime: ",
+            {{source_location(loc), "the next token is not a offset_datetime"}}));
+    }
+}
+
+inline result<std::pair<key, region>, std::string>
+parse_simple_key(location& loc)
+{
+    if(const auto bstr = parse_basic_string(loc))
+    {
+        return ok(std::make_pair(bstr.unwrap().first.str, bstr.unwrap().second));
+    }
+    if(const auto lstr = parse_literal_string(loc))
+    {
+        return ok(std::make_pair(lstr.unwrap().first.str, lstr.unwrap().second));
+    }
+    if(const auto bare = lex_unquoted_key::invoke(loc))
+    {
+        const auto reg = bare.unwrap();
+        return ok(std::make_pair(reg.str(), reg));
+    }
+    return err(format_underline("toml::parse_simple_key: ",
+            {{source_location(loc), "the next token is not a simple key"}}));
+}
+
+// dotted key become vector of keys
+inline result<std::pair<std::vector<key>, region>, std::string>
+parse_key(location& loc)
+{
+    const auto first = loc.iter();
+    // dotted key -> `foo.bar.baz` where several single keys are chained by
+    // dots. Whitespaces between keys and dots are allowed.
+    if(const auto token = lex_dotted_key::invoke(loc))
+    {
+        const auto reg = token.unwrap();
+        location inner_loc(loc.name(), reg.str());
+        std::vector<key> keys;
+
+        while(inner_loc.iter() != inner_loc.end())
+        {
+            lex_ws::invoke(inner_loc);
+            if(const auto k = parse_simple_key(inner_loc))
+            {
+                keys.push_back(k.unwrap().first);
+            }
+            else
+            {
+                throw internal_error(format_underline(
+                    "toml::detail::parse_key: dotted key contains invalid key",
+                    {{source_location(inner_loc), k.unwrap_err()}}),
+                    source_location(inner_loc));
+            }
+
+            lex_ws::invoke(inner_loc);
+            if(inner_loc.iter() == inner_loc.end())
+            {
+                break;
+            }
+            else if(*inner_loc.iter() == '.')
+            {
+                inner_loc.advance(); // to skip `.`
+            }
+            else
+            {
+                throw internal_error(format_underline("toml::parse_key: "
+                    "dotted key contains invalid key ",
+                    {{source_location(inner_loc), "should be `.`"}}),
+                    source_location(inner_loc));
+            }
+        }
+        return ok(std::make_pair(keys, reg));
+    }
+    loc.reset(first);
+
+    // simple_key: a single (basic_string|literal_string|bare key)
+    if(const auto smpl = parse_simple_key(loc))
+    {
+        return ok(std::make_pair(std::vector<key>(1, smpl.unwrap().first),
+                                 smpl.unwrap().second));
+    }
+    return err(format_underline("toml::parse_key: an invalid key appeared.",
+                {{source_location(loc), "is not a valid key"}}, {
+                "bare keys  : non-empty strings composed only of [A-Za-z0-9_-].",
+                "quoted keys: same as \"basic strings\" or 'literal strings'.",
+                "dotted keys: sequence of bare or quoted keys joined with a dot."
+                }));
+}
+
+// forward-decl to implement parse_array and parse_table
+template<typename Value>
+result<Value, std::string> parse_value(location&);
+
+template<typename Value>
+result<std::pair<typename Value::array_type, region>, std::string>
+parse_array(location& loc)
+{
+    using value_type = Value;
+    using array_type = typename value_type::array_type;
+
+    const auto first = loc.iter();
+    if(loc.iter() == loc.end())
+    {
+        return err("toml::parse_array: input is empty");
+    }
+    if(*loc.iter() != '[')
+    {
+        return err("toml::parse_array: token is not an array");
+    }
+    loc.advance();
+
+    using lex_ws_comment_newline = repeat<
+        either<lex_wschar, lex_newline, lex_comment>, unlimited>;
+
+    array_type retval;
+    while(loc.iter() != loc.end())
+    {
+        lex_ws_comment_newline::invoke(loc); // skip
+
+        if(loc.iter() != loc.end() && *loc.iter() == ']')
+        {
+            loc.advance(); // skip ']'
+            return ok(std::make_pair(retval,
+                      region(loc, first, loc.iter())));
+        }
+
+        if(auto val = parse_value<value_type>(loc))
+        {
+            // After TOML v1.0.0-rc.1, array becomes to be able to have values
+            // with different types. So here we will omit this by default.
+            //
+            // But some of the test-suite checks if the parser accepts a hetero-
+            // geneous arrays, so we keep this for a while.
+#ifdef TOML11_DISALLOW_HETEROGENEOUS_ARRAYS
+            if(!retval.empty() && retval.front().type() != val.as_ok().type())
+            {
+                auto array_start_loc = loc;
+                array_start_loc.reset(first);
+
+                throw syntax_error(format_underline("toml::parse_array: "
+                    "type of elements should be the same each other.", {
+                        {source_location(array_start_loc), "array starts here"},
+                        {
+                            retval.front().location(),
+                            "value has type " + stringize(retval.front().type())
+                        },
+                        {
+                            val.unwrap().location(),
+                            "value has different type, " + stringize(val.unwrap().type())
+                        }
+                    }), source_location(loc));
+            }
+#endif
+            retval.push_back(std::move(val.unwrap()));
+        }
+        else
+        {
+            auto array_start_loc = loc;
+            array_start_loc.reset(first);
+
+            throw syntax_error(format_underline("toml::parse_array: "
+                "value having invalid format appeared in an array", {
+                    {source_location(array_start_loc), "array starts here"},
+                    {source_location(loc), "it is not a valid value."}
+                }), source_location(loc));
+        }
+
+        using lex_array_separator = sequence<maybe<lex_ws_comment_newline>, character<','>>;
+        const auto sp = lex_array_separator::invoke(loc);
+        if(!sp)
+        {
+            lex_ws_comment_newline::invoke(loc);
+            if(loc.iter() != loc.end() && *loc.iter() == ']')
+            {
+                loc.advance(); // skip ']'
+                return ok(std::make_pair(retval,
+                          region(loc, first, loc.iter())));
+            }
+            else
+            {
+                auto array_start_loc = loc;
+                array_start_loc.reset(first);
+
+                throw syntax_error(format_underline("toml::parse_array:"
+                    " missing array separator `,` after a value", {
+                        {source_location(array_start_loc), "array starts here"},
+                        {source_location(loc),             "should be `,`"}
+                    }), source_location(loc));
+            }
+        }
+    }
+    loc.reset(first);
+    throw syntax_error(format_underline("toml::parse_array: "
+            "array did not closed by `]`",
+            {{source_location(loc), "should be closed"}}),
+            source_location(loc));
+}
+
+template<typename Value>
+result<std::pair<std::pair<std::vector<key>, region>, Value>, std::string>
+parse_key_value_pair(location& loc)
+{
+    using value_type = Value;
+
+    const auto first = loc.iter();
+    auto key_reg = parse_key(loc);
+    if(!key_reg)
+    {
+        std::string msg = std::move(key_reg.unwrap_err());
+        // if the next token is keyvalue-separator, it means that there are no
+        // key. then we need to show error as "empty key is not allowed".
+        if(const auto keyval_sep = lex_keyval_sep::invoke(loc))
+        {
+            loc.reset(first);
+            msg = format_underline("toml::parse_key_value_pair: "
+                "empty key is not allowed.",
+                {{source_location(loc), "key expected before '='"}});
+        }
+        return err(std::move(msg));
+    }
+
+    const auto kvsp = lex_keyval_sep::invoke(loc);
+    if(!kvsp)
+    {
+        std::string msg;
+        // if the line contains '=' after the invalid sequence, possibly the
+        // error is in the key (like, invalid character in bare key).
+        const auto line_end = std::find(loc.iter(), loc.end(), '\n');
+        if(std::find(loc.iter(), line_end, '=') != line_end)
+        {
+            msg = format_underline("toml::parse_key_value_pair: "
+                "invalid format for key",
+                {{source_location(loc), "invalid character in key"}},
+                {"Did you forget '.' to separate dotted-key?",
+                "Allowed characters for bare key are [0-9a-zA-Z_-]."});
+        }
+        else // if not, the error is lack of key-value separator.
+        {
+            msg = format_underline("toml::parse_key_value_pair: "
+                "missing key-value separator `=`",
+                {{source_location(loc), "should be `=`"}});
+        }
+        loc.reset(first);
+        return err(std::move(msg));
+    }
+
+    const auto after_kvsp = loc.iter(); // err msg
+    auto val = parse_value<value_type>(loc);
+    if(!val)
+    {
+        std::string msg;
+        loc.reset(after_kvsp);
+        // check there is something not a comment/whitespace after `=`
+        if(sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>::invoke(loc))
+        {
+            loc.reset(after_kvsp);
+            msg = format_underline("toml::parse_key_value_pair: "
+                    "missing value after key-value separator '='",
+                    {{source_location(loc), "expected value, but got nothing"}});
+        }
+        else // there is something not a comment/whitespace, so invalid format.
+        {
+            msg = std::move(val.unwrap_err());
+        }
+        loc.reset(first);
+        return err(msg);
+    }
+    return ok(std::make_pair(std::move(key_reg.unwrap()),
+                             std::move(val.unwrap())));
+}
+
+// for error messages.
+template<typename InputIterator>
+std::string format_dotted_keys(InputIterator first, const InputIterator last)
+{
+    static_assert(std::is_same<key,
+        typename std::iterator_traits<InputIterator>::value_type>::value,"");
+
+    std::string retval(*first++);
+    for(; first != last; ++first)
+    {
+        retval += '.';
+        retval += *first;
+    }
+    return retval;
+}
+
+// forward decl for is_valid_forward_table_definition
+result<std::pair<std::vector<key>, region>, std::string>
+parse_table_key(location& loc);
+template<typename Value>
+result<std::pair<typename Value::table_type, region>, std::string>
+parse_inline_table(location& loc);
+
+// The following toml file is allowed.
+// ```toml
+// [a.b.c]     # here, table `a` has element `b`.
+// foo = "bar"
+// [a]         # merge a = {baz = "qux"} to a = {b = {...}}
+// baz = "qux"
+// ```
+// But the following is not allowed.
+// ```toml
+// [a]
+// b.c.foo = "bar"
+// [a]             # error! the same table [a] defined!
+// baz = "qux"
+// ```
+// The following is neither allowed.
+// ```toml
+// a = { b.c.foo = "bar"}
+// [a]             # error! the same table [a] defined!
+// baz = "qux"
+// ```
+// Here, it parses region of `tab->at(k)` as a table key and check the depth
+// of the key. If the key region points deeper node, it would be allowed.
+// Otherwise, the key points the same node. It would be rejected.
+template<typename Value, typename Iterator>
+bool is_valid_forward_table_definition(const Value& fwd, const Value& inserting,
+        Iterator key_first, Iterator key_curr, Iterator key_last)
+{
+    // ------------------------------------------------------------------------
+    // check type of the value to be inserted/merged
+
+    std::string inserting_reg = "";
+    if(const auto ptr = detail::get_region(inserting))
+    {
+        inserting_reg = ptr->str();
+    }
+    location inserting_def("internal", std::move(inserting_reg));
+    if(const auto inlinetable = parse_inline_table<Value>(inserting_def))
+    {
+        // check if we are overwriting existing table.
+        // ```toml
+        // # NG
+        // a.b = 42
+        // a = {d = 3.14}
+        // ```
+        // Inserting an inline table to a existing super-table is not allowed in
+        // any case. If we found it, we can reject it without further checking.
+        return false;
+    }
+
+    // ------------------------------------------------------------------------
+    // check table defined before
+
+    std::string internal = "";
+    if(const auto ptr = detail::get_region(fwd))
+    {
+        internal = ptr->str();
+    }
+    location def("internal", std::move(internal));
+    if(const auto tabkeys = parse_table_key(def)) // [table.key]
+    {
+        // table keys always contains all the nodes from the root.
+        const auto& tks = tabkeys.unwrap().first;
+        if(std::size_t(std::distance(key_first, key_last)) == tks.size() &&
+           std::equal(tks.begin(), tks.end(), key_first))
+        {
+            // the keys are equivalent. it is not allowed.
+            return false;
+        }
+        // the keys are not equivalent. it is allowed.
+        return true;
+    }
+    if(const auto dotkeys = parse_key(def))
+    {
+        // consider the following case.
+        // [a]
+        // b.c = {d = 42}
+        // [a.b.c]
+        // e = 2.71
+        // this defines the table [a.b.c] twice. no?
+
+        // a dotted key starts from the node representing a table in which the
+        // dotted key belongs to.
+        const auto& dks = dotkeys.unwrap().first;
+        if(std::size_t(std::distance(key_curr, key_last)) == dks.size() &&
+           std::equal(dks.begin(), dks.end(), key_curr))
+        {
+            // the keys are equivalent. it is not allowed.
+            return false;
+        }
+        // the keys are not equivalent. it is allowed.
+        return true;
+    }
+    return false;
+}
+
+template<typename Value, typename InputIterator>
+result<bool, std::string>
+insert_nested_key(typename Value::table_type& root, const Value& v,
+                  InputIterator iter, const InputIterator last,
+                  region key_reg,
+                  const bool is_array_of_table = false)
+{
+    static_assert(std::is_same<key,
+        typename std::iterator_traits<InputIterator>::value_type>::value,"");
+
+    using value_type = Value;
+    using table_type = typename value_type::table_type;
+    using array_type = typename value_type::array_type;
+
+    const auto first = iter;
+    assert(iter != last);
+
+    table_type* tab = std::addressof(root);
+    for(; iter != last; ++iter) // search recursively
+    {
+        const key& k = *iter;
+        if(std::next(iter) == last) // k is the last key
+        {
+            // XXX if the value is array-of-tables, there can be several
+            //     tables that are in the same array. in that case, we need to
+            //     find the last element and insert it to there.
+            if(is_array_of_table)
+            {
+                if(tab->count(k) == 1) // there is already an array of table
+                {
+                    if(tab->at(k).is_table())
+                    {
+                        // show special err msg for conflicting table
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: array of table (\"",
+                            format_dotted_keys(first, last),
+                            "\") cannot be defined"), {
+                                {tab->at(k).location(), "table already defined"},
+                                {v.location(), "this conflicts with the previous table"}
+                            }), v.location());
+                    }
+                    else if(!(tab->at(k).is_array()))
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: array of table (\"",
+                            format_dotted_keys(first, last), "\") collides with"
+                            " existing value"), {
+                                {tab->at(k).location(),
+                                 concat_to_string("this ", tab->at(k).type(),
+                                                  " value already exists")},
+                                {v.location(),
+                                 "while inserting this array-of-tables"}
+                            }), v.location());
+                    }
+                    // the above if-else-if checks tab->at(k) is an array
+                    auto& a = tab->at(k).as_array();
+                    // If table element is defined as [[array_of_tables]], it
+                    // cannot be an empty array. If an array of tables is
+                    // defined as `aot = []`, it cannot be appended.
+                    if(a.empty() || !(a.front().is_table()))
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: array of table (\"",
+                            format_dotted_keys(first, last), "\") collides with"
+                            " existing value"), {
+                                {tab->at(k).location(),
+                                 concat_to_string("this ", tab->at(k).type(),
+                                                  " value already exists")},
+                                {v.location(),
+                                 "while inserting this array-of-tables"}
+                            }), v.location());
+                    }
+                    // avoid conflicting array of table like the following.
+                    // ```toml
+                    // a = [{b = 42}] # define a as an array of *inline* tables
+                    // [[a]]          # a is an array of *multi-line* tables
+                    // b = 54
+                    // ```
+                    // Here, from the type information, these cannot be detected
+                    // because inline table is also a table.
+                    // But toml v0.5.0 explicitly says it is invalid. The above
+                    // array-of-tables has a static size and appending to the
+                    // array is invalid.
+                    // In this library, multi-line table value has a region
+                    // that points to the key of the table (e.g. [[a]]). By
+                    // comparing the first two letters in key, we can detect
+                    // the array-of-table is inline or multiline.
+                    if(const auto ptr = detail::get_region(a.front()))
+                    {
+                        if(ptr->str().substr(0,2) != "[[")
+                        {
+                            throw syntax_error(format_underline(concat_to_string(
+                                "toml::insert_value: array of table (\"",
+                                format_dotted_keys(first, last), "\") collides "
+                                "with existing array-of-tables"), {
+                                    {tab->at(k).location(),
+                                     concat_to_string("this ", tab->at(k).type(),
+                                                      " value has static size")},
+                                    {v.location(),
+                                     "appending it to the statically sized array"}
+                                }), v.location());
+                        }
+                    }
+                    a.push_back(v);
+                    return ok(true);
+                }
+                else // if not, we need to create the array of table
+                {
+                    // XXX: Consider the following array of tables.
+                    // ```toml
+                    // # This is a comment.
+                    // [[aot]]
+                    // foo = "bar"
+                    // ```
+                    // Here, the comment is for `aot`. But here, actually two
+                    // values are defined. An array that contains tables, named
+                    // `aot`, and the 0th element of the `aot`, `{foo = "bar"}`.
+                    // Those two are different from each other. But both of them
+                    // points to the same portion of the TOML file, `[[aot]]`,
+                    // so `key_reg.comments()` returns `# This is a comment`.
+                    // If it is assigned as a comment of `aot` defined here, the
+                    // comment will be duplicated. Both the `aot` itself and
+                    // the 0-th element will have the same comment. This causes
+                    // "duplication of the same comments" bug when the data is
+                    // serialized.
+                    //     Next, consider the following.
+                    // ```toml
+                    // # comment 1
+                    // aot = [
+                    //     # comment 2
+                    //     {foo = "bar"},
+                    // ]
+                    // ```
+                    // In this case, we can distinguish those two comments. So
+                    // here we need to add "comment 1" to the `aot` and
+                    // "comment 2" to the 0th element of that.
+                    //     To distinguish those two, we check the key region.
+                    std::vector<std::string> comments{/* empty by default */};
+                    if(key_reg.str().substr(0, 2) != "[[")
+                    {
+                        comments = key_reg.comments();
+                    }
+                    value_type aot(array_type(1, v), key_reg, std::move(comments));
+                    tab->insert(std::make_pair(k, aot));
+                    return ok(true);
+                }
+            } // end if(array of table)
+
+            if(tab->count(k) == 1)
+            {
+                if(tab->at(k).is_table() && v.is_table())
+                {
+                    if(!is_valid_forward_table_definition(
+                                tab->at(k), v, first, iter, last))
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: table (\"",
+                            format_dotted_keys(first, last),
+                            "\") already exists."), {
+                                {tab->at(k).location(), "table already exists here"},
+                                {v.location(), "table defined twice"}
+                            }), v.location());
+                    }
+                    // to allow the following toml file.
+                    // [a.b.c]
+                    // d = 42
+                    // [a]
+                    // e = 2.71
+                    auto& t = tab->at(k).as_table();
+                    for(const auto& kv : v.as_table())
+                    {
+                        if(tab->at(k).contains(kv.first))
+                        {
+                            throw syntax_error(format_underline(concat_to_string(
+                                "toml::insert_value: value (\"",
+                                format_dotted_keys(first, last),
+                                "\") already exists."), {
+                                    {t.at(kv.first).location(), "already exists here"},
+                                    {v.location(), "this defined twice"}
+                                }), v.location());
+                        }
+                        t[kv.first] = kv.second;
+                    }
+                    detail::change_region(tab->at(k), key_reg);
+                    return ok(true);
+                }
+                else if(v.is_table()                     &&
+                        tab->at(k).is_array()            &&
+                        tab->at(k).as_array().size() > 0 &&
+                        tab->at(k).as_array().front().is_table())
+                {
+                    throw syntax_error(format_underline(concat_to_string(
+                        "toml::insert_value: array of tables (\"",
+                        format_dotted_keys(first, last), "\") already exists."), {
+                            {tab->at(k).location(), "array of tables defined here"},
+                            {v.location(), "table conflicts with the previous array of table"}
+                        }), v.location());
+                }
+                else
+                {
+                    throw syntax_error(format_underline(concat_to_string(
+                        "toml::insert_value: value (\"",
+                        format_dotted_keys(first, last), "\") already exists."), {
+                            {tab->at(k).location(), "value already exists here"},
+                            {v.location(), "value defined twice"}
+                        }), v.location());
+                }
+            }
+            tab->insert(std::make_pair(k, v));
+            return ok(true);
+        }
+        else // k is not the last one, we should insert recursively
+        {
+            // if there is no corresponding value, insert it first.
+            // related: you don't need to write
+            // # [x]
+            // # [x.y]
+            // to write
+            // [x.y.z]
+            if(tab->count(k) == 0)
+            {
+                // a table that is defined implicitly doesn't have any comments.
+                (*tab)[k] = value_type(table_type{}, key_reg, {/*no comment*/});
+            }
+
+            // type checking...
+            if(tab->at(k).is_table())
+            {
+                // According to toml-lang/toml:36d3091b3 "Clarify that inline
+                // tables are immutable", check if it adds key-value pair to an
+                // inline table.
+                if(const auto* ptr = get_region(tab->at(k)))
+                {
+                    // here, if the value is a (multi-line) table, the region
+                    // should be something like `[table-name]`.
+                    if(ptr->front() == '{')
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: inserting to an inline table (",
+                            format_dotted_keys(first, std::next(iter)),
+                            ") but inline tables are immutable"), {
+                                {tab->at(k).location(), "inline tables are immutable"},
+                                {v.location(), "inserting this"}
+                            }), v.location());
+                    }
+                }
+                tab = std::addressof((*tab)[k].as_table());
+            }
+            else if(tab->at(k).is_array()) // inserting to array-of-tables?
+            {
+                auto& a = (*tab)[k].as_array();
+                if(!a.back().is_table())
+                {
+                    throw syntax_error(format_underline(concat_to_string(
+                        "toml::insert_value: target (",
+                        format_dotted_keys(first, std::next(iter)),
+                        ") is neither table nor an array of tables"), {
+                            {a.back().location(), concat_to_string(
+                                    "actual type is ", a.back().type())},
+                            {v.location(), "inserting this"}
+                        }), v.location());
+                }
+                tab = std::addressof(a.back().as_table());
+            }
+            else
+            {
+                throw syntax_error(format_underline(concat_to_string(
+                    "toml::insert_value: target (",
+                    format_dotted_keys(first, std::next(iter)),
+                    ") is neither table nor an array of tables"), {
+                        {tab->at(k).location(), concat_to_string(
+                                "actual type is ", tab->at(k).type())},
+                        {v.location(), "inserting this"}
+                    }), v.location());
+            }
+        }
+    }
+    return err(std::string("toml::detail::insert_nested_key: never reach here"));
+}
+
+template<typename Value>
+result<std::pair<typename Value::table_type, region>, std::string>
+parse_inline_table(location& loc)
+{
+    using value_type = Value;
+    using table_type = typename value_type::table_type;
+
+    const auto first = loc.iter();
+    table_type retval;
+    if(!(loc.iter() != loc.end() && *loc.iter() == '{'))
+    {
+        return err(format_underline("toml::parse_inline_table: ",
+            {{source_location(loc), "the next token is not an inline table"}}));
+    }
+    loc.advance();
+
+    // check if the inline table is an empty table = { }
+    maybe<lex_ws>::invoke(loc);
+    if(loc.iter() != loc.end() && *loc.iter() == '}')
+    {
+        loc.advance(); // skip `}`
+        return ok(std::make_pair(retval, region(loc, first, loc.iter())));
+    }
+
+    // it starts from "{". it should be formatted as inline-table
+    while(loc.iter() != loc.end())
+    {
+        const auto kv_r = parse_key_value_pair<value_type>(loc);
+        if(!kv_r)
+        {
+            return err(kv_r.unwrap_err());
+        }
+
+        const auto&              kvpair  = kv_r.unwrap();
+        const std::vector<key>&  keys    = kvpair.first.first;
+        const auto&              key_reg = kvpair.first.second;
+        const value_type&        val     = kvpair.second;
+
+        const auto inserted =
+            insert_nested_key(retval, val, keys.begin(), keys.end(), key_reg);
+        if(!inserted)
+        {
+            throw internal_error("toml::parse_inline_table: "
+                "failed to insert value into table: " + inserted.unwrap_err(),
+                source_location(loc));
+        }
+
+        using lex_table_separator = sequence<maybe<lex_ws>, character<','>>;
+        const auto sp = lex_table_separator::invoke(loc);
+
+        if(!sp)
+        {
+            maybe<lex_ws>::invoke(loc);
+
+            if(loc.iter() == loc.end())
+            {
+                throw syntax_error(format_underline(
+                    "toml::parse_inline_table: missing table separator `}` ",
+                    {{source_location(loc), "should be `}`"}}),
+                    source_location(loc));
+            }
+            else if(*loc.iter() == '}')
+            {
+                loc.advance(); // skip `}`
+                return ok(std::make_pair(
+                            retval, region(loc, first, loc.iter())));
+            }
+            else if(*loc.iter() == '#' || *loc.iter() == '\r' || *loc.iter() == '\n')
+            {
+                throw syntax_error(format_underline(
+                    "toml::parse_inline_table: missing curly brace `}`",
+                    {{source_location(loc), "should be `}`"}}),
+                    source_location(loc));
+            }
+            else
+            {
+                throw syntax_error(format_underline(
+                    "toml::parse_inline_table: missing table separator `,` ",
+                    {{source_location(loc), "should be `,`"}}),
+                    source_location(loc));
+            }
+        }
+        else // `,` is found
+        {
+            maybe<lex_ws>::invoke(loc);
+            if(loc.iter() != loc.end() && *loc.iter() == '}')
+            {
+                throw syntax_error(format_underline(
+                    "toml::parse_inline_table: trailing comma is not allowed in"
+                    " an inline table",
+                    {{source_location(loc), "should be `}`"}}),
+                    source_location(loc));
+            }
+        }
+    }
+    loc.reset(first);
+    throw syntax_error(format_underline("toml::parse_inline_table: "
+            "inline table did not closed by `}`",
+            {{source_location(loc), "should be closed"}}),
+            source_location(loc));
+}
+
+inline result<value_t, std::string> guess_number_type(const location& l)
+{
+    // This function tries to find some (common) mistakes by checking characters
+    // that follows the last character of a value. But it is often difficult
+    // because some non-newline characters can appear after a value. E.g.
+    // spaces, tabs, commas (in an array or inline table), closing brackets
+    // (of an array or inline table), comment-sign (#). Since this function
+    // does not parse further, those characters are always allowed to be there.
+    location loc = l;
+
+    if(lex_offset_date_time::invoke(loc)) {return ok(value_t::offset_datetime);}
+    loc.reset(l.iter());
+
+    if(lex_local_date_time::invoke(loc))
+    {
+        // bad offset may appear after this.
+        if(loc.iter() != loc.end() && (*loc.iter() == '+' || *loc.iter() == '-'
+                    || *loc.iter() == 'Z' || *loc.iter() == 'z'))
+        {
+            return err(format_underline("bad offset: should be [+-]HH:MM or Z",
+                        {{source_location(loc), "[+-]HH:MM or Z"}},
+                        {"pass: +09:00, -05:30", "fail: +9:00, -5:30"}));
+        }
+        return ok(value_t::local_datetime);
+    }
+    loc.reset(l.iter());
+
+    if(lex_local_date::invoke(loc))
+    {
+        // bad time may appear after this.
+        // A space is allowed as a delimiter between local time. But there are
+        // both cases in which a space becomes valid or invalid.
+        // - invalid: 2019-06-16 7:00:00
+        // - valid  : 2019-06-16 07:00:00
+        if(loc.iter() != loc.end())
+        {
+            const auto c = *loc.iter();
+            if(c == 'T' || c == 't')
+            {
+                return err(format_underline("bad time: should be HH:MM:SS.subsec",
+                        {{source_location(loc), "HH:MM:SS.subsec"}},
+                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
+                         "fail: 1979-05-27T7:32:00, 1979-05-27 17:32"}));
+            }
+            if('0' <= c && c <= '9')
+            {
+                return err(format_underline("bad time: missing T",
+                        {{source_location(loc), "T or space required here"}},
+                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
+                         "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
+            }
+            if(c == ' ' && std::next(loc.iter()) != loc.end() &&
+                ('0' <= *std::next(loc.iter()) && *std::next(loc.iter())<= '9'))
+            {
+                loc.advance();
+                return err(format_underline("bad time: should be HH:MM:SS.subsec",
+                        {{source_location(loc), "HH:MM:SS.subsec"}},
+                        {"pass: 1979-05-27T07:32:00, 1979-05-27 07:32:00.999999",
+                         "fail: 1979-05-27T7:32:00, 1979-05-27 7:32"}));
+            }
+        }
+        return ok(value_t::local_date);
+    }
+    loc.reset(l.iter());
+
+    if(lex_local_time::invoke(loc)) {return ok(value_t::local_time);}
+    loc.reset(l.iter());
+
+    if(lex_float::invoke(loc))
+    {
+        if(loc.iter() != loc.end() && *loc.iter() == '_')
+        {
+            return err(format_underline("bad float: `_` should be surrounded by digits",
+                        {{source_location(loc), "here"}},
+                        {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
+                         "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
+        }
+        return ok(value_t::floating);
+    }
+    loc.reset(l.iter());
+
+    if(lex_integer::invoke(loc))
+    {
+        if(loc.iter() != loc.end())
+        {
+            const auto c = *loc.iter();
+            if(c == '_')
+            {
+                return err(format_underline("bad integer: `_` should be surrounded by digits",
+                            {{source_location(loc), "here"}},
+                            {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
+                             "fail: 1__000, 0123"}));
+            }
+            if('0' <= c && c <= '9')
+            {
+                // leading zero. point '0'
+                loc.retrace();
+                return err(format_underline("bad integer: leading zero",
+                            {{source_location(loc), "here"}},
+                            {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
+                             "fail: 1__000, 0123"}));
+            }
+            if(c == ':' || c == '-')
+            {
+                return err(format_underline("bad datetime: invalid format",
+                            {{source_location(loc), "here"}},
+                            {"pass: 1979-05-27T07:32:00-07:00, 1979-05-27 07:32:00.999999Z",
+                             "fail: 1979-05-27T7:32:00-7:00, 1979-05-27 7:32-00:30"}));
+            }
+            if(c == '.' || c == 'e' || c == 'E')
+            {
+                return err(format_underline("bad float: invalid format",
+                            {{source_location(loc), "here"}},
+                            {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
+                             "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
+            }
+        }
+        return ok(value_t::integer);
+    }
+    if(loc.iter() != loc.end() && *loc.iter() == '.')
+    {
+        return err(format_underline("bad float: invalid format",
+                {{source_location(loc), "integer part required before this"}},
+                {"pass: +1.0, -2e-2, 3.141_592_653_589, inf, nan",
+                 "fail: .0, 1., _1.0, 1.0_, 1_.0, 1.0__0"}));
+    }
+    if(loc.iter() != loc.end() && *loc.iter() == '_')
+    {
+        return err(format_underline("bad number: `_` should be surrounded by digits",
+                {{source_location(loc), "`_` is not surrounded by digits"}},
+                {"pass: -42, 1_000, 1_2_3_4_5, 0xC0FFEE, 0b0010, 0o755",
+                 "fail: 1__000, 0123"}));
+    }
+    return err(format_underline("bad format: unknown value appeared",
+                {{source_location(loc), "here"}}));
+}
+
+inline result<value_t, std::string> guess_value_type(const location& loc)
+{
+    switch(*loc.iter())
+    {
+        case '"' : {return ok(value_t::string);  }
+        case '\'': {return ok(value_t::string);  }
+        case 't' : {return ok(value_t::boolean); }
+        case 'f' : {return ok(value_t::boolean); }
+        case '[' : {return ok(value_t::array);   }
+        case '{' : {return ok(value_t::table);   }
+        case 'i' : {return ok(value_t::floating);} // inf.
+        case 'n' : {return ok(value_t::floating);} // nan.
+        default  : {return guess_number_type(loc);}
+    }
+}
+
+template<typename Value, typename T>
+result<Value, std::string>
+parse_value_helper(result<std::pair<T, region>, std::string> rslt)
+{
+    if(rslt.is_ok())
+    {
+        auto comments = rslt.as_ok().second.comments();
+        return ok(Value(std::move(rslt.as_ok()), std::move(comments)));
+    }
+    else
+    {
+        return err(std::move(rslt.as_err()));
+    }
+}
+
+template<typename Value>
+result<Value, std::string> parse_value(location& loc)
+{
+    const auto first = loc.iter();
+    if(first == loc.end())
+    {
+        return err(format_underline("toml::parse_value: input is empty",
+                   {{source_location(loc), ""}}));
+    }
+
+    const auto type = guess_value_type(loc);
+    if(!type)
+    {
+        return err(type.unwrap_err());
+    }
+
+    switch(type.unwrap())
+    {
+        case value_t::boolean        : {return parse_value_helper<Value>(parse_boolean(loc)            );}
+        case value_t::integer        : {return parse_value_helper<Value>(parse_integer(loc)            );}
+        case value_t::floating       : {return parse_value_helper<Value>(parse_floating(loc)           );}
+        case value_t::string         : {return parse_value_helper<Value>(parse_string(loc)             );}
+        case value_t::offset_datetime: {return parse_value_helper<Value>(parse_offset_datetime(loc)    );}
+        case value_t::local_datetime : {return parse_value_helper<Value>(parse_local_datetime(loc)     );}
+        case value_t::local_date     : {return parse_value_helper<Value>(parse_local_date(loc)         );}
+        case value_t::local_time     : {return parse_value_helper<Value>(parse_local_time(loc)         );}
+        case value_t::array          : {return parse_value_helper<Value>(parse_array<Value>(loc)       );}
+        case value_t::table          : {return parse_value_helper<Value>(parse_inline_table<Value>(loc));}
+        default:
+        {
+            const auto msg = format_underline("toml::parse_value: "
+                    "unknown token appeared", {{source_location(loc), "unknown"}});
+            loc.reset(first);
+            return err(msg);
+        }
+    }
+}
+
+inline result<std::pair<std::vector<key>, region>, std::string>
+parse_table_key(location& loc)
+{
+    if(auto token = lex_std_table::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto open = lex_std_table_open::invoke(inner_loc);
+        if(!open || inner_loc.iter() == inner_loc.end())
+        {
+            throw internal_error(format_underline(
+                "toml::parse_table_key: no `[`",
+                {{source_location(inner_loc), "should be `[`"}}),
+                source_location(inner_loc));
+        }
+        // to skip [ a . b . c ]
+        //          ^----------- this whitespace
+        lex_ws::invoke(inner_loc);
+        const auto keys = parse_key(inner_loc);
+        if(!keys)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_table_key: invalid key",
+                {{source_location(inner_loc), "not key"}}),
+                source_location(inner_loc));
+        }
+        // to skip [ a . b . c ]
+        //                    ^-- this whitespace
+        lex_ws::invoke(inner_loc);
+        const auto close = lex_std_table_close::invoke(inner_loc);
+        if(!close)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_table_key: no `]`",
+                {{source_location(inner_loc), "should be `]`"}}),
+                source_location(inner_loc));
+        }
+
+        // after [table.key], newline or EOF(empty table) required.
+        if(loc.iter() != loc.end())
+        {
+            using lex_newline_after_table_key =
+                sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>;
+            const auto nl = lex_newline_after_table_key::invoke(loc);
+            if(!nl)
+            {
+                throw syntax_error(format_underline(
+                    "toml::parse_table_key: newline required after [table.key]",
+                    {{source_location(loc), "expected newline"}}),
+                    source_location(loc));
+            }
+        }
+        return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
+    }
+    else
+    {
+        return err(format_underline("toml::parse_table_key: "
+            "not a valid table key", {{source_location(loc), "here"}}));
+    }
+}
+
+inline result<std::pair<std::vector<key>, region>, std::string>
+parse_array_table_key(location& loc)
+{
+    if(auto token = lex_array_table::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+
+        const auto open = lex_array_table_open::invoke(inner_loc);
+        if(!open || inner_loc.iter() == inner_loc.end())
+        {
+            throw internal_error(format_underline(
+                "toml::parse_array_table_key: no `[[`",
+                {{source_location(inner_loc), "should be `[[`"}}),
+                source_location(inner_loc));
+        }
+        lex_ws::invoke(inner_loc);
+        const auto keys = parse_key(inner_loc);
+        if(!keys)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_array_table_key: invalid key",
+                {{source_location(inner_loc), "not a key"}}),
+                source_location(inner_loc));
+        }
+        lex_ws::invoke(inner_loc);
+        const auto close = lex_array_table_close::invoke(inner_loc);
+        if(!close)
+        {
+            throw internal_error(format_underline(
+                "toml::parse_table_key: no `]]`",
+                {{source_location(inner_loc), "should be `]]`"}}),
+                source_location(inner_loc));
+        }
+
+        // after [[table.key]], newline or EOF(empty table) required.
+        if(loc.iter() != loc.end())
+        {
+            using lex_newline_after_table_key =
+                sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>;
+            const auto nl = lex_newline_after_table_key::invoke(loc);
+            if(!nl)
+            {
+                throw syntax_error(format_underline("toml::"
+                    "parse_array_table_key: newline required after [[table.key]]",
+                    {{source_location(loc), "expected newline"}}),
+                    source_location(loc));
+            }
+        }
+        return ok(std::make_pair(keys.unwrap().first, token.unwrap()));
+    }
+    else
+    {
+        return err(format_underline("toml::parse_array_table_key: "
+            "not a valid table key", {{source_location(loc), "here"}}));
+    }
+}
+
+// parse table body (key-value pairs until the iter hits the next [tablekey])
+template<typename Value>
+result<typename Value::table_type, std::string>
+parse_ml_table(location& loc)
+{
+    using value_type = Value;
+    using table_type = typename value_type::table_type;
+
+    const auto first = loc.iter();
+    if(first == loc.end())
+    {
+        return ok(table_type{});
+    }
+
+    // XXX at lest one newline is needed.
+    using skip_line = repeat<
+        sequence<maybe<lex_ws>, maybe<lex_comment>, lex_newline>, at_least<1>>;
+    skip_line::invoke(loc);
+    lex_ws::invoke(loc);
+
+    table_type tab;
+    while(loc.iter() != loc.end())
+    {
+        lex_ws::invoke(loc);
+        const auto before = loc.iter();
+        if(const auto tmp = parse_array_table_key(loc)) // next table found
+        {
+            loc.reset(before);
+            return ok(tab);
+        }
+        if(const auto tmp = parse_table_key(loc)) // next table found
+        {
+            loc.reset(before);
+            return ok(tab);
+        }
+
+        if(const auto kv = parse_key_value_pair<value_type>(loc))
+        {
+            const auto&              kvpair  = kv.unwrap();
+            const std::vector<key>&  keys    = kvpair.first.first;
+            const auto&              key_reg = kvpair.first.second;
+            const value_type&        val     = kvpair.second;
+            const auto inserted =
+                insert_nested_key(tab, val, keys.begin(), keys.end(), key_reg);
+            if(!inserted)
+            {
+                return err(inserted.unwrap_err());
+            }
+        }
+        else
+        {
+            return err(kv.unwrap_err());
+        }
+
+        // comment lines are skipped by the above function call.
+        // However, since the `skip_line` requires at least 1 newline, it fails
+        // if the file ends with ws and/or comment without newline.
+        // `skip_line` matches `ws? + comment? + newline`, not `ws` or `comment`
+        // itself. To skip the last ws and/or comment, call lexers.
+        // It does not matter if these fails, so the return value is discarded.
+        lex_ws::invoke(loc);
+        lex_comment::invoke(loc);
+
+        // skip_line is (whitespace? comment? newline)_{1,}. multiple empty lines
+        // and comments after the last key-value pairs are allowed.
+        const auto newline = skip_line::invoke(loc);
+        if(!newline && loc.iter() != loc.end())
+        {
+            const auto before2 = loc.iter();
+            lex_ws::invoke(loc); // skip whitespace
+            const auto msg = format_underline("toml::parse_table: "
+                "invalid line format", {{source_location(loc), concat_to_string(
+                "expected newline, but got '", show_char(*loc.iter()), "'.")}});
+            loc.reset(before2);
+            return err(msg);
+        }
+
+        // the skip_lines only matches with lines that includes newline.
+        // to skip the last line that includes comment and/or whitespace
+        // but no newline, call them one more time.
+        lex_ws::invoke(loc);
+        lex_comment::invoke(loc);
+    }
+    return ok(tab);
+}
+
+template<typename Value>
+result<Value, std::string> parse_toml_file(location& loc)
+{
+    using value_type = Value;
+    using table_type = typename value_type::table_type;
+
+    const auto first = loc.iter();
+    if(first == loc.end())
+    {
+        // For empty files, return an empty table with an empty region (zero-length).
+        // Without the region, error messages would miss the filename.
+        return ok(value_type(table_type{}, region(loc, first, first), {}));
+    }
+
+    // put the first line as a region of a file
+    // Here first != loc.end(), so taking std::next is okay
+    const region file(loc, first, std::next(loc.iter()));
+
+    // The first successive comments that are separated from the first value
+    // by an empty line are for a file itself.
+    // ```toml
+    // # this is a comment for a file.
+    //
+    // key = "the first value"
+    // ```
+    // ```toml
+    // # this is a comment for "the first value".
+    // key = "the first value"
+    // ```
+    std::vector<std::string> comments;
+    using lex_first_comments = sequence<
+        repeat<sequence<maybe<lex_ws>, lex_comment, lex_newline>, at_least<1>>,
+        sequence<maybe<lex_ws>, lex_newline>
+        >;
+    if(const auto token = lex_first_comments::invoke(loc))
+    {
+        location inner_loc(loc.name(), token.unwrap().str());
+        while(inner_loc.iter() != inner_loc.end())
+        {
+            maybe<lex_ws>::invoke(inner_loc); // remove ws if exists
+            if(lex_newline::invoke(inner_loc))
+            {
+                assert(inner_loc.iter() == inner_loc.end());
+                break; // empty line found.
+            }
+            auto com = lex_comment::invoke(inner_loc).unwrap().str();
+            com.erase(com.begin()); // remove # sign
+            comments.push_back(std::move(com));
+            lex_newline::invoke(inner_loc);
+        }
+    }
+
+    table_type data;
+    // root object is also a table, but without [tablename]
+    if(const auto tab = parse_ml_table<value_type>(loc))
+    {
+        data = std::move(tab.unwrap());
+    }
+    else // failed (empty table is regarded as success in parse_ml_table)
+    {
+        return err(tab.unwrap_err());
+    }
+    while(loc.iter() != loc.end())
+    {
+        // here, the region of [table] is regarded as the table-key because
+        // the table body is normally too big and it is not so informative
+        // if the first key-value pair of the table is shown in the error
+        // message.
+        if(const auto tabkey = parse_array_table_key(loc))
+        {
+            const auto tab = parse_ml_table<value_type>(loc);
+            if(!tab){return err(tab.unwrap_err());}
+
+            const auto& tk   = tabkey.unwrap();
+            const auto& keys = tk.first;
+            const auto& reg  = tk.second;
+
+            const auto inserted = insert_nested_key(data,
+                    value_type(tab.unwrap(), reg, reg.comments()),
+                    keys.begin(), keys.end(), reg,
+                    /*is_array_of_table=*/ true);
+            if(!inserted) {return err(inserted.unwrap_err());}
+
+            continue;
+        }
+        if(const auto tabkey = parse_table_key(loc))
+        {
+            const auto tab = parse_ml_table<value_type>(loc);
+            if(!tab){return err(tab.unwrap_err());}
+
+            const auto& tk   = tabkey.unwrap();
+            const auto& keys = tk.first;
+            const auto& reg  = tk.second;
+
+            const auto inserted = insert_nested_key(data,
+                value_type(tab.unwrap(), reg, reg.comments()),
+                keys.begin(), keys.end(), reg);
+            if(!inserted) {return err(inserted.unwrap_err());}
+
+            continue;
+        }
+        return err(format_underline("toml::parse_toml_file: "
+            "unknown line appeared", {{source_location(loc), "unknown format"}}));
+    }
+
+    return ok(Value(std::move(data), file, comments));
+}
+
+} // detail
+
+template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
+         template<typename ...> class Table   = std::unordered_map,
+         template<typename ...> class Array   = std::vector>
+basic_value<Comment, Table, Array>
+parse(std::istream& is, const std::string& fname = "unknown file")
+{
+    using value_type = basic_value<Comment, Table, Array>;
+
+    const auto beg = is.tellg();
+    is.seekg(0, std::ios::end);
+    const auto end = is.tellg();
+    const auto fsize = end - beg;
+    is.seekg(beg);
+
+    // read whole file as a sequence of char
+    assert(fsize >= 0);
+    std::vector<char> letters(static_cast<std::size_t>(fsize));
+    is.read(letters.data(), fsize);
+
+    // append LF.
+    // Although TOML does not require LF at the EOF, to make parsing logic
+    // simpler, we "normalize" the content by adding LF if it does not exist.
+    // It also checks if the last char is CR, to avoid changing the meaning.
+    // This is not the *best* way to deal with the last character, but is a
+    // simple and quick fix.
+    if(!letters.empty() && letters.back() != '\n' && letters.back() != '\r')
+    {
+        letters.push_back('\n');
+    }
+
+    detail::location loc(std::move(fname), std::move(letters));
+
+    // skip BOM if exists.
+    // XXX component of BOM (like 0xEF) exceeds the representable range of
+    // signed char, so on some (actually, most) of the environment, these cannot
+    // be compared to char. However, since we are always out of luck, we need to
+    // check our chars are equivalent to BOM. To do this, first we need to
+    // convert char to unsigned char to guarantee the comparability.
+    if(loc.source()->size() >= 3)
+    {
+        std::array<unsigned char, 3> BOM;
+        std::memcpy(BOM.data(), loc.source()->data(), 3);
+        if(BOM[0] == 0xEF && BOM[1] == 0xBB && BOM[2] == 0xBF)
+        {
+            loc.advance(3); // BOM found. skip.
+        }
+    }
+
+    const auto data = detail::parse_toml_file<value_type>(loc);
+    if(!data)
+    {
+        throw syntax_error(data.unwrap_err(), source_location(loc));
+    }
+    return data.unwrap();
+}
+
+template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
+         template<typename ...> class Table   = std::unordered_map,
+         template<typename ...> class Array   = std::vector>
+basic_value<Comment, Table, Array> parse(const std::string& fname)
+{
+    std::ifstream ifs(fname.c_str(), std::ios_base::binary);
+    if(!ifs.good())
+    {
+        throw std::runtime_error("toml::parse: file open error -> " + fname);
+    }
+    return parse<Comment, Table, Array>(ifs, fname);
+}
+
+#ifdef TOML11_HAS_STD_FILESYSTEM
+// This function just forwards `parse("filename.toml")` to std::string version
+// to avoid the ambiguity in overload resolution.
+//
+// Both std::string and std::filesystem::path are convertible from const char*.
+// Without this, both parse(std::string) and parse(std::filesystem::path)
+// matches to parse("filename.toml"). This breaks the existing code.
+//
+// This function exactly matches to the invocation with c-string.
+// So this function is preferred than others and the ambiguity disappears.
+template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
+         template<typename ...> class Table   = std::unordered_map,
+         template<typename ...> class Array   = std::vector>
+basic_value<Comment, Table, Array> parse(const char* fname)
+{
+    return parse<Comment, Table, Array>(std::string(fname));
+}
+
+template<typename                     Comment = TOML11_DEFAULT_COMMENT_STRATEGY,
+         template<typename ...> class Table   = std::unordered_map,
+         template<typename ...> class Array   = std::vector>
+basic_value<Comment, Table, Array> parse(const std::filesystem::path& fpath)
+{
+    std::ifstream ifs(fpath, std::ios_base::binary);
+    if(!ifs.good())
+    {
+        throw std::runtime_error("toml::parse: file open error -> " +
+                                 fpath.string());
+    }
+    return parse<Comment, Table, Array>(ifs, fpath.string());
+}
+#endif // TOML11_HAS_STD_FILESYSTEM
+
+} // toml
+#endif// TOML11_PARSER_HPP
diff --git a/src/toml11/toml/region.hpp b/src/toml11/toml/region.hpp
new file mode 100644
index 000000000..2e01e51d0
--- /dev/null
+++ b/src/toml11/toml/region.hpp
@@ -0,0 +1,417 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_REGION_HPP
+#define TOML11_REGION_HPP
+#include <memory>
+#include <vector>
+#include <algorithm>
+#include <initializer_list>
+#include <iterator>
+#include <iomanip>
+#include <cassert>
+#include "color.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+// helper function to avoid std::string(0, 'c') or std::string(iter, iter)
+template<typename Iterator>
+std::string make_string(Iterator first, Iterator last)
+{
+    if(first == last) {return "";}
+    return std::string(first, last);
+}
+inline std::string make_string(std::size_t len, char c)
+{
+    if(len == 0) {return "";}
+    return std::string(len, c);
+}
+
+// region_base is a base class of location and region that are defined below.
+// it will be used to generate better error messages.
+struct region_base
+{
+    region_base() = default;
+    virtual ~region_base() = default;
+    region_base(const region_base&) = default;
+    region_base(region_base&&     ) = default;
+    region_base& operator=(const region_base&) = default;
+    region_base& operator=(region_base&&     ) = default;
+
+    virtual bool is_ok()           const noexcept {return false;}
+    virtual char front()           const noexcept {return '\0';}
+
+    virtual std::string str()      const {return std::string("unknown region");}
+    virtual std::string name()     const {return std::string("unknown file");}
+    virtual std::string line()     const {return std::string("unknown line");}
+    virtual std::string line_num() const {return std::string("?");}
+
+    // length of the region
+    virtual std::size_t size()     const noexcept {return 0;}
+    // number of characters in the line before the region
+    virtual std::size_t before()   const noexcept {return 0;}
+    // number of characters in the line after the region
+    virtual std::size_t after()    const noexcept {return 0;}
+
+    virtual std::vector<std::string> comments() const {return {};}
+    // ```toml
+    // # comment_before
+    // key = "value" # comment_inline
+    // ```
+};
+
+// location represents a position in a container, which contains a file content.
+// it can be considered as a region that contains only one character.
+//
+// it contains pointer to the file content and iterator that points the current
+// location.
+struct location final : public region_base
+{
+    using const_iterator  = typename std::vector<char>::const_iterator;
+    using difference_type = typename const_iterator::difference_type;
+    using source_ptr      = std::shared_ptr<const std::vector<char>>;
+
+    location(std::string source_name, std::vector<char> cont)
+      : source_(std::make_shared<std::vector<char>>(std::move(cont))),
+        line_number_(1), source_name_(std::move(source_name)), iter_(source_->cbegin())
+    {}
+    location(std::string source_name, const std::string& cont)
+      : source_(std::make_shared<std::vector<char>>(cont.begin(), cont.end())),
+        line_number_(1), source_name_(std::move(source_name)), iter_(source_->cbegin())
+    {}
+
+    location(const location&) = default;
+    location(location&&)      = default;
+    location& operator=(const location&) = default;
+    location& operator=(location&&)      = default;
+    ~location() = default;
+
+    bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *iter_;}
+
+    // this const prohibits codes like `++(loc.iter())`.
+    const const_iterator iter()  const noexcept {return iter_;}
+
+    const_iterator begin() const noexcept {return source_->cbegin();}
+    const_iterator end()   const noexcept {return source_->cend();}
+
+    // XXX `location::line_num()` used to be implemented using `std::count` to
+    // count a number of '\n'. But with a long toml file (typically, 10k lines),
+    // it becomes intolerably slow because each time it generates error messages,
+    // it counts '\n' from thousands of characters. To workaround it, I decided
+    // to introduce `location::line_number_` member variable and synchronize it
+    // to the location changes the point to look. So an overload of `iter()`
+    // which returns mutable reference is removed and `advance()`, `retrace()`
+    // and `reset()` is added.
+    void advance(difference_type n = 1) noexcept
+    {
+        this->line_number_ += static_cast<std::size_t>(
+                std::count(this->iter_, std::next(this->iter_, n), '\n'));
+        this->iter_ += n;
+        return;
+    }
+    void retrace(difference_type n = 1) noexcept
+    {
+        this->line_number_ -= static_cast<std::size_t>(
+                std::count(std::prev(this->iter_, n), this->iter_, '\n'));
+        this->iter_ -= n;
+        return;
+    }
+    void reset(const_iterator rollback) noexcept
+    {
+        // since c++11, std::distance works in both ways for random-access
+        // iterators and returns a negative value if `first > last`.
+        if(0 <= std::distance(rollback, this->iter_)) // rollback < iter
+        {
+            this->line_number_ -= static_cast<std::size_t>(
+                    std::count(rollback, this->iter_, '\n'));
+        }
+        else // iter < rollback [[unlikely]]
+        {
+            this->line_number_ += static_cast<std::size_t>(
+                    std::count(this->iter_, rollback, '\n'));
+        }
+        this->iter_ = rollback;
+        return;
+    }
+
+    std::string str()  const override {return make_string(1, *this->iter());}
+    std::string name() const override {return source_name_;}
+
+    std::string line_num() const override
+    {
+        return std::to_string(this->line_number_);
+    }
+
+    std::string line() const override
+    {
+        return make_string(this->line_begin(), this->line_end());
+    }
+
+    const_iterator line_begin() const noexcept
+    {
+        using reverse_iterator = std::reverse_iterator<const_iterator>;
+        return std::find(reverse_iterator(this->iter()),
+                         reverse_iterator(this->begin()), '\n').base();
+    }
+    const_iterator line_end() const noexcept
+    {
+        return std::find(this->iter(), this->end(), '\n');
+    }
+
+    // location is always points a character. so the size is 1.
+    std::size_t size() const noexcept override
+    {
+        return 1u;
+    }
+    std::size_t before() const noexcept override
+    {
+        const auto sz = std::distance(this->line_begin(), this->iter());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
+    }
+    std::size_t after() const noexcept override
+    {
+        const auto sz = std::distance(this->iter(), this->line_end());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
+    }
+
+    source_ptr const& source() const& noexcept {return source_;}
+    source_ptr&&      source() &&     noexcept {return std::move(source_);}
+
+  private:
+
+    source_ptr     source_;
+    std::size_t    line_number_;
+    std::string    source_name_;
+    const_iterator iter_;
+};
+
+// region represents a range in a container, which contains a file content.
+//
+// it contains pointer to the file content and iterator that points the first
+// and last location.
+struct region final : public region_base
+{
+    using const_iterator = typename std::vector<char>::const_iterator;
+    using source_ptr     = std::shared_ptr<const std::vector<char>>;
+
+    // delete default constructor. source_ never be null.
+    region() = delete;
+
+    explicit region(const location& loc)
+      : source_(loc.source()), source_name_(loc.name()),
+        first_(loc.iter()), last_(loc.iter())
+    {}
+    explicit region(location&& loc)
+      : source_(loc.source()), source_name_(loc.name()),
+        first_(loc.iter()), last_(loc.iter())
+    {}
+
+    region(const location& loc, const_iterator f, const_iterator l)
+      : source_(loc.source()), source_name_(loc.name()), first_(f), last_(l)
+    {}
+    region(location&& loc, const_iterator f, const_iterator l)
+      : source_(loc.source()), source_name_(loc.name()), first_(f), last_(l)
+    {}
+
+    region(const region&) = default;
+    region(region&&)      = default;
+    region& operator=(const region&) = default;
+    region& operator=(region&&)      = default;
+    ~region() = default;
+
+    region& operator+=(const region& other)
+    {
+        // different regions cannot be concatenated
+        assert(this->begin() == other.begin() && this->end() == other.end() &&
+               this->last_   == other.first_);
+
+        this->last_ = other.last_;
+        return *this;
+    }
+
+    bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *first_;}
+
+    std::string str()  const override {return make_string(first_, last_);}
+    std::string line() const override
+    {
+        if(this->contain_newline())
+        {
+            return make_string(this->line_begin(),
+                    std::find(this->line_begin(), this->last(), '\n'));
+        }
+        return make_string(this->line_begin(), this->line_end());
+    }
+    std::string line_num() const override
+    {
+        return std::to_string(1 + std::count(this->begin(), this->first(), '\n'));
+    }
+
+    std::size_t size() const noexcept override
+    {
+        const auto sz = std::distance(first_, last_);
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
+    }
+    std::size_t before() const noexcept override
+    {
+        const auto sz = std::distance(this->line_begin(), this->first());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
+    }
+    std::size_t after() const noexcept override
+    {
+        const auto sz = std::distance(this->last(), this->line_end());
+        assert(sz >= 0);
+        return static_cast<std::size_t>(sz);
+    }
+
+    bool contain_newline() const noexcept
+    {
+        return std::find(this->first(), this->last(), '\n') != this->last();
+    }
+
+    const_iterator line_begin() const noexcept
+    {
+        using reverse_iterator = std::reverse_iterator<const_iterator>;
+        return std::find(reverse_iterator(this->first()),
+                         reverse_iterator(this->begin()), '\n').base();
+    }
+    const_iterator line_end() const noexcept
+    {
+        return std::find(this->last(), this->end(), '\n');
+    }
+
+    const_iterator begin() const noexcept {return source_->cbegin();}
+    const_iterator end()   const noexcept {return source_->cend();}
+    const_iterator first() const noexcept {return first_;}
+    const_iterator last()  const noexcept {return last_;}
+
+    source_ptr const& source() const& noexcept {return source_;}
+    source_ptr&&      source() &&     noexcept {return std::move(source_);}
+
+    std::string name() const override {return source_name_;}
+
+    std::vector<std::string> comments() const override
+    {
+        // assuming the current region (`*this`) points a value.
+        // ```toml
+        // a = "value"
+        //     ^^^^^^^- this region
+        // ```
+        using rev_iter = std::reverse_iterator<const_iterator>;
+
+        std::vector<std::string> com{};
+        {
+            // find comments just before the current region.
+            // ```toml
+            // # this should be collected.
+            // # this also.
+            // a = value # not this.
+            // ```
+
+            // # this is a comment for `a`, not array elements.
+            // a = [1, 2, 3, 4, 5]
+            if(this->first() == std::find_if(this->line_begin(), this->first(),
+                [](const char c) noexcept -> bool {return c == '[' || c == '{';}))
+            {
+                auto iter = this->line_begin(); // points the first character
+                while(iter != this->begin())
+                {
+                    iter = std::prev(iter);
+
+                    // range [line_start, iter) represents the previous line
+                    const auto line_start   = std::find(
+                            rev_iter(iter), rev_iter(this->begin()), '\n').base();
+                    const auto comment_found = std::find(line_start, iter, '#');
+                    if(comment_found == iter)
+                    {
+                        break; // comment not found.
+                    }
+
+                    // exclude the following case.
+                    // > a = "foo" # comment // <-- this is not a comment for b but a.
+                    // > b = "current value"
+                    if(std::all_of(line_start, comment_found,
+                            [](const char c) noexcept -> bool {
+                                return c == ' ' || c == '\t';
+                            }))
+                    {
+                        // unwrap the first '#' by std::next.
+                        auto s = make_string(std::next(comment_found), iter);
+                        if(!s.empty() && s.back() == '\r') {s.pop_back();}
+                        com.push_back(std::move(s));
+                    }
+                    else
+                    {
+                        break;
+                    }
+                    iter = line_start;
+                }
+            }
+        }
+
+        if(com.size() > 1)
+        {
+            std::reverse(com.begin(), com.end());
+        }
+
+        {
+            // find comments just after the current region.
+            // ```toml
+            // # not this.
+            // a = value # this one.
+            // a = [ # not this (technically difficult)
+            //
+            // ] # and this.
+            // ```
+            // The reason why it's difficult is that it requires parsing in the
+            // following case.
+            // ```toml
+            // a = [ 10 # this comment is for `10`. not for `a` but `a[0]`.
+            // # ...
+            // ] # this is apparently a comment for a.
+            //
+            // b = [
+            // 3.14 ] # there is no way to add a comment to `3.14` currently.
+            //
+            // c = [
+            //   3.14 # do this if you need a comment here.
+            // ]
+            // ```
+            const auto comment_found =
+                std::find(this->last(), this->line_end(), '#');
+            if(comment_found != this->line_end()) // '#' found
+            {
+                // table = {key = "value"} # what is this for?
+                // the above comment is not for "value", but {key="value"}.
+                if(comment_found == std::find_if(this->last(), comment_found,
+                    [](const char c) noexcept -> bool {
+                        return !(c == ' ' || c == '\t' || c == ',');
+                    }))
+                {
+                    // unwrap the first '#' by std::next.
+                    auto s = make_string(std::next(comment_found), this->line_end());
+                    if(!s.empty() && s.back() == '\r') {s.pop_back();}
+                    com.push_back(std::move(s));
+                }
+            }
+        }
+        return com;
+    }
+
+  private:
+
+    source_ptr     source_;
+    std::string    source_name_;
+    const_iterator first_, last_;
+};
+
+} // detail
+} // toml
+#endif// TOML11_REGION_H
diff --git a/src/toml11/toml/result.hpp b/src/toml11/toml/result.hpp
new file mode 100644
index 000000000..77cd46c64
--- /dev/null
+++ b/src/toml11/toml/result.hpp
@@ -0,0 +1,717 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_RESULT_HPP
+#define TOML11_RESULT_HPP
+#include "traits.hpp"
+#include <type_traits>
+#include <stdexcept>
+#include <utility>
+#include <new>
+#include <string>
+#include <sstream>
+#include <cassert>
+
+namespace toml
+{
+
+template<typename T>
+struct success
+{
+    using value_type = T;
+    value_type value;
+
+    explicit success(const value_type& v)
+        noexcept(std::is_nothrow_copy_constructible<value_type>::value)
+        : value(v)
+    {}
+    explicit success(value_type&& v)
+        noexcept(std::is_nothrow_move_constructible<value_type>::value)
+        : value(std::move(v))
+    {}
+
+    template<typename U>
+    explicit success(U&& v): value(std::forward<U>(v)) {}
+
+    template<typename U>
+    explicit success(const success<U>& v): value(v.value) {}
+    template<typename U>
+    explicit success(success<U>&& v): value(std::move(v.value)) {}
+
+    ~success() = default;
+    success(const success&) = default;
+    success(success&&)      = default;
+    success& operator=(const success&) = default;
+    success& operator=(success&&)      = default;
+};
+
+template<typename T>
+struct failure
+{
+    using value_type = T;
+    value_type value;
+
+    explicit failure(const value_type& v)
+        noexcept(std::is_nothrow_copy_constructible<value_type>::value)
+        : value(v)
+    {}
+    explicit failure(value_type&& v)
+        noexcept(std::is_nothrow_move_constructible<value_type>::value)
+        : value(std::move(v))
+    {}
+
+    template<typename U>
+    explicit failure(U&& v): value(std::forward<U>(v)) {}
+
+    template<typename U>
+    explicit failure(const failure<U>& v): value(v.value) {}
+    template<typename U>
+    explicit failure(failure<U>&& v): value(std::move(v.value)) {}
+
+    ~failure() = default;
+    failure(const failure&) = default;
+    failure(failure&&)      = default;
+    failure& operator=(const failure&) = default;
+    failure& operator=(failure&&)      = default;
+};
+
+template<typename T>
+success<typename std::remove_cv<typename std::remove_reference<T>::type>::type>
+ok(T&& v)
+{
+    return success<
+        typename std::remove_cv<typename std::remove_reference<T>::type>::type
+        >(std::forward<T>(v));
+}
+template<typename T>
+failure<typename std::remove_cv<typename std::remove_reference<T>::type>::type>
+err(T&& v)
+{
+    return failure<
+        typename std::remove_cv<typename std::remove_reference<T>::type>::type
+        >(std::forward<T>(v));
+}
+
+inline success<std::string> ok(const char* literal)
+{
+    return success<std::string>(std::string(literal));
+}
+inline failure<std::string> err(const char* literal)
+{
+    return failure<std::string>(std::string(literal));
+}
+
+
+template<typename T, typename E>
+struct result
+{
+    using value_type = T;
+    using error_type = E;
+    using success_type = success<value_type>;
+    using failure_type = failure<error_type>;
+
+    result(const success_type& s): is_ok_(true)
+    {
+        auto tmp = ::new(std::addressof(this->succ)) success_type(s);
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+    }
+    result(const failure_type& f): is_ok_(false)
+    {
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(f);
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+    }
+    result(success_type&& s): is_ok_(true)
+    {
+        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s));
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+    }
+    result(failure_type&& f): is_ok_(false)
+    {
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f));
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+    }
+
+    template<typename U>
+    result(const success<U>& s): is_ok_(true)
+    {
+        auto tmp = ::new(std::addressof(this->succ)) success_type(s.value);
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+    }
+    template<typename U>
+    result(const failure<U>& f): is_ok_(false)
+    {
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(f.value);
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+    }
+    template<typename U>
+    result(success<U>&& s): is_ok_(true)
+    {
+        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s.value));
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+    }
+    template<typename U>
+    result(failure<U>&& f): is_ok_(false)
+    {
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f.value));
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+    }
+
+    result& operator=(const success_type& s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ)) success_type(s);
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+        return *this;
+    }
+    result& operator=(const failure_type& f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(f);
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+        return *this;
+    }
+    result& operator=(success_type&& s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s));
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+        return *this;
+    }
+    result& operator=(failure_type&& f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f));
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+        return *this;
+    }
+
+    template<typename U>
+    result& operator=(const success<U>& s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ)) success_type(s.value);
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+        return *this;
+    }
+    template<typename U>
+    result& operator=(const failure<U>& f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(f.value);
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+        return *this;
+    }
+    template<typename U>
+    result& operator=(success<U>&& s)
+    {
+        this->cleanup();
+        this->is_ok_ = true;
+        auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(s.value));
+        assert(tmp == std::addressof(this->succ));
+        (void)tmp;
+        return *this;
+    }
+    template<typename U>
+    result& operator=(failure<U>&& f)
+    {
+        this->cleanup();
+        this->is_ok_ = false;
+        auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(f.value));
+        assert(tmp == std::addressof(this->fail));
+        (void)tmp;
+        return *this;
+    }
+
+    ~result() noexcept {this->cleanup();}
+
+    result(const result& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+    }
+    result(result&& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+    }
+
+    template<typename U, typename F>
+    result(const result<U, F>& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+    }
+    template<typename U, typename F>
+    result(result<U, F>&& other): is_ok_(other.is_ok())
+    {
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+    }
+
+    result& operator=(const result& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+    result& operator=(result&& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+
+    template<typename U, typename F>
+    result& operator=(const result<U, F>& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(other.as_ok());
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(other.as_err());
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+    template<typename U, typename F>
+    result& operator=(result<U, F>&& other)
+    {
+        this->cleanup();
+        if(other.is_ok())
+        {
+            auto tmp = ::new(std::addressof(this->succ)) success_type(std::move(other.as_ok()));
+            assert(tmp == std::addressof(this->succ));
+            (void)tmp;
+        }
+        else
+        {
+            auto tmp = ::new(std::addressof(this->fail)) failure_type(std::move(other.as_err()));
+            assert(tmp == std::addressof(this->fail));
+            (void)tmp;
+        }
+        is_ok_ = other.is_ok();
+        return *this;
+    }
+
+    bool is_ok()  const noexcept {return is_ok_;}
+    bool is_err() const noexcept {return !is_ok_;}
+
+    operator bool() const noexcept {return is_ok_;}
+
+    value_type&       unwrap() &
+    {
+        if(is_err())
+        {
+            throw std::runtime_error("toml::result: bad unwrap: " +
+                                     format_error(this->as_err()));
+        }
+        return this->succ.value;
+    }
+    value_type const& unwrap() const&
+    {
+        if(is_err())
+        {
+            throw std::runtime_error("toml::result: bad unwrap: " +
+                                     format_error(this->as_err()));
+        }
+        return this->succ.value;
+    }
+    value_type&&      unwrap() &&
+    {
+        if(is_err())
+        {
+            throw std::runtime_error("toml::result: bad unwrap: " +
+                                     format_error(this->as_err()));
+        }
+        return std::move(this->succ.value);
+    }
+
+    value_type&       unwrap_or(value_type& opt) &
+    {
+        if(is_err()) {return opt;}
+        return this->succ.value;
+    }
+    value_type const& unwrap_or(value_type const& opt) const&
+    {
+        if(is_err()) {return opt;}
+        return this->succ.value;
+    }
+    value_type        unwrap_or(value_type opt) &&
+    {
+        if(is_err()) {return opt;}
+        return this->succ.value;
+    }
+
+    error_type&       unwrap_err() &
+    {
+        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
+        return this->fail.value;
+    }
+    error_type const& unwrap_err() const&
+    {
+        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
+        return this->fail.value;
+    }
+    error_type&&      unwrap_err() &&
+    {
+        if(is_ok()) {throw std::runtime_error("toml::result: bad unwrap_err");}
+        return std::move(this->fail.value);
+    }
+
+    value_type&       as_ok() &      noexcept {return this->succ.value;}
+    value_type const& as_ok() const& noexcept {return this->succ.value;}
+    value_type&&      as_ok() &&     noexcept {return std::move(this->succ.value);}
+
+    error_type&       as_err() &      noexcept {return this->fail.value;}
+    error_type const& as_err() const& noexcept {return this->fail.value;}
+    error_type&&      as_err() &&     noexcept {return std::move(this->fail.value);}
+
+
+    // prerequisities
+    // F: T -> U
+    // retval: result<U, E>
+    template<typename F>
+    result<detail::return_type_of_t<F, value_type&>, error_type>
+    map(F&& f) &
+    {
+        if(this->is_ok()){return ok(f(this->as_ok()));}
+        return err(this->as_err());
+    }
+    template<typename F>
+    result<detail::return_type_of_t<F, value_type const&>, error_type>
+    map(F&& f) const&
+    {
+        if(this->is_ok()){return ok(f(this->as_ok()));}
+        return err(this->as_err());
+    }
+    template<typename F>
+    result<detail::return_type_of_t<F, value_type &&>, error_type>
+    map(F&& f) &&
+    {
+        if(this->is_ok()){return ok(f(std::move(this->as_ok())));}
+        return err(std::move(this->as_err()));
+    }
+
+    // prerequisities
+    // F: E -> F
+    // retval: result<T, F>
+    template<typename F>
+    result<value_type, detail::return_type_of_t<F, error_type&>>
+    map_err(F&& f) &
+    {
+        if(this->is_err()){return err(f(this->as_err()));}
+        return ok(this->as_ok());
+    }
+    template<typename F>
+    result<value_type, detail::return_type_of_t<F, error_type const&>>
+    map_err(F&& f) const&
+    {
+        if(this->is_err()){return err(f(this->as_err()));}
+        return ok(this->as_ok());
+    }
+    template<typename F>
+    result<value_type, detail::return_type_of_t<F, error_type&&>>
+    map_err(F&& f) &&
+    {
+        if(this->is_err()){return err(f(std::move(this->as_err())));}
+        return ok(std::move(this->as_ok()));
+    }
+
+    // prerequisities
+    // F: T -> U
+    // retval: U
+    template<typename F, typename U>
+    detail::return_type_of_t<F, value_type&>
+    map_or_else(F&& f, U&& opt) &
+    {
+        if(this->is_err()){return std::forward<U>(opt);}
+        return f(this->as_ok());
+    }
+    template<typename F, typename U>
+    detail::return_type_of_t<F, value_type const&>
+    map_or_else(F&& f, U&& opt) const&
+    {
+        if(this->is_err()){return std::forward<U>(opt);}
+        return f(this->as_ok());
+    }
+    template<typename F, typename U>
+    detail::return_type_of_t<F, value_type&&>
+    map_or_else(F&& f, U&& opt) &&
+    {
+        if(this->is_err()){return std::forward<U>(opt);}
+        return f(std::move(this->as_ok()));
+    }
+
+    // prerequisities
+    // F: E -> U
+    // retval: U
+    template<typename F, typename U>
+    detail::return_type_of_t<F, error_type&>
+    map_err_or_else(F&& f, U&& opt) &
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(this->as_err());
+    }
+    template<typename F, typename U>
+    detail::return_type_of_t<F, error_type const&>
+    map_err_or_else(F&& f, U&& opt) const&
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(this->as_err());
+    }
+    template<typename F, typename U>
+    detail::return_type_of_t<F, error_type&&>
+    map_err_or_else(F&& f, U&& opt) &&
+    {
+        if(this->is_ok()){return std::forward<U>(opt);}
+        return f(std::move(this->as_err()));
+    }
+
+    // prerequisities:
+    // F: func T -> U
+    // toml::err(error_type) should be convertible to U.
+    // normally, type U is another result<S, F> and E is convertible to F
+    template<typename F>
+    detail::return_type_of_t<F, value_type&>
+    and_then(F&& f) &
+    {
+        if(this->is_ok()){return f(this->as_ok());}
+        return err(this->as_err());
+    }
+    template<typename F>
+    detail::return_type_of_t<F, value_type const&>
+    and_then(F&& f) const&
+    {
+        if(this->is_ok()){return f(this->as_ok());}
+        return err(this->as_err());
+    }
+    template<typename F>
+    detail::return_type_of_t<F, value_type&&>
+    and_then(F&& f) &&
+    {
+        if(this->is_ok()){return f(std::move(this->as_ok()));}
+        return err(std::move(this->as_err()));
+    }
+
+    // prerequisities:
+    // F: func E -> U
+    // toml::ok(value_type) should be convertible to U.
+    // normally, type U is another result<S, F> and T is convertible to S
+    template<typename F>
+    detail::return_type_of_t<F, error_type&>
+    or_else(F&& f) &
+    {
+        if(this->is_err()){return f(this->as_err());}
+        return ok(this->as_ok());
+    }
+    template<typename F>
+    detail::return_type_of_t<F, error_type const&>
+    or_else(F&& f) const&
+    {
+        if(this->is_err()){return f(this->as_err());}
+        return ok(this->as_ok());
+    }
+    template<typename F>
+    detail::return_type_of_t<F, error_type&&>
+    or_else(F&& f) &&
+    {
+        if(this->is_err()){return f(std::move(this->as_err()));}
+        return ok(std::move(this->as_ok()));
+    }
+
+    // if *this is error, returns *this. otherwise, returns other.
+    result and_other(const result& other) const&
+    {
+        return this->is_err() ? *this : other;
+    }
+    result and_other(result&& other) &&
+    {
+        return this->is_err() ? std::move(*this) : std::move(other);
+    }
+
+    // if *this is okay, returns *this. otherwise, returns other.
+    result or_other(const result& other) const&
+    {
+        return this->is_ok() ? *this : other;
+    }
+    result or_other(result&& other) &&
+    {
+        return this->is_ok() ? std::move(*this) : std::move(other);
+    }
+
+    void swap(result<T, E>& other)
+    {
+        result<T, E> tmp(std::move(*this));
+        *this = std::move(other);
+        other = std::move(tmp);
+        return ;
+    }
+
+  private:
+
+    static std::string format_error(std::exception const& excpt)
+    {
+        return std::string(excpt.what());
+    }
+    template<typename U, typename std::enable_if<!std::is_base_of<
+        std::exception, U>::value, std::nullptr_t>::type = nullptr>
+    static std::string format_error(U const& others)
+    {
+        std::ostringstream oss; oss << others;
+        return oss.str();
+    }
+
+    void cleanup() noexcept
+    {
+        if(this->is_ok_) {this->succ.~success_type();}
+        else             {this->fail.~failure_type();}
+        return;
+    }
+
+  private:
+
+    bool      is_ok_;
+    union
+    {
+        success_type succ;
+        failure_type fail;
+    };
+};
+
+template<typename T, typename E>
+void swap(result<T, E>& lhs, result<T, E>& rhs)
+{
+    lhs.swap(rhs);
+    return;
+}
+
+// this might be confusing because it eagerly evaluated, while in the other
+// cases operator && and || are short-circuited.
+//
+// template<typename T, typename E>
+// inline result<T, E>
+// operator&&(const result<T, E>& lhs, const result<T, E>& rhs) noexcept
+// {
+//     return lhs.is_ok() ? rhs : lhs;
+// }
+//
+// template<typename T, typename E>
+// inline result<T, E>
+// operator||(const result<T, E>& lhs, const result<T, E>& rhs) noexcept
+// {
+//     return lhs.is_ok() ? lhs : rhs;
+// }
+
+// ----------------------------------------------------------------------------
+// re-use result<T, E> as a optional<T> with none_t
+
+namespace detail
+{
+struct none_t {};
+inline bool operator==(const none_t&, const none_t&) noexcept {return true;}
+inline bool operator!=(const none_t&, const none_t&) noexcept {return false;}
+inline bool operator< (const none_t&, const none_t&) noexcept {return false;}
+inline bool operator<=(const none_t&, const none_t&) noexcept {return true;}
+inline bool operator> (const none_t&, const none_t&) noexcept {return false;}
+inline bool operator>=(const none_t&, const none_t&) noexcept {return true;}
+template<typename charT, typename traitsT>
+std::basic_ostream<charT, traitsT>&
+operator<<(std::basic_ostream<charT, traitsT>& os, const none_t&)
+{
+    os << "none";
+    return os;
+}
+inline failure<none_t> none() noexcept {return failure<none_t>{none_t{}};}
+} // detail
+} // toml11
+#endif// TOML11_RESULT_H
diff --git a/src/toml11/toml/serializer.hpp b/src/toml11/toml/serializer.hpp
new file mode 100644
index 000000000..88ae775a8
--- /dev/null
+++ b/src/toml11/toml/serializer.hpp
@@ -0,0 +1,922 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_SERIALIZER_HPP
+#define TOML11_SERIALIZER_HPP
+#include <cmath>
+#include <cstdio>
+
+#include <limits>
+
+#include "lexer.hpp"
+#include "value.hpp"
+
+namespace toml
+{
+
+// This function serialize a key. It checks a string is a bare key and
+// escapes special characters if the string is not compatible to a bare key.
+// ```cpp
+// std::string k("non.bare.key"); // the key itself includes `.`s.
+// std::string formatted = toml::format_key(k);
+// assert(formatted == "\"non.bare.key\"");
+// ```
+//
+// This function is exposed to make it easy to write a user-defined serializer.
+// Since toml restricts characters available in a bare key, generally a string
+// should be escaped. But checking whether a string needs to be surrounded by
+// a `"` and escaping some special character is boring.
+template<typename charT, typename traits, typename Alloc>
+std::basic_string<charT, traits, Alloc>
+format_key(const std::basic_string<charT, traits, Alloc>& k)
+{
+    if(k.empty())
+    {
+        return std::string("\"\"");
+    }
+
+    // check the key can be a bare (unquoted) key
+    detail::location loc(k, std::vector<char>(k.begin(), k.end()));
+    detail::lex_unquoted_key::invoke(loc);
+    if(loc.iter() == loc.end())
+    {
+        return k; // all the tokens are consumed. the key is unquoted-key.
+    }
+
+    //if it includes special characters, then format it in a "quoted" key.
+    std::basic_string<charT, traits, Alloc> serialized("\"");
+    for(const char c : k)
+    {
+        switch(c)
+        {
+            case '\\': {serialized += "\\\\"; break;}
+            case '\"': {serialized += "\\\""; break;}
+            case '\b': {serialized += "\\b";  break;}
+            case '\t': {serialized += "\\t";  break;}
+            case '\f': {serialized += "\\f";  break;}
+            case '\n': {serialized += "\\n";  break;}
+            case '\r': {serialized += "\\r";  break;}
+            default  : {serialized += c;      break;}
+        }
+    }
+    serialized += "\"";
+    return serialized;
+}
+
+template<typename charT, typename traits, typename Alloc>
+std::basic_string<charT, traits, Alloc>
+format_keys(const std::vector<std::basic_string<charT, traits, Alloc>>& keys)
+{
+    if(keys.empty())
+    {
+        return std::string("\"\"");
+    }
+
+    std::basic_string<charT, traits, Alloc> serialized;
+    for(const auto& ky : keys)
+    {
+        serialized += format_key(ky);
+        serialized += charT('.');
+    }
+    serialized.pop_back(); // remove the last dot '.'
+    return serialized;
+}
+
+template<typename Value>
+struct serializer
+{
+    static_assert(detail::is_basic_value<Value>::value,
+                  "toml::serializer is for toml::value and its variants, "
+                  "toml::basic_value<...>.");
+
+    using value_type           = Value;
+    using key_type             = typename value_type::key_type            ;
+    using comment_type         = typename value_type::comment_type        ;
+    using boolean_type         = typename value_type::boolean_type        ;
+    using integer_type         = typename value_type::integer_type        ;
+    using floating_type        = typename value_type::floating_type       ;
+    using string_type          = typename value_type::string_type         ;
+    using local_time_type      = typename value_type::local_time_type     ;
+    using local_date_type      = typename value_type::local_date_type     ;
+    using local_datetime_type  = typename value_type::local_datetime_type ;
+    using offset_datetime_type = typename value_type::offset_datetime_type;
+    using array_type           = typename value_type::array_type          ;
+    using table_type           = typename value_type::table_type          ;
+
+    serializer(const std::size_t w              = 80u,
+               const int         float_prec     = std::numeric_limits<toml::floating>::max_digits10,
+               const bool        can_be_inlined = false,
+               const bool        no_comment     = false,
+               std::vector<toml::key> ks        = {},
+               const bool     value_has_comment = false)
+        : can_be_inlined_(can_be_inlined), no_comment_(no_comment),
+          value_has_comment_(value_has_comment && !no_comment),
+          float_prec_(float_prec), width_(w), keys_(std::move(ks))
+    {}
+    ~serializer() = default;
+
+    std::string operator()(const boolean_type& b) const
+    {
+        return b ? "true" : "false";
+    }
+    std::string operator()(const integer_type i) const
+    {
+        return std::to_string(i);
+    }
+    std::string operator()(const floating_type f) const
+    {
+        if(std::isnan(f))
+        {
+            if(std::signbit(f))
+            {
+                return std::string("-nan");
+            }
+            else
+            {
+                return std::string("nan");
+            }
+        }
+        else if(!std::isfinite(f))
+        {
+            if(std::signbit(f))
+            {
+                return std::string("-inf");
+            }
+            else
+            {
+                return std::string("inf");
+            }
+        }
+
+        const auto fmt = "%.*g";
+        const auto bsz = std::snprintf(nullptr, 0, fmt, this->float_prec_, f);
+        // +1 for null character(\0)
+        std::vector<char> buf(static_cast<std::size_t>(bsz + 1), '\0');
+        std::snprintf(buf.data(), buf.size(), fmt, this->float_prec_, f);
+
+        std::string token(buf.begin(), std::prev(buf.end()));
+        if(!token.empty() && token.back() == '.') // 1. => 1.0
+        {
+            token += '0';
+        }
+
+        const auto e = std::find_if(
+            token.cbegin(), token.cend(), [](const char c) noexcept -> bool {
+                return c == 'e' || c == 'E';
+            });
+        const auto has_exponent = (token.cend() != e);
+        const auto has_fraction = (token.cend() != std::find(
+            token.cbegin(), token.cend(), '.'));
+
+        if(!has_exponent && !has_fraction)
+        {
+            // the resulting value does not have any float specific part!
+            token += ".0";
+        }
+        return token;
+    }
+    std::string operator()(const string_type& s) const
+    {
+        if(s.kind == string_t::basic)
+        {
+            if((std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
+                std::find(s.str.cbegin(), s.str.cend(), '\"') != s.str.cend()) &&
+               this->width_ != (std::numeric_limits<std::size_t>::max)())
+            {
+                // if linefeed or double-quote is contained,
+                // make it multiline basic string.
+                const auto escaped = this->escape_ml_basic_string(s.str);
+                std::string open("\"\"\"");
+                std::string close("\"\"\"");
+                if(escaped.find('\n') != std::string::npos ||
+                   this->width_ < escaped.size() + 6)
+                {
+                    // if the string body contains newline or is enough long,
+                    // add newlines after and before delimiters.
+                    open += "\n";
+                    close = std::string("\\\n") + close;
+                }
+                return open + escaped + close;
+            }
+
+            // no linefeed. try to make it oneline-string.
+            std::string oneline = this->escape_basic_string(s.str);
+            if(oneline.size() + 2 < width_ || width_ < 2)
+            {
+                const std::string quote("\"");
+                return quote + oneline + quote;
+            }
+
+            // the line is too long compared to the specified width.
+            // split it into multiple lines.
+            std::string token("\"\"\"\n");
+            while(!oneline.empty())
+            {
+                if(oneline.size() < width_)
+                {
+                    token += oneline;
+                    oneline.clear();
+                }
+                else if(oneline.at(width_-2) == '\\')
+                {
+                    token += oneline.substr(0, width_-2);
+                    token += "\\\n";
+                    oneline.erase(0, width_-2);
+                }
+                else
+                {
+                    token += oneline.substr(0, width_-1);
+                    token += "\\\n";
+                    oneline.erase(0, width_-1);
+                }
+            }
+            return token + std::string("\\\n\"\"\"");
+        }
+        else // the string `s` is literal-string.
+        {
+            if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
+               std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
+            {
+                std::string open("'''");
+                if(this->width_ + 6 < s.str.size())
+                {
+                    open += '\n'; // the first newline is ignored by TOML spec
+                }
+                const std::string close("'''");
+                return open + s.str + close;
+            }
+            else
+            {
+                const std::string quote("'");
+                return quote + s.str + quote;
+            }
+        }
+    }
+
+    std::string operator()(const local_date_type& d) const
+    {
+        std::ostringstream oss;
+        oss << d;
+        return oss.str();
+    }
+    std::string operator()(const local_time_type& t) const
+    {
+        std::ostringstream oss;
+        oss << t;
+        return oss.str();
+    }
+    std::string operator()(const local_datetime_type& dt) const
+    {
+        std::ostringstream oss;
+        oss << dt;
+        return oss.str();
+    }
+    std::string operator()(const offset_datetime_type& odt) const
+    {
+        std::ostringstream oss;
+        oss << odt;
+        return oss.str();
+    }
+
+    std::string operator()(const array_type& v) const
+    {
+        if(v.empty())
+        {
+            return std::string("[]");
+        }
+        if(this->is_array_of_tables(v))
+        {
+            return make_array_of_tables(v);
+        }
+
+        // not an array of tables. normal array.
+        // first, try to make it inline if none of the elements have a comment.
+        if( ! this->has_comment_inside(v))
+        {
+            const auto inl = this->make_inline_array(v);
+            if(inl.size() < this->width_ &&
+               std::find(inl.cbegin(), inl.cend(), '\n') == inl.cend())
+            {
+                return inl;
+            }
+        }
+
+        // if the length exceeds this->width_, print multiline array.
+        // key = [
+        //   # ...
+        //   42,
+        //   ...
+        // ]
+        std::string token;
+        std::string current_line;
+        token += "[\n";
+        for(const auto& item : v)
+        {
+            if( ! item.comments().empty() && !no_comment_)
+            {
+                // if comment exists, the element must be the only element in the line.
+                // e.g. the following is not allowed.
+                // ```toml
+                // array = [
+                // # comment for what?
+                // 1, 2, 3, 4, 5
+                // ]
+                // ```
+                if(!current_line.empty())
+                {
+                    if(current_line.back() != '\n')
+                    {
+                        current_line += '\n';
+                    }
+                    token += current_line;
+                    current_line.clear();
+                }
+                for(const auto& c : item.comments())
+                {
+                    token += '#';
+                    token += c;
+                    token += '\n';
+                }
+                token += toml::visit(*this, item);
+                if(!token.empty() && token.back() == '\n') {token.pop_back();}
+                token += ",\n";
+                continue;
+            }
+            std::string next_elem;
+            if(item.is_table())
+            {
+                serializer ser(*this);
+                ser.can_be_inlined_ = true;
+                ser.width_ = (std::numeric_limits<std::size_t>::max)();
+                next_elem += toml::visit(ser, item);
+            }
+            else
+            {
+                next_elem += toml::visit(*this, item);
+            }
+
+            // comma before newline.
+            if(!next_elem.empty() && next_elem.back() == '\n') {next_elem.pop_back();}
+
+            // if current line does not exceeds the width limit, continue.
+            if(current_line.size() + next_elem.size() + 1 < this->width_)
+            {
+                current_line += next_elem;
+                current_line += ',';
+            }
+            else if(current_line.empty())
+            {
+                // if current line was empty, force put the next_elem because
+                // next_elem is not splittable
+                token += next_elem;
+                token += ",\n";
+                // current_line is kept empty
+            }
+            else // reset current_line
+            {
+                assert(current_line.back() == ',');
+                token += current_line;
+                token += '\n';
+                current_line = next_elem;
+                current_line += ',';
+            }
+        }
+        if(!current_line.empty())
+        {
+            if(!current_line.empty() && current_line.back() != '\n')
+            {
+                current_line += '\n';
+            }
+            token += current_line;
+        }
+        token += "]\n";
+        return token;
+    }
+
+    // templatize for any table-like container
+    std::string operator()(const table_type& v) const
+    {
+        // if an element has a comment, then it can't be inlined.
+        // table = {# how can we write a comment for this? key = "value"}
+        if(this->can_be_inlined_ && !(this->has_comment_inside(v)))
+        {
+            std::string token;
+            if(!this->keys_.empty())
+            {
+                token += format_key(this->keys_.back());
+                token += " = ";
+            }
+            token += this->make_inline_table(v);
+            if(token.size() < this->width_ &&
+               token.end() == std::find(token.begin(), token.end(), '\n'))
+            {
+                return token;
+            }
+        }
+
+        std::string token;
+        if(!keys_.empty())
+        {
+            token += '[';
+            token += format_keys(keys_);
+            token += "]\n";
+        }
+        token += this->make_multiline_table(v);
+        return token;
+    }
+
+  private:
+
+    std::string escape_basic_string(const std::string& s) const
+    {
+        //XXX assuming `s` is a valid utf-8 sequence.
+        std::string retval;
+        for(const char c : s)
+        {
+            switch(c)
+            {
+                case '\\': {retval += "\\\\"; break;}
+                case '\"': {retval += "\\\""; break;}
+                case '\b': {retval += "\\b";  break;}
+                case '\t': {retval += "\\t";  break;}
+                case '\f': {retval += "\\f";  break;}
+                case '\n': {retval += "\\n";  break;}
+                case '\r': {retval += "\\r";  break;}
+                default  :
+                {
+                    if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
+                    {
+                        retval += "\\u00";
+                        retval += char(48 + (c / 16));
+                        retval += char((c % 16 < 10 ? 48 : 55) + (c % 16));
+                    }
+                    else
+                    {
+                        retval += c;
+                    }
+                }
+            }
+        }
+        return retval;
+    }
+
+    std::string escape_ml_basic_string(const std::string& s) const
+    {
+        std::string retval;
+        for(auto i=s.cbegin(), e=s.cend(); i!=e; ++i)
+        {
+            switch(*i)
+            {
+                case '\\': {retval += "\\\\"; break;}
+                // One or two consecutive "s are allowed.
+                // Later we will check there are no three consecutive "s.
+                //   case '\"': {retval += "\\\""; break;}
+                case '\b': {retval += "\\b";  break;}
+                case '\t': {retval += "\\t";  break;}
+                case '\f': {retval += "\\f";  break;}
+                case '\n': {retval += "\n";   break;}
+                case '\r':
+                {
+                    if(std::next(i) != e && *std::next(i) == '\n')
+                    {
+                        retval += "\r\n";
+                        ++i;
+                    }
+                    else
+                    {
+                        retval += "\\r";
+                    }
+                    break;
+                }
+                default  :
+                {
+                    const auto c = *i;
+                    if((0x00 <= c && c <= 0x08) || (0x0A <= c && c <= 0x1F) || c == 0x7F)
+                    {
+                        retval += "\\u00";
+                        retval += char(48 + (c / 16));
+                        retval += char((c % 16 < 10 ? 48 : 55) + (c % 16));
+                    }
+                    else
+                    {
+                        retval += c;
+                    }
+                }
+
+            }
+        }
+        // Only 1 or 2 consecutive `"`s are allowed in multiline basic string.
+        // 3 consecutive `"`s are considered as a closing delimiter.
+        // We need to check if there are 3 or more consecutive `"`s and insert
+        // backslash to break them down into several short `"`s like the `str6`
+        // in the following example.
+        // ```toml
+        // str4 = """Here are two quotation marks: "". Simple enough."""
+        // # str5 = """Here are three quotation marks: """."""  # INVALID
+        // str5 = """Here are three quotation marks: ""\"."""
+        // str6 = """Here are fifteen quotation marks: ""\"""\"""\"""\"""\"."""
+        // ```
+        auto found_3_quotes = retval.find("\"\"\"");
+        while(found_3_quotes != std::string::npos)
+        {
+            retval.replace(found_3_quotes, 3, "\"\"\\\"");
+            found_3_quotes = retval.find("\"\"\"");
+        }
+        return retval;
+    }
+
+    // if an element of a table or an array has a comment, it cannot be inlined.
+    bool has_comment_inside(const array_type& a) const noexcept
+    {
+        // if no_comment is set, comments would not be written.
+        if(this->no_comment_) {return false;}
+
+        for(const auto& v : a)
+        {
+            if(!v.comments().empty()) {return true;}
+        }
+        return false;
+    }
+    bool has_comment_inside(const table_type& t) const noexcept
+    {
+        // if no_comment is set, comments would not be written.
+        if(this->no_comment_) {return false;}
+
+        for(const auto& kv : t)
+        {
+            if(!kv.second.comments().empty()) {return true;}
+        }
+        return false;
+    }
+
+    std::string make_inline_array(const array_type& v) const
+    {
+        assert(!has_comment_inside(v));
+        std::string token;
+        token += '[';
+        bool is_first = true;
+        for(const auto& item : v)
+        {
+            if(is_first) {is_first = false;} else {token += ',';}
+            token += visit(serializer(
+                (std::numeric_limits<std::size_t>::max)(), this->float_prec_,
+                /* inlined */ true, /*no comment*/ false, /*keys*/ {},
+                /*has_comment*/ !item.comments().empty()), item);
+        }
+        token += ']';
+        return token;
+    }
+
+    std::string make_inline_table(const table_type& v) const
+    {
+        assert(!has_comment_inside(v));
+        assert(this->can_be_inlined_);
+        std::string token;
+        token += '{';
+        bool is_first = true;
+        for(const auto& kv : v)
+        {
+            // in inline tables, trailing comma is not allowed (toml-lang #569).
+            if(is_first) {is_first = false;} else {token += ',';}
+            token += format_key(kv.first);
+            token += '=';
+            token += visit(serializer(
+                (std::numeric_limits<std::size_t>::max)(), this->float_prec_,
+                /* inlined */ true, /*no comment*/ false, /*keys*/ {},
+                /*has_comment*/ !kv.second.comments().empty()), kv.second);
+        }
+        token += '}';
+        return token;
+    }
+
+    std::string make_multiline_table(const table_type& v) const
+    {
+        std::string token;
+
+        // print non-table elements first.
+        // ```toml
+        // [foo]         # a table we're writing now here
+        // key = "value" # <- non-table element, "key"
+        // # ...
+        // [foo.bar] # <- table element, "bar"
+        // ```
+        // because after printing [foo.bar], the remaining non-table values will
+        // be assigned into [foo.bar], not [foo]. Those values should be printed
+        // earlier.
+        for(const auto& kv : v)
+        {
+            if(kv.second.is_table() || is_array_of_tables(kv.second))
+            {
+                continue;
+            }
+
+            token += write_comments(kv.second);
+
+            const auto key_and_sep    = format_key(kv.first) + " = ";
+            const auto residual_width = (this->width_ > key_and_sep.size()) ?
+                                        this->width_ - key_and_sep.size() : 0;
+            token += key_and_sep;
+            token += visit(serializer(residual_width, this->float_prec_,
+                /*can be inlined*/ true, /*no comment*/ false, /*keys*/ {},
+                /*has_comment*/ !kv.second.comments().empty()), kv.second);
+
+            if(token.back() != '\n')
+            {
+                token += '\n';
+            }
+        }
+
+        // normal tables / array of tables
+
+        // after multiline table appeared, the other tables cannot be inline
+        // because the table would be assigned into the table.
+        // [foo]
+        // ...
+        // bar = {...} # <- bar will be a member of [foo].
+        bool multiline_table_printed = false;
+        for(const auto& kv : v)
+        {
+            if(!kv.second.is_table() && !is_array_of_tables(kv.second))
+            {
+                continue; // other stuff are already serialized. skip them.
+            }
+
+            std::vector<toml::key> ks(this->keys_);
+            ks.push_back(kv.first);
+
+            auto tmp = visit(serializer(this->width_, this->float_prec_,
+                !multiline_table_printed, this->no_comment_, ks,
+                /*has_comment*/ !kv.second.comments().empty()), kv.second);
+
+            // If it is the first time to print a multi-line table, it would be
+            // helpful to separate normal key-value pair and subtables by a
+            // newline.
+            // (this checks if the current key-value pair contains newlines.
+            //  but it is not perfect because multi-line string can also contain
+            //  a newline. in such a case, an empty line will be written) TODO
+            if((!multiline_table_printed) &&
+               std::find(tmp.cbegin(), tmp.cend(), '\n') != tmp.cend())
+            {
+                multiline_table_printed = true;
+                token += '\n'; // separate key-value pairs and subtables
+
+                token += write_comments(kv.second);
+                token += tmp;
+
+                // care about recursive tables (all tables in each level prints
+                // newline and there will be a full of newlines)
+                if(tmp.substr(tmp.size() - 2, 2) != "\n\n" &&
+                   tmp.substr(tmp.size() - 4, 4) != "\r\n\r\n" )
+                {
+                    token += '\n';
+                }
+            }
+            else
+            {
+                token += write_comments(kv.second);
+                token += tmp;
+                token += '\n';
+            }
+        }
+        return token;
+    }
+
+    std::string make_array_of_tables(const array_type& v) const
+    {
+        // if it's not inlined, we need to add `[[table.key]]`.
+        // but if it can be inlined, we can format it as the following.
+        // ```
+        // table.key = [
+        //   {...},
+        //   # comment
+        //   {...},
+        // ]
+        // ```
+        // This function checks if inlinization is possible or not, and then
+        // format the array-of-tables in a proper way.
+        //
+        // Note about comments:
+        //
+        // If the array itself has a comment (value_has_comment_ == true), we
+        // should try to make it inline.
+        // ```toml
+        // # comment about array
+        // array = [
+        //   # comment about table element
+        //   {of = "table"}
+        // ]
+        // ```
+        // If it is formatted as a multiline table, the two comments becomes
+        // indistinguishable.
+        // ```toml
+        // # comment about array
+        // # comment about table element
+        // [[array]]
+        // of = "table"
+        // ```
+        // So we need to try to make it inline, and it force-inlines regardless
+        // of the line width limit.
+        //     It may fail if the element of a table has comment. In that case,
+        // the array-of-tables will be formatted as a multiline table.
+        if(this->can_be_inlined_ || this->value_has_comment_)
+        {
+            std::string token;
+            if(!keys_.empty())
+            {
+                token += format_key(keys_.back());
+                token += " = ";
+            }
+
+            bool failed = false;
+            token += "[\n";
+            for(const auto& item : v)
+            {
+                // if an element of the table has a comment, the table
+                // cannot be inlined.
+                if(this->has_comment_inside(item.as_table()))
+                {
+                    failed = true;
+                    break;
+                }
+                // write comments for the table itself
+                token += write_comments(item);
+
+                const auto t = this->make_inline_table(item.as_table());
+
+                if(t.size() + 1 > width_ || // +1 for the last comma {...},
+                   std::find(t.cbegin(), t.cend(), '\n') != t.cend())
+                {
+                    // if the value itself has a comment, ignore the line width limit
+                    if( ! this->value_has_comment_)
+                    {
+                        failed = true;
+                        break;
+                    }
+                }
+                token += t;
+                token += ",\n";
+            }
+
+            if( ! failed)
+            {
+                token += "]\n";
+                return token;
+            }
+            // if failed, serialize them as [[array.of.tables]].
+        }
+
+        std::string token;
+        for(const auto& item : v)
+        {
+            token += write_comments(item);
+            token += "[[";
+            token += format_keys(keys_);
+            token += "]]\n";
+            token += this->make_multiline_table(item.as_table());
+        }
+        return token;
+    }
+
+    std::string write_comments(const value_type& v) const
+    {
+        std::string retval;
+        if(this->no_comment_) {return retval;}
+
+        for(const auto& c : v.comments())
+        {
+            retval += '#';
+            retval += c;
+            retval += '\n';
+        }
+        return retval;
+    }
+
+    bool is_array_of_tables(const value_type& v) const
+    {
+        if(!v.is_array() || v.as_array().empty()) {return false;}
+        return is_array_of_tables(v.as_array());
+    }
+    bool is_array_of_tables(const array_type& v) const
+    {
+        // Since TOML v0.5.0, heterogeneous arrays are allowed. So we need to
+        // check all the element in an array to check if the array is an array
+        // of tables.
+        return std::all_of(v.begin(), v.end(), [](const value_type& elem) {
+                return elem.is_table();
+            });
+    }
+
+  private:
+
+    bool        can_be_inlined_;
+    bool        no_comment_;
+    bool        value_has_comment_;
+    int         float_prec_;
+    std::size_t width_;
+    std::vector<toml::key> keys_;
+};
+
+template<typename C,
+         template<typename ...> class M, template<typename ...> class V>
+std::string
+format(const basic_value<C, M, V>& v, std::size_t w = 80u,
+       int fprec = std::numeric_limits<toml::floating>::max_digits10,
+       bool no_comment = false, bool force_inline = false)
+{
+    using value_type = basic_value<C, M, V>;
+    // if value is a table, it is considered to be a root object.
+    // the root object can't be an inline table.
+    if(v.is_table())
+    {
+        std::ostringstream oss;
+        if(!v.comments().empty())
+        {
+            oss << v.comments();
+            oss << '\n'; // to split the file comment from the first element
+        }
+        const auto serialized = visit(serializer<value_type>(w, fprec, false, no_comment), v);
+        oss << serialized;
+        return oss.str();
+    }
+    return visit(serializer<value_type>(w, fprec, force_inline), v);
+}
+
+namespace detail
+{
+template<typename charT, typename traits>
+int comment_index(std::basic_ostream<charT, traits>&)
+{
+    static const int index = std::ios_base::xalloc();
+    return index;
+}
+} // detail
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+nocomment(std::basic_ostream<charT, traits>& os)
+{
+    // by default, it is zero. and by default, it shows comments.
+    os.iword(detail::comment_index(os)) = 1;
+    return os;
+}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+showcomment(std::basic_ostream<charT, traits>& os)
+{
+    // by default, it is zero. and by default, it shows comments.
+    os.iword(detail::comment_index(os)) = 0;
+    return os;
+}
+
+template<typename charT, typename traits, typename C,
+         template<typename ...> class M, template<typename ...> class V>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const basic_value<C, M, V>& v)
+{
+    using value_type = basic_value<C, M, V>;
+
+    // get status of std::setw().
+    const auto w     = static_cast<std::size_t>(os.width());
+    const int  fprec = static_cast<int>(os.precision());
+    os.width(0);
+
+    // by default, iword is initialized by 0. And by default, toml11 outputs
+    // comments. So `0` means showcomment. 1 means nocommnet.
+    const bool no_comment = (1 == os.iword(detail::comment_index(os)));
+
+    if(!no_comment && v.is_table() && !v.comments().empty())
+    {
+        os << v.comments();
+        os << '\n'; // to split the file comment from the first element
+    }
+    // the root object can't be an inline table. so pass `false`.
+    const auto serialized = visit(serializer<value_type>(w, fprec, no_comment, false), v);
+    os << serialized;
+
+    // if v is a non-table value, and has only one comment, then
+    // put a comment just after a value. in the following way.
+    //
+    // ```toml
+    // key = "value" # comment.
+    // ```
+    //
+    // Since the top-level toml object is a table, one who want to put a
+    // non-table toml value must use this in a following way.
+    //
+    // ```cpp
+    // toml::value v;
+    // std::cout << "user-defined-key = " << v << std::endl;
+    // ```
+    //
+    // In this case, it is impossible to put comments before key-value pair.
+    // The only way to preserve comments is to put all of them after a value.
+    if(!no_comment && !v.is_table() && !v.comments().empty())
+    {
+        os << " #";
+        for(const auto& c : v.comments()) {os << c;}
+    }
+    return os;
+}
+
+} // toml
+#endif// TOML11_SERIALIZER_HPP
diff --git a/src/toml11/toml/source_location.hpp b/src/toml11/toml/source_location.hpp
new file mode 100644
index 000000000..fa175b5b4
--- /dev/null
+++ b/src/toml11/toml/source_location.hpp
@@ -0,0 +1,233 @@
+//     Copyright Toru Niina 2019.
+// Distributed under the MIT License.
+#ifndef TOML11_SOURCE_LOCATION_HPP
+#define TOML11_SOURCE_LOCATION_HPP
+#include <cstdint>
+#include <sstream>
+
+#include "region.hpp"
+
+namespace toml
+{
+
+// A struct to contain location in a toml file.
+// The interface imitates std::experimental::source_location,
+// but not completely the same.
+//
+// It would be constructed by toml::value. It can be used to generate
+// user-defined error messages.
+//
+// - std::uint_least32_t line() const noexcept
+//   - returns the line number where the region is on.
+// - std::uint_least32_t column() const noexcept
+//   - returns the column number where the region starts.
+// - std::uint_least32_t region() const noexcept
+//   - returns the size of the region.
+//
+// +-- line()       +-- region of interest (region() == 9)
+// v            .---+---.
+// 12 | value = "foo bar"
+//              ^
+//              +-- column()
+//
+// - std::string const& file_name() const noexcept;
+//   - name of the file.
+// - std::string const& line_str() const noexcept;
+//   - the whole line that contains the region of interest.
+//
+struct source_location
+{
+  public:
+
+    source_location()
+        : line_num_(1), column_num_(1), region_size_(1),
+          file_name_("unknown file"), line_str_("")
+    {}
+
+    explicit source_location(const detail::region_base* reg)
+        : line_num_(1), column_num_(1), region_size_(1),
+          file_name_("unknown file"), line_str_("")
+    {
+        if(reg)
+        {
+            if(reg->line_num() != detail::region_base().line_num())
+            {
+                line_num_ = static_cast<std::uint_least32_t>(
+                        std::stoul(reg->line_num()));
+            }
+            column_num_  = static_cast<std::uint_least32_t>(reg->before() + 1);
+            region_size_ = static_cast<std::uint_least32_t>(reg->size());
+            file_name_   = reg->name();
+            line_str_    = reg->line();
+        }
+    }
+
+    explicit source_location(const detail::region& reg)
+        : line_num_(static_cast<std::uint_least32_t>(std::stoul(reg.line_num()))),
+          column_num_(static_cast<std::uint_least32_t>(reg.before() + 1)),
+          region_size_(static_cast<std::uint_least32_t>(reg.size())),
+          file_name_(reg.name()),
+          line_str_ (reg.line())
+    {}
+    explicit source_location(const detail::location& loc)
+        : line_num_(static_cast<std::uint_least32_t>(std::stoul(loc.line_num()))),
+          column_num_(static_cast<std::uint_least32_t>(loc.before() + 1)),
+          region_size_(static_cast<std::uint_least32_t>(loc.size())),
+          file_name_(loc.name()),
+          line_str_ (loc.line())
+    {}
+
+    ~source_location() = default;
+    source_location(source_location const&) = default;
+    source_location(source_location &&)     = default;
+    source_location& operator=(source_location const&) = default;
+    source_location& operator=(source_location &&)     = default;
+
+    std::uint_least32_t line()      const noexcept {return line_num_;}
+    std::uint_least32_t column()    const noexcept {return column_num_;}
+    std::uint_least32_t region()    const noexcept {return region_size_;}
+
+    std::string const&  file_name() const noexcept {return file_name_;}
+    std::string const&  line_str()  const noexcept {return line_str_;}
+
+  private:
+
+    std::uint_least32_t line_num_;
+    std::uint_least32_t column_num_;
+    std::uint_least32_t region_size_;
+    std::string         file_name_;
+    std::string         line_str_;
+};
+
+namespace detail
+{
+
+// internal error message generation.
+inline std::string format_underline(const std::string& message,
+        const std::vector<std::pair<source_location, std::string>>& loc_com,
+        const std::vector<std::string>& helps = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
+{
+    std::size_t line_num_width = 0;
+    for(const auto& lc : loc_com)
+    {
+        std::uint_least32_t line = lc.first.line();
+        std::size_t        digit = 0;
+        while(line != 0)
+        {
+            line  /= 10;
+            digit +=  1;
+        }
+        line_num_width = (std::max)(line_num_width, digit);
+    }
+    // 1 is the minimum width
+    line_num_width = std::max<std::size_t>(line_num_width, 1);
+
+    std::ostringstream retval;
+
+    if(colorize)
+    {
+        retval << color::colorize; // turn on ANSI color
+    }
+
+    // XXX
+    // Here, before `colorize` support, it does not output `[error]` prefix
+    // automatically. So some user may output it manually and this change may
+    // duplicate the prefix. To avoid it, check the first 7 characters and
+    // if it is "[error]", it removes that part from the message shown.
+    if(message.size() > 7 && message.substr(0, 7) == "[error]")
+    {
+        retval << color::bold << color::red << "[error]" << color::reset
+               << color::bold << message.substr(7) << color::reset << '\n';
+    }
+    else
+    {
+        retval << color::bold << color::red << "[error] " << color::reset
+               << color::bold << message << color::reset << '\n';
+    }
+
+    const auto format_one_location = [line_num_width]
+        (std::ostringstream& oss,
+         const source_location& loc, const std::string& comment) -> void
+        {
+            oss << ' ' << color::bold << color::blue
+                << std::setw(static_cast<int>(line_num_width))
+                << std::right << loc.line() << " | "  << color::reset
+                << loc.line_str() << '\n';
+
+            oss << make_string(line_num_width + 1, ' ')
+                << color::bold << color::blue << " | " << color::reset
+                << make_string(loc.column()-1 /*1-origin*/, ' ');
+
+            if(loc.region() == 1)
+            {
+                // invalid
+                // ^------
+                oss << color::bold << color::red << "^---" << color::reset;
+            }
+            else
+            {
+                // invalid
+                // ~~~~~~~
+                const auto underline_len = (std::min)(
+                    static_cast<std::size_t>(loc.region()), loc.line_str().size());
+                oss << color::bold << color::red
+                    << make_string(underline_len, '~') << color::reset;
+            }
+            oss << ' ';
+            oss << comment;
+            return;
+        };
+
+    assert(!loc_com.empty());
+
+    // --> example.toml
+    //   |
+    retval << color::bold << color::blue << " --> " << color::reset
+           << loc_com.front().first.file_name() << '\n';
+    retval << make_string(line_num_width + 1, ' ')
+           << color::bold << color::blue << " |\n"  << color::reset;
+    // 1 | key value
+    //   |    ^--- missing =
+    format_one_location(retval, loc_com.front().first, loc_com.front().second);
+
+    // process the rest of the locations
+    for(std::size_t i=1; i<loc_com.size(); ++i)
+    {
+        const auto& prev = loc_com.at(i-1);
+        const auto& curr = loc_com.at(i);
+
+        retval << '\n';
+        // if the filenames are the same, print "..."
+        if(prev.first.file_name() == curr.first.file_name())
+        {
+            retval << color::bold << color::blue << " ...\n" << color::reset;
+        }
+        else // if filename differs, print " --> filename.toml" again
+        {
+            retval << color::bold << color::blue << " --> " << color::reset
+                   << curr.first.file_name() << '\n';
+            retval << make_string(line_num_width + 1, ' ')
+                   << color::bold << color::blue << " |\n"  << color::reset;
+        }
+
+        format_one_location(retval, curr.first, curr.second);
+    }
+
+    if(!helps.empty())
+    {
+        retval << '\n';
+        retval << make_string(line_num_width + 1, ' ');
+        retval << color::bold << color::blue << " |" << color::reset;
+        for(const auto& help : helps)
+        {
+            retval << color::bold << "\nHint: " << color::reset;
+            retval << help;
+        }
+    }
+    return retval.str();
+}
+
+} // detail
+} // toml
+#endif// TOML11_SOURCE_LOCATION_HPP
diff --git a/src/toml11/toml/storage.hpp b/src/toml11/toml/storage.hpp
new file mode 100644
index 000000000..202f9035f
--- /dev/null
+++ b/src/toml11/toml/storage.hpp
@@ -0,0 +1,43 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_STORAGE_HPP
+#define TOML11_STORAGE_HPP
+#include "utility.hpp"
+
+namespace toml
+{
+namespace detail
+{
+
+// this contains pointer and deep-copy the content if copied.
+// to avoid recursive pointer.
+template<typename T>
+struct storage
+{
+    using value_type = T;
+
+    explicit storage(value_type const& v): ptr(toml::make_unique<T>(v)) {}
+    explicit storage(value_type&&      v): ptr(toml::make_unique<T>(std::move(v))) {}
+    ~storage() = default;
+    storage(const storage& rhs): ptr(toml::make_unique<T>(*rhs.ptr)) {}
+    storage& operator=(const storage& rhs)
+    {
+        this->ptr = toml::make_unique<T>(*rhs.ptr);
+        return *this;
+    }
+    storage(storage&&) = default;
+    storage& operator=(storage&&) = default;
+
+    bool is_ok() const noexcept {return static_cast<bool>(ptr);}
+
+    value_type&       value() &      noexcept {return *ptr;}
+    value_type const& value() const& noexcept {return *ptr;}
+    value_type&&      value() &&     noexcept {return std::move(*ptr);}
+
+  private:
+    std::unique_ptr<value_type> ptr;
+};
+
+} // detail
+} // toml
+#endif// TOML11_STORAGE_HPP
diff --git a/src/toml11/toml/string.hpp b/src/toml11/toml/string.hpp
new file mode 100644
index 000000000..5136d8c56
--- /dev/null
+++ b/src/toml11/toml/string.hpp
@@ -0,0 +1,225 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_STRING_HPP
+#define TOML11_STRING_HPP
+#include <cstdint>
+
+#include <algorithm>
+#include <string>
+
+#if __cplusplus >= 201703L
+#if __has_include(<string_view>)
+#define TOML11_USING_STRING_VIEW 1
+#include <string_view>
+#endif
+#endif
+
+namespace toml
+{
+
+enum class string_t : std::uint8_t
+{
+    basic   = 0,
+    literal = 1,
+};
+
+struct string
+{
+    string()  = default;
+    ~string() = default;
+    string(const string& s) = default;
+    string(string&& s)      = default;
+    string& operator=(const string& s) = default;
+    string& operator=(string&& s)      = default;
+
+    string(const std::string& s): kind(string_t::basic), str(s){}
+    string(const std::string& s, string_t k):   kind(k), str(s){}
+    string(const char* s):        kind(string_t::basic), str(s){}
+    string(const char* s,        string_t k):   kind(k), str(s){}
+
+    string(std::string&& s): kind(string_t::basic), str(std::move(s)){}
+    string(std::string&& s, string_t k):   kind(k), str(std::move(s)){}
+
+    string& operator=(const std::string& s)
+    {kind = string_t::basic; str = s; return *this;}
+    string& operator=(std::string&& s)
+    {kind = string_t::basic; str = std::move(s); return *this;}
+
+    operator std::string&       () &      noexcept {return str;}
+    operator std::string const& () const& noexcept {return str;}
+    operator std::string&&      () &&     noexcept {return std::move(str);}
+
+    string& operator+=(const char*        rhs) {str += rhs; return *this;}
+    string& operator+=(const char         rhs) {str += rhs; return *this;}
+    string& operator+=(const std::string& rhs) {str += rhs; return *this;}
+    string& operator+=(const string&      rhs) {str += rhs.str; return *this;}
+
+#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
+    explicit string(std::string_view s): kind(string_t::basic), str(s){}
+    string(std::string_view s, string_t k): kind(k), str(s){}
+
+    string& operator=(std::string_view s)
+    {kind = string_t::basic; str = s; return *this;}
+
+    explicit operator std::string_view() const noexcept
+    {return std::string_view(str);}
+
+    string& operator+=(const std::string_view& rhs) {str += rhs; return *this;}
+#endif
+
+    string_t    kind;
+    std::string str;
+};
+
+inline bool operator==(const string& lhs, const string& rhs)
+{
+    return lhs.kind == rhs.kind && lhs.str == rhs.str;
+}
+inline bool operator!=(const string& lhs, const string& rhs)
+{
+    return !(lhs == rhs);
+}
+inline bool operator<(const string& lhs, const string& rhs)
+{
+    return (lhs.kind == rhs.kind) ? (lhs.str < rhs.str) : (lhs.kind < rhs.kind);
+}
+inline bool operator>(const string& lhs, const string& rhs)
+{
+    return rhs < lhs;
+}
+inline bool operator<=(const string& lhs, const string& rhs)
+{
+    return !(rhs < lhs);
+}
+inline bool operator>=(const string& lhs, const string& rhs)
+{
+    return !(lhs < rhs);
+}
+
+inline bool
+operator==(const string& lhs, const std::string& rhs) {return lhs.str == rhs;}
+inline bool
+operator!=(const string& lhs, const std::string& rhs) {return lhs.str != rhs;}
+inline bool
+operator< (const string& lhs, const std::string& rhs) {return lhs.str <  rhs;}
+inline bool
+operator> (const string& lhs, const std::string& rhs) {return lhs.str >  rhs;}
+inline bool
+operator<=(const string& lhs, const std::string& rhs) {return lhs.str <= rhs;}
+inline bool
+operator>=(const string& lhs, const std::string& rhs) {return lhs.str >= rhs;}
+
+inline bool
+operator==(const std::string& lhs, const string& rhs) {return lhs == rhs.str;}
+inline bool
+operator!=(const std::string& lhs, const string& rhs) {return lhs != rhs.str;}
+inline bool
+operator< (const std::string& lhs, const string& rhs) {return lhs <  rhs.str;}
+inline bool
+operator> (const std::string& lhs, const string& rhs) {return lhs >  rhs.str;}
+inline bool
+operator<=(const std::string& lhs, const string& rhs) {return lhs <= rhs.str;}
+inline bool
+operator>=(const std::string& lhs, const string& rhs) {return lhs >= rhs.str;}
+
+inline bool
+operator==(const string& lhs, const char* rhs) {return lhs.str == std::string(rhs);}
+inline bool
+operator!=(const string& lhs, const char* rhs) {return lhs.str != std::string(rhs);}
+inline bool
+operator< (const string& lhs, const char* rhs) {return lhs.str <  std::string(rhs);}
+inline bool
+operator> (const string& lhs, const char* rhs) {return lhs.str >  std::string(rhs);}
+inline bool
+operator<=(const string& lhs, const char* rhs) {return lhs.str <= std::string(rhs);}
+inline bool
+operator>=(const string& lhs, const char* rhs) {return lhs.str >= std::string(rhs);}
+
+inline bool
+operator==(const char* lhs, const string& rhs) {return std::string(lhs) == rhs.str;}
+inline bool
+operator!=(const char* lhs, const string& rhs) {return std::string(lhs) != rhs.str;}
+inline bool
+operator< (const char* lhs, const string& rhs) {return std::string(lhs) <  rhs.str;}
+inline bool
+operator> (const char* lhs, const string& rhs) {return std::string(lhs) >  rhs.str;}
+inline bool
+operator<=(const char* lhs, const string& rhs) {return std::string(lhs) <= rhs.str;}
+inline bool
+operator>=(const char* lhs, const string& rhs) {return std::string(lhs) >= rhs.str;}
+
+template<typename charT, typename traits>
+std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, const string& s)
+{
+    if(s.kind == string_t::basic)
+    {
+        if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend())
+        {
+            // it contains newline. make it multiline string.
+            os << "\"\"\"\n";
+            for(auto i=s.str.cbegin(), e=s.str.cend(); i!=e; ++i)
+            {
+                switch(*i)
+                {
+                    case '\\': {os << "\\\\"; break;}
+                    case '\"': {os << "\\\""; break;}
+                    case '\b': {os << "\\b";  break;}
+                    case '\t': {os << "\\t";  break;}
+                    case '\f': {os << "\\f";  break;}
+                    case '\n': {os << '\n';   break;}
+                    case '\r':
+                    {
+                        // since it is a multiline string,
+                        // CRLF is not needed to be escaped.
+                        if(std::next(i) != e && *std::next(i) == '\n')
+                        {
+                            os << "\r\n";
+                            ++i;
+                        }
+                        else
+                        {
+                            os << "\\r";
+                        }
+                        break;
+                    }
+                    default: {os << *i; break;}
+                }
+            }
+            os << "\\\n\"\"\"";
+            return os;
+        }
+        // no newline. make it inline.
+        os << "\"";
+        for(const auto c : s.str)
+        {
+            switch(c)
+            {
+                case '\\': {os << "\\\\"; break;}
+                case '\"': {os << "\\\""; break;}
+                case '\b': {os << "\\b";  break;}
+                case '\t': {os << "\\t";  break;}
+                case '\f': {os << "\\f";  break;}
+                case '\n': {os << "\\n";  break;}
+                case '\r': {os << "\\r";  break;}
+                default  : {os << c;      break;}
+            }
+        }
+        os << "\"";
+        return os;
+    }
+    // the string `s` is literal-string.
+    if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
+       std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
+    {
+        // contains newline or single quote. make it multiline.
+        os << "'''\n" << s.str << "'''";
+        return os;
+    }
+    // normal literal string
+    os << '\'' << s.str << '\'';
+    return os;
+}
+
+} // toml
+#endif// TOML11_STRING_H
diff --git a/src/toml11/toml/traits.hpp b/src/toml11/toml/traits.hpp
new file mode 100644
index 000000000..5495c93b2
--- /dev/null
+++ b/src/toml11/toml/traits.hpp
@@ -0,0 +1,327 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_TRAITS_HPP
+#define TOML11_TRAITS_HPP
+
+#include "from.hpp"
+#include "into.hpp"
+
+#include <chrono>
+#include <forward_list>
+#include <string>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+#if __cplusplus >= 201703L
+#if __has_include(<string_view>)
+#include <string_view>
+#endif // has_include(<string_view>)
+#endif // cplusplus   >= C++17
+
+namespace toml
+{
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+class basic_value;
+
+namespace detail
+{
+// ---------------------------------------------------------------------------
+// check whether type T is a kind of container/map class
+
+struct has_iterator_impl
+{
+    template<typename T> static std::true_type  check(typename T::iterator*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_value_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::value_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_key_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::key_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_mapped_type_impl
+{
+    template<typename T> static std::true_type  check(typename T::mapped_type*);
+    template<typename T> static std::false_type check(...);
+};
+struct has_reserve_method_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>().reserve(std::declval<std::size_t>()))*);
+};
+struct has_push_back_method_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>().push_back(std::declval<typename T::value_type>()))*);
+};
+struct is_comparable_impl
+{
+    template<typename T> static std::false_type check(...);
+    template<typename T> static std::true_type  check(
+        decltype(std::declval<T>() < std::declval<T>())*);
+};
+
+struct has_from_toml_method_impl
+{
+    template<typename T, typename C,
+             template<typename ...> class Tb, template<typename ...> class A>
+    static std::true_type  check(
+        decltype(std::declval<T>().from_toml(
+                std::declval<::toml::basic_value<C, Tb, A>>()))*);
+
+    template<typename T, typename C,
+             template<typename ...> class Tb, template<typename ...> class A>
+    static std::false_type check(...);
+};
+struct has_into_toml_method_impl
+{
+    template<typename T>
+    static std::true_type  check(decltype(std::declval<T>().into_toml())*);
+    template<typename T>
+    static std::false_type check(...);
+};
+
+struct has_specialized_from_impl
+{
+    template<typename T>
+    static std::false_type check(...);
+    template<typename T, std::size_t S = sizeof(::toml::from<T>)>
+    static std::true_type check(::toml::from<T>*);
+};
+struct has_specialized_into_impl
+{
+    template<typename T>
+    static std::false_type check(...);
+    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
+    static std::true_type check(::toml::from<T>*);
+};
+
+
+/// Intel C++ compiler can not use decltype in parent class declaration, here
+/// is a hack to work around it. https://stackoverflow.com/a/23953090/4692076
+#ifdef __INTEL_COMPILER
+#define decltype(...) std::enable_if<true, decltype(__VA_ARGS__)>::type
+#endif
+
+template<typename T>
+struct has_iterator    : decltype(has_iterator_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_value_type  : decltype(has_value_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_key_type    : decltype(has_key_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_mapped_type : decltype(has_mapped_type_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_reserve_method : decltype(has_reserve_method_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_push_back_method : decltype(has_push_back_method_impl::check<T>(nullptr)){};
+template<typename T>
+struct is_comparable : decltype(is_comparable_impl::check<T>(nullptr)){};
+
+template<typename T, typename C,
+         template<typename ...> class Tb, template<typename ...> class A>
+struct has_from_toml_method
+: decltype(has_from_toml_method_impl::check<T, C, Tb, A>(nullptr)){};
+
+template<typename T>
+struct has_into_toml_method
+: decltype(has_into_toml_method_impl::check<T>(nullptr)){};
+
+template<typename T>
+struct has_specialized_from : decltype(has_specialized_from_impl::check<T>(nullptr)){};
+template<typename T>
+struct has_specialized_into : decltype(has_specialized_into_impl::check<T>(nullptr)){};
+
+#ifdef __INTEL_COMPILER
+#undef decltype
+#endif
+
+// ---------------------------------------------------------------------------
+// C++17 and/or/not
+
+#if __cplusplus >= 201703L
+
+using std::conjunction;
+using std::disjunction;
+using std::negation;
+
+#else
+
+template<typename ...> struct conjunction : std::true_type{};
+template<typename T>   struct conjunction<T> : T{};
+template<typename T, typename ... Ts>
+struct conjunction<T, Ts...> :
+    std::conditional<static_cast<bool>(T::value), conjunction<Ts...>, T>::type
+{};
+
+template<typename ...> struct disjunction : std::false_type{};
+template<typename T>   struct disjunction<T> : T {};
+template<typename T, typename ... Ts>
+struct disjunction<T, Ts...> :
+    std::conditional<static_cast<bool>(T::value), T, disjunction<Ts...>>::type
+{};
+
+template<typename T>
+struct negation : std::integral_constant<bool, !static_cast<bool>(T::value)>{};
+
+#endif
+
+// ---------------------------------------------------------------------------
+// type checkers
+
+template<typename T> struct is_std_pair : std::false_type{};
+template<typename T1, typename T2>
+struct is_std_pair<std::pair<T1, T2>> : std::true_type{};
+
+template<typename T> struct is_std_tuple : std::false_type{};
+template<typename ... Ts>
+struct is_std_tuple<std::tuple<Ts...>> : std::true_type{};
+
+template<typename T> struct is_std_forward_list : std::false_type{};
+template<typename T>
+struct is_std_forward_list<std::forward_list<T>> : std::true_type{};
+
+template<typename T> struct is_chrono_duration: std::false_type{};
+template<typename Rep, typename Period>
+struct is_chrono_duration<std::chrono::duration<Rep, Period>>: std::true_type{};
+
+template<typename T>
+struct is_map : conjunction< // map satisfies all the following conditions
+    has_iterator<T>,         // has T::iterator
+    has_value_type<T>,       // has T::value_type
+    has_key_type<T>,         // has T::key_type
+    has_mapped_type<T>       // has T::mapped_type
+    >{};
+template<typename T> struct is_map<T&>                : is_map<T>{};
+template<typename T> struct is_map<T const&>          : is_map<T>{};
+template<typename T> struct is_map<T volatile&>       : is_map<T>{};
+template<typename T> struct is_map<T const volatile&> : is_map<T>{};
+
+template<typename T>
+struct is_container : conjunction<
+    negation<is_map<T>>,                         // not a map
+    negation<std::is_same<T, std::string>>,      // not a std::string
+#if __cplusplus >= 201703L
+#if __has_include(<string_view>)
+    negation<std::is_same<T, std::string_view>>, // not a std::string_view
+#endif // has_include(<string_view>)
+#endif
+    has_iterator<T>,                             // has T::iterator
+    has_value_type<T>                            // has T::value_type
+    >{};
+template<typename T> struct is_container<T&>                : is_container<T>{};
+template<typename T> struct is_container<T const&>          : is_container<T>{};
+template<typename T> struct is_container<T volatile&>       : is_container<T>{};
+template<typename T> struct is_container<T const volatile&> : is_container<T>{};
+
+template<typename T>
+struct is_basic_value: std::false_type{};
+template<typename T> struct is_basic_value<T&>                : is_basic_value<T>{};
+template<typename T> struct is_basic_value<T const&>          : is_basic_value<T>{};
+template<typename T> struct is_basic_value<T volatile&>       : is_basic_value<T>{};
+template<typename T> struct is_basic_value<T const volatile&> : is_basic_value<T>{};
+template<typename C, template<typename ...> class M, template<typename ...> class V>
+struct is_basic_value<::toml::basic_value<C, M, V>>: std::true_type{};
+
+// ---------------------------------------------------------------------------
+// C++14 index_sequence
+
+#if __cplusplus >= 201402L
+
+using std::index_sequence;
+using std::make_index_sequence;
+
+#else
+
+template<std::size_t ... Ns> struct index_sequence{};
+
+template<typename IS, std::size_t N> struct push_back_index_sequence{};
+template<std::size_t N, std::size_t ... Ns>
+struct push_back_index_sequence<index_sequence<Ns...>, N>
+{
+    typedef index_sequence<Ns..., N> type;
+};
+
+template<std::size_t N>
+struct index_sequence_maker
+{
+    typedef typename push_back_index_sequence<
+        typename index_sequence_maker<N-1>::type, N>::type type;
+};
+template<>
+struct index_sequence_maker<0>
+{
+    typedef index_sequence<0> type;
+};
+template<std::size_t N>
+using make_index_sequence = typename index_sequence_maker<N-1>::type;
+
+#endif // __cplusplus >= 2014
+
+// ---------------------------------------------------------------------------
+// C++14 enable_if_t
+
+#if __cplusplus >= 201402L
+
+using std::enable_if_t;
+
+#else
+
+template<bool B, typename T>
+using enable_if_t = typename std::enable_if<B, T>::type;
+
+#endif // __cplusplus >= 2014
+
+// ---------------------------------------------------------------------------
+// return_type_of_t
+
+#if __cplusplus >= 201703L && defined(__cpp_lib_is_invocable) && __cpp_lib_is_invocable>=201703
+
+template<typename F, typename ... Args>
+using return_type_of_t = std::invoke_result_t<F, Args...>;
+
+#else
+// result_of is deprecated after C++17
+template<typename F, typename ... Args>
+using return_type_of_t = typename std::result_of<F(Args...)>::type;
+
+#endif
+
+// ---------------------------------------------------------------------------
+// is_string_literal
+//
+// to use this, pass `typename remove_reference<T>::type` to T.
+
+template<typename T>
+struct is_string_literal:
+disjunction<
+    std::is_same<const char*, T>,
+    conjunction<
+        std::is_array<T>,
+        std::is_same<const char, typename std::remove_extent<T>::type>
+        >
+    >{};
+
+// ---------------------------------------------------------------------------
+// C++20 remove_cvref_t
+
+template<typename T>
+struct remove_cvref
+{
+    using type = typename std::remove_cv<
+        typename std::remove_reference<T>::type>::type;
+};
+
+template<typename T>
+using remove_cvref_t = typename remove_cvref<T>::type;
+
+}// detail
+}//toml
+#endif // TOML_TRAITS
diff --git a/src/toml11/toml/types.hpp b/src/toml11/toml/types.hpp
new file mode 100644
index 000000000..1e420e7fd
--- /dev/null
+++ b/src/toml11/toml/types.hpp
@@ -0,0 +1,173 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_TYPES_HPP
+#define TOML11_TYPES_HPP
+#include <unordered_map>
+#include <vector>
+
+#include "comments.hpp"
+#include "datetime.hpp"
+#include "string.hpp"
+#include "traits.hpp"
+
+namespace toml
+{
+
+template<typename Comment, // discard/preserve_comment
+         template<typename ...> class Table, // map-like class
+         template<typename ...> class Array> // vector-like class
+class basic_value;
+
+using character = char;
+using key = std::string;
+
+#if !defined(__clang__) && defined(__GNUC__) && __GNUC__ <= 4
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wshadow"
+#endif
+
+using boolean        = bool;
+using integer        = std::int64_t;
+using floating       = double; // "float" is a keyword, cannot use it here.
+// the following stuffs are structs defined here, so aliases are not needed.
+// - string
+// - offset_datetime
+// - offset_datetime
+// - local_datetime
+// - local_date
+// - local_time
+
+#if defined(__GNUC__) && !defined(__clang__)
+#  pragma GCC diagnostic pop
+#endif
+
+// default toml::value and default array/table. these are defined after defining
+// basic_value itself.
+// using value = basic_value<discard_comments, std::unordered_map, std::vector>;
+// using array = typename value::array_type;
+// using table = typename value::table_type;
+
+// to avoid warnings about `value_t::integer` is "shadowing" toml::integer in
+// GCC -Wshadow=global.
+#if defined(__GNUC__) && !defined(__clang__)
+#  pragma GCC diagnostic push
+#  if 7 <= __GNUC__
+#    pragma GCC diagnostic ignored "-Wshadow=global"
+#  else // gcc-6 or older
+#    pragma GCC diagnostic ignored "-Wshadow"
+#  endif
+#endif
+enum class value_t : std::uint8_t
+{
+    empty           =  0,
+    boolean         =  1,
+    integer         =  2,
+    floating        =  3,
+    string          =  4,
+    offset_datetime =  5,
+    local_datetime  =  6,
+    local_date      =  7,
+    local_time      =  8,
+    array           =  9,
+    table           = 10,
+};
+#if defined(__GNUC__) && !defined(__clang__)
+#  pragma GCC diagnostic pop
+#endif
+
+template<typename charT, typename traits>
+inline std::basic_ostream<charT, traits>&
+operator<<(std::basic_ostream<charT, traits>& os, value_t t)
+{
+    switch(t)
+    {
+        case value_t::boolean         : os << "boolean";         return os;
+        case value_t::integer         : os << "integer";         return os;
+        case value_t::floating        : os << "floating";        return os;
+        case value_t::string          : os << "string";          return os;
+        case value_t::offset_datetime : os << "offset_datetime"; return os;
+        case value_t::local_datetime  : os << "local_datetime";  return os;
+        case value_t::local_date      : os << "local_date";      return os;
+        case value_t::local_time      : os << "local_time";      return os;
+        case value_t::array           : os << "array";           return os;
+        case value_t::table           : os << "table";           return os;
+        case value_t::empty           : os << "empty";           return os;
+        default                       : os << "unknown";         return os;
+    }
+}
+
+template<typename charT = char,
+         typename traits = std::char_traits<charT>,
+         typename alloc = std::allocator<charT>>
+inline std::basic_string<charT, traits, alloc> stringize(value_t t)
+{
+    std::basic_ostringstream<charT, traits, alloc> oss;
+    oss << t;
+    return oss.str();
+}
+
+namespace detail
+{
+
+// helper to define a type that represents a value_t value.
+template<value_t V>
+using value_t_constant = std::integral_constant<value_t, V>;
+
+// meta-function that convertes from value_t to the exact toml type that corresponds to.
+// It takes toml::basic_value type because array and table types depend on it.
+template<value_t t, typename Value> struct enum_to_type                      {using type = void                      ;};
+template<typename Value> struct enum_to_type<value_t::empty          , Value>{using type = void                      ;};
+template<typename Value> struct enum_to_type<value_t::boolean        , Value>{using type = boolean                   ;};
+template<typename Value> struct enum_to_type<value_t::integer        , Value>{using type = integer                   ;};
+template<typename Value> struct enum_to_type<value_t::floating       , Value>{using type = floating                  ;};
+template<typename Value> struct enum_to_type<value_t::string         , Value>{using type = string                    ;};
+template<typename Value> struct enum_to_type<value_t::offset_datetime, Value>{using type = offset_datetime           ;};
+template<typename Value> struct enum_to_type<value_t::local_datetime , Value>{using type = local_datetime            ;};
+template<typename Value> struct enum_to_type<value_t::local_date     , Value>{using type = local_date                ;};
+template<typename Value> struct enum_to_type<value_t::local_time     , Value>{using type = local_time                ;};
+template<typename Value> struct enum_to_type<value_t::array          , Value>{using type = typename Value::array_type;};
+template<typename Value> struct enum_to_type<value_t::table          , Value>{using type = typename Value::table_type;};
+
+// meta-function that converts from an exact toml type to the enum that corresponds to.
+template<typename T, typename Value>
+struct type_to_enum : std::conditional<
+    std::is_same<T, typename Value::array_type>::value, // if T == array_type,
+    value_t_constant<value_t::array>,                   // then value_t::array
+    typename std::conditional<                          // else...
+        std::is_same<T, typename Value::table_type>::value, // if T == table_type
+        value_t_constant<value_t::table>,               // then value_t::table
+        value_t_constant<value_t::empty>                // else value_t::empty
+        >::type
+    >::type {};
+template<typename Value> struct type_to_enum<boolean        , Value>: value_t_constant<value_t::boolean        > {};
+template<typename Value> struct type_to_enum<integer        , Value>: value_t_constant<value_t::integer        > {};
+template<typename Value> struct type_to_enum<floating       , Value>: value_t_constant<value_t::floating       > {};
+template<typename Value> struct type_to_enum<string         , Value>: value_t_constant<value_t::string         > {};
+template<typename Value> struct type_to_enum<offset_datetime, Value>: value_t_constant<value_t::offset_datetime> {};
+template<typename Value> struct type_to_enum<local_datetime , Value>: value_t_constant<value_t::local_datetime > {};
+template<typename Value> struct type_to_enum<local_date     , Value>: value_t_constant<value_t::local_date     > {};
+template<typename Value> struct type_to_enum<local_time     , Value>: value_t_constant<value_t::local_time     > {};
+
+// meta-function that checks the type T is the same as one of the toml::* types.
+template<typename T, typename Value>
+struct is_exact_toml_type : disjunction<
+    std::is_same<T, boolean        >,
+    std::is_same<T, integer        >,
+    std::is_same<T, floating       >,
+    std::is_same<T, string         >,
+    std::is_same<T, offset_datetime>,
+    std::is_same<T, local_datetime >,
+    std::is_same<T, local_date     >,
+    std::is_same<T, local_time     >,
+    std::is_same<T, typename Value::array_type>,
+    std::is_same<T, typename Value::table_type>
+    >{};
+template<typename T, typename V> struct is_exact_toml_type<T&, V>               : is_exact_toml_type<T, V>{};
+template<typename T, typename V> struct is_exact_toml_type<T const&, V>         : is_exact_toml_type<T, V>{};
+template<typename T, typename V> struct is_exact_toml_type<T volatile&, V>      : is_exact_toml_type<T, V>{};
+template<typename T, typename V> struct is_exact_toml_type<T const volatile&, V>: is_exact_toml_type<T, V>{};
+
+} // detail
+} // toml
+
+#endif// TOML11_TYPES_H
diff --git a/src/toml11/toml/utility.hpp b/src/toml11/toml/utility.hpp
new file mode 100644
index 000000000..4a6b4309d
--- /dev/null
+++ b/src/toml11/toml/utility.hpp
@@ -0,0 +1,149 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_UTILITY_HPP
+#define TOML11_UTILITY_HPP
+#include <memory>
+#include <sstream>
+#include <utility>
+
+#include "traits.hpp"
+
+#if __cplusplus >= 201402L
+#  define TOML11_MARK_AS_DEPRECATED(msg) [[deprecated(msg)]]
+#elif defined(__GNUC__)
+#  define TOML11_MARK_AS_DEPRECATED(msg) __attribute__((deprecated(msg)))
+#elif defined(_MSC_VER)
+#  define TOML11_MARK_AS_DEPRECATED(msg) __declspec(deprecated(msg))
+#else
+#  define TOML11_MARK_AS_DEPRECATED
+#endif
+
+namespace toml
+{
+
+#if __cplusplus >= 201402L
+
+using std::make_unique;
+
+#else
+
+template<typename T, typename ... Ts>
+inline std::unique_ptr<T> make_unique(Ts&& ... args)
+{
+    return std::unique_ptr<T>(new T(std::forward<Ts>(args)...));
+}
+
+#endif // __cplusplus >= 2014
+
+namespace detail
+{
+template<typename Container>
+void try_reserve_impl(Container& container, std::size_t N, std::true_type)
+{
+    container.reserve(N);
+    return;
+}
+template<typename Container>
+void try_reserve_impl(Container&, std::size_t, std::false_type) noexcept
+{
+    return;
+}
+} // detail
+
+template<typename Container>
+void try_reserve(Container& container, std::size_t N)
+{
+    if(N <= container.size()) {return;}
+    detail::try_reserve_impl(container, N, detail::has_reserve_method<Container>{});
+    return;
+}
+
+namespace detail
+{
+inline std::string concat_to_string_impl(std::ostringstream& oss)
+{
+    return oss.str();
+}
+template<typename T, typename ... Ts>
+std::string concat_to_string_impl(std::ostringstream& oss, T&& head, Ts&& ... tail)
+{
+    oss << std::forward<T>(head);
+    return concat_to_string_impl(oss, std::forward<Ts>(tail) ... );
+}
+} // detail
+
+template<typename ... Ts>
+std::string concat_to_string(Ts&& ... args)
+{
+    std::ostringstream oss;
+    oss << std::boolalpha << std::fixed;
+    return detail::concat_to_string_impl(oss, std::forward<Ts>(args) ...);
+}
+
+template<typename T>
+T from_string(const std::string& str, T opt)
+{
+    T v(opt);
+    std::istringstream iss(str);
+    iss >> v;
+    return v;
+}
+
+namespace detail
+{
+#if __cplusplus >= 201402L
+template<typename T>
+decltype(auto) last_one(T&& tail) noexcept
+{
+    return std::forward<T>(tail);
+}
+
+template<typename T, typename ... Ts>
+decltype(auto) last_one(T&& /*head*/, Ts&& ... tail) noexcept
+{
+    return last_one(std::forward<Ts>(tail)...);
+}
+#else // C++11
+// The following code
+// ```cpp
+//  1 | template<typename T, typename ... Ts>
+//  2 | auto last_one(T&& /*head*/, Ts&& ... tail)
+//  3 |  -> decltype(last_one(std::forward<Ts>(tail)...))
+//  4 | {
+//  5 |     return last_one(std::forward<Ts>(tail)...);
+//  6 | }
+// ```
+// does not work because the function `last_one(...)` is not yet defined at
+// line #3, so `decltype()` cannot deduce the type returned from `last_one`.
+// So we need to determine return type in a different way, like a meta func.
+
+template<typename T, typename ... Ts>
+struct last_one_in_pack
+{
+    using type = typename last_one_in_pack<Ts...>::type;
+};
+template<typename T>
+struct last_one_in_pack<T>
+{
+    using type = T;
+};
+template<typename ... Ts>
+using last_one_in_pack_t = typename last_one_in_pack<Ts...>::type;
+
+template<typename T>
+T&& last_one(T&& tail) noexcept
+{
+    return std::forward<T>(tail);
+}
+template<typename T, typename ... Ts>
+enable_if_t<(sizeof...(Ts) > 0), last_one_in_pack_t<Ts&& ...>>
+last_one(T&& /*head*/, Ts&& ... tail)
+{
+    return last_one(std::forward<Ts>(tail)...);
+}
+
+#endif
+} // detail
+
+}// toml
+#endif // TOML11_UTILITY
diff --git a/src/toml11/toml/value.hpp b/src/toml11/toml/value.hpp
new file mode 100644
index 000000000..1b43db8d4
--- /dev/null
+++ b/src/toml11/toml/value.hpp
@@ -0,0 +1,2035 @@
+//     Copyright Toru Niina 2017.
+// Distributed under the MIT License.
+#ifndef TOML11_VALUE_HPP
+#define TOML11_VALUE_HPP
+#include <cassert>
+
+#include "comments.hpp"
+#include "exception.hpp"
+#include "into.hpp"
+#include "region.hpp"
+#include "source_location.hpp"
+#include "storage.hpp"
+#include "traits.hpp"
+#include "types.hpp"
+#include "utility.hpp"
+
+namespace toml
+{
+
+namespace detail
+{
+
+// to show error messages. not recommended for users.
+template<typename Value>
+inline region_base const* get_region(const Value& v)
+{
+    return v.region_info_.get();
+}
+
+template<typename Value>
+void change_region(Value& v, region reg)
+{
+    v.region_info_ = std::make_shared<region>(std::move(reg));
+    return;
+}
+
+template<value_t Expected, typename Value>
+[[noreturn]] inline void
+throw_bad_cast(const std::string& funcname, value_t actual, const Value& v)
+{
+    throw type_error(detail::format_underline(
+        concat_to_string(funcname, "bad_cast to ", Expected), {
+            {v.location(), concat_to_string("the actual type is ", actual)}
+        }), v.location());
+}
+
+// Throw `out_of_range` from `toml::value::at()` and `toml::find()`
+// after generating an error message.
+//
+// The implementation is a bit complicated and there are many edge-cases.
+// If you are not interested in the error message generation, just skip this.
+template<typename Value>
+[[noreturn]] void
+throw_key_not_found_error(const Value& v, const key& ky)
+{
+    // The top-level table has its region at the first character of the file.
+    // That means that, in the case when a key is not found in the top-level
+    // table, the error message points to the first character. If the file has
+    // its first table at the first line, the error message would be like this.
+    // ```console
+    // [error] key "a" not found
+    //  --> example.toml
+    //    |
+    //  1 | [table]
+    //    | ^------ in this table
+    // ```
+    // It actually points to the top-level table at the first character,
+    // not `[table]`. But it is too confusing. To avoid the confusion, the error
+    // message should explicitly say "key not found in the top-level table",
+    // or "the parsed file is empty" if there is no content at all (0 bytes in file).
+    const auto loc = v.location();
+    if(loc.line() == 1 && loc.region() == 0)
+    {
+        // First line with a zero-length region means "empty file".
+        // The region will be generated at `parse_toml_file` function
+        // if the file contains no bytes.
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found in the top-level table"), {
+                {loc, "the parsed file is empty"}
+            }));
+    }
+    else if(loc.line() == 1 && loc.region() == 1)
+    {
+        // Here it assumes that top-level table starts at the first character.
+        // The region corresponds to the top-level table will be generated at
+        // `parse_toml_file` function.
+        //     It also assumes that the top-level table size is just one and
+        // the line number is `1`. It is always satisfied. And those conditions
+        // are satisfied only if the table is the top-level table.
+        //
+        // 1. one-character dot-key at the first line
+        // ```toml
+        // a.b = "c"
+        // ```
+        // toml11 counts whole key as the table key. Here, `a.b` is the region
+        // of the table "a". It could be counter intuitive, but it works.
+        // The size of the region is 3, not 1. The above example is the shortest
+        // dot-key example. The size cannot be 1.
+        //
+        // 2. one-character inline-table at the first line
+        // ```toml
+        // a = {b = "c"}
+        // ```
+        // toml11 considers the inline table body as the table region. Here,
+        // `{b = "c"}` is the region of the table "a". The size of the region
+        // is 9, not 1. The shotest inline table still has two characters, `{`
+        // and `}`. The size cannot be 1.
+        //
+        // 3. one-character table declaration at the first line
+        // ```toml
+        // [a]
+        // ```
+        // toml11 considers the whole table key as the table region. Here,
+        // `[a]` is the table region. The size is 3, not 1.
+        //
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found in the top-level table"), {
+                {loc, "the top-level table starts here"}
+            }));
+    }
+    else
+    {
+        // normal table.
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found"), { {loc, "in this table"} }));
+    }
+}
+
+// switch by `value_t` at the compile time.
+template<value_t T>
+struct switch_cast {};
+#define TOML11_GENERATE_SWITCH_CASTER(TYPE) \
+    template<>                                                           \
+    struct switch_cast<value_t::TYPE>                                    \
+    {                                                                    \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type& invoke(Value& v)             \
+        {                                                                \
+            return v.as_##TYPE();                                        \
+        }                                                                \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type const& invoke(const Value& v) \
+        {                                                                \
+            return v.as_##TYPE();                                        \
+        }                                                                \
+        template<typename Value>                                         \
+        static typename Value::TYPE##_type&& invoke(Value&& v)           \
+        {                                                                \
+            return std::move(v).as_##TYPE();                             \
+        }                                                                \
+    };                                                                   \
+    /**/
+TOML11_GENERATE_SWITCH_CASTER(boolean)
+TOML11_GENERATE_SWITCH_CASTER(integer)
+TOML11_GENERATE_SWITCH_CASTER(floating)
+TOML11_GENERATE_SWITCH_CASTER(string)
+TOML11_GENERATE_SWITCH_CASTER(offset_datetime)
+TOML11_GENERATE_SWITCH_CASTER(local_datetime)
+TOML11_GENERATE_SWITCH_CASTER(local_date)
+TOML11_GENERATE_SWITCH_CASTER(local_time)
+TOML11_GENERATE_SWITCH_CASTER(array)
+TOML11_GENERATE_SWITCH_CASTER(table)
+
+#undef TOML11_GENERATE_SWITCH_CASTER
+
+}// detail
+
+template<typename Comment, // discard/preserve_comment
+         template<typename ...> class Table = std::unordered_map,
+         template<typename ...> class Array = std::vector>
+class basic_value
+{
+    template<typename T, typename U>
+    static void assigner(T& dst, U&& v)
+    {
+        const auto tmp = ::new(std::addressof(dst)) T(std::forward<U>(v));
+        assert(tmp == std::addressof(dst));
+        (void)tmp;
+    }
+
+    using region_base = detail::region_base;
+
+    template<typename C, template<typename ...> class T,
+             template<typename ...> class A>
+    friend class basic_value;
+
+  public:
+
+    using comment_type         = Comment;
+    using key_type             = ::toml::key;
+    using value_type           = basic_value<comment_type, Table, Array>;
+    using boolean_type         = ::toml::boolean;
+    using integer_type         = ::toml::integer;
+    using floating_type        = ::toml::floating;
+    using string_type          = ::toml::string;
+    using local_time_type      = ::toml::local_time;
+    using local_date_type      = ::toml::local_date;
+    using local_datetime_type  = ::toml::local_datetime;
+    using offset_datetime_type = ::toml::offset_datetime;
+    using array_type           = Array<value_type>;
+    using table_type           = Table<key_type, value_type>;
+
+  public:
+
+    basic_value() noexcept
+        : type_(value_t::empty),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {}
+    ~basic_value() noexcept {this->cleanup();}
+
+    basic_value(const basic_value& v)
+        : type_(v.type()), region_info_(v.region_info_), comments_(v.comments_)
+    {
+        switch(v.type())
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          : assigner(array_          , v.array_          ); break;
+            case value_t::table          : assigner(table_          , v.table_          ); break;
+            default: break;
+        }
+    }
+    basic_value(basic_value&& v)
+        : type_(v.type()), region_info_(std::move(v.region_info_)),
+          comments_(std::move(v.comments_))
+    {
+        switch(this->type_) // here this->type_ is already initialized
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default: break;
+        }
+    }
+    basic_value& operator=(const basic_value& v)
+    {
+        this->cleanup();
+        this->region_info_ = v.region_info_;
+        this->comments_ = v.comments_;
+        this->type_ = v.type();
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          : assigner(array_          , v.array_          ); break;
+            case value_t::table          : assigner(table_          , v.table_          ); break;
+            default: break;
+        }
+        return *this;
+    }
+    basic_value& operator=(basic_value&& v)
+    {
+        this->cleanup();
+        this->region_info_ = std::move(v.region_info_);
+        this->comments_ = std::move(v.comments_);
+        this->type_ = v.type();
+        switch(this->type_)
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default: break;
+        }
+        return *this;
+    }
+
+    // overwrite comments ----------------------------------------------------
+
+    basic_value(const basic_value& v, std::vector<std::string> com)
+        : type_(v.type()), region_info_(v.region_info_),
+          comments_(std::move(com))
+    {
+        switch(v.type())
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          : assigner(array_          , v.array_          ); break;
+            case value_t::table          : assigner(table_          , v.table_          ); break;
+            default: break;
+        }
+    }
+
+    basic_value(basic_value&& v, std::vector<std::string> com)
+        : type_(v.type()), region_info_(std::move(v.region_info_)),
+          comments_(std::move(com))
+    {
+        switch(this->type_) // here this->type_ is already initialized
+        {
+            case value_t::boolean        : assigner(boolean_        , std::move(v.boolean_        )); break;
+            case value_t::integer        : assigner(integer_        , std::move(v.integer_        )); break;
+            case value_t::floating       : assigner(floating_       , std::move(v.floating_       )); break;
+            case value_t::string         : assigner(string_         , std::move(v.string_         )); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, std::move(v.offset_datetime_)); break;
+            case value_t::local_datetime : assigner(local_datetime_ , std::move(v.local_datetime_ )); break;
+            case value_t::local_date     : assigner(local_date_     , std::move(v.local_date_     )); break;
+            case value_t::local_time     : assigner(local_time_     , std::move(v.local_time_     )); break;
+            case value_t::array          : assigner(array_          , std::move(v.array_          )); break;
+            case value_t::table          : assigner(table_          , std::move(v.table_          )); break;
+            default: break;
+        }
+    }
+
+    // -----------------------------------------------------------------------
+    // conversion between different basic_values.
+    template<typename C,
+             template<typename ...> class T,
+             template<typename ...> class A>
+    basic_value(const basic_value<C, T, A>& v)
+        : type_(v.type()), region_info_(v.region_info_), comments_(v.comments())
+    {
+        switch(v.type())
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          :
+            {
+                array_type tmp(v.as_array(std::nothrow).begin(),
+                               v.as_array(std::nothrow).end());
+                assigner(array_, std::move(tmp));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(v.as_table(std::nothrow).begin(),
+                               v.as_table(std::nothrow).end());
+                assigner(table_, std::move(tmp));
+                break;
+            }
+            default: break;
+        }
+    }
+    template<typename C,
+             template<typename ...> class T,
+             template<typename ...> class A>
+    basic_value(const basic_value<C, T, A>& v, std::vector<std::string> com)
+        : type_(v.type()), region_info_(v.region_info_),
+          comments_(std::move(com))
+    {
+        switch(v.type())
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          :
+            {
+                array_type tmp(v.as_array(std::nothrow).begin(),
+                               v.as_array(std::nothrow).end());
+                assigner(array_, std::move(tmp));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(v.as_table(std::nothrow).begin(),
+                               v.as_table(std::nothrow).end());
+                assigner(table_, std::move(tmp));
+                break;
+            }
+            default: break;
+        }
+    }
+    template<typename C,
+             template<typename ...> class T,
+             template<typename ...> class A>
+    basic_value& operator=(const basic_value<C, T, A>& v)
+    {
+        this->region_info_ = v.region_info_;
+        this->comments_    = comment_type(v.comments());
+        this->type_        = v.type();
+        switch(v.type())
+        {
+            case value_t::boolean        : assigner(boolean_        , v.boolean_        ); break;
+            case value_t::integer        : assigner(integer_        , v.integer_        ); break;
+            case value_t::floating       : assigner(floating_       , v.floating_       ); break;
+            case value_t::string         : assigner(string_         , v.string_         ); break;
+            case value_t::offset_datetime: assigner(offset_datetime_, v.offset_datetime_); break;
+            case value_t::local_datetime : assigner(local_datetime_ , v.local_datetime_ ); break;
+            case value_t::local_date     : assigner(local_date_     , v.local_date_     ); break;
+            case value_t::local_time     : assigner(local_time_     , v.local_time_     ); break;
+            case value_t::array          :
+            {
+                array_type tmp(v.as_array(std::nothrow).begin(),
+                               v.as_array(std::nothrow).end());
+                assigner(array_, std::move(tmp));
+                break;
+            }
+            case value_t::table          :
+            {
+                table_type tmp(v.as_table(std::nothrow).begin(),
+                               v.as_table(std::nothrow).end());
+                assigner(table_, std::move(tmp));
+                break;
+            }
+            default: break;
+        }
+        return *this;
+    }
+
+    // boolean ==============================================================
+
+    basic_value(boolean b)
+        : type_(value_t::boolean),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->boolean_, b);
+    }
+    basic_value& operator=(boolean b)
+    {
+        this->cleanup();
+        this->type_ = value_t::boolean;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->boolean_, b);
+        return *this;
+    }
+    basic_value(boolean b, std::vector<std::string> com)
+        : type_(value_t::boolean),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->boolean_, b);
+    }
+
+    // integer ==============================================================
+
+    template<typename T, typename std::enable_if<detail::conjunction<
+        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value(T i)
+        : type_(value_t::integer),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->integer_, static_cast<integer>(i));
+    }
+
+    template<typename T, typename std::enable_if<detail::conjunction<
+        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value& operator=(T i)
+    {
+        this->cleanup();
+        this->type_ = value_t::integer;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->integer_, static_cast<integer>(i));
+        return *this;
+    }
+
+    template<typename T, typename std::enable_if<detail::conjunction<
+        std::is_integral<T>, detail::negation<std::is_same<T, boolean>>>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value(T i, std::vector<std::string> com)
+        : type_(value_t::integer),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->integer_, static_cast<integer>(i));
+    }
+
+    // floating =============================================================
+
+    template<typename T, typename std::enable_if<
+        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value(T f)
+        : type_(value_t::floating),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->floating_, static_cast<floating>(f));
+    }
+
+
+    template<typename T, typename std::enable_if<
+        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value& operator=(T f)
+    {
+        this->cleanup();
+        this->type_ = value_t::floating;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->floating_, static_cast<floating>(f));
+        return *this;
+    }
+
+    template<typename T, typename std::enable_if<
+        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value(T f, std::vector<std::string> com)
+        : type_(value_t::floating),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->floating_, f);
+    }
+
+    // string ===============================================================
+
+    basic_value(toml::string s)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, std::move(s));
+    }
+    basic_value& operator=(toml::string s)
+    {
+        this->cleanup();
+        this->type_ = value_t::string ;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->string_, s);
+        return *this;
+    }
+    basic_value(toml::string s, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, std::move(s));
+    }
+
+    basic_value(std::string s)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(std::move(s)));
+    }
+    basic_value& operator=(std::string s)
+    {
+        this->cleanup();
+        this->type_ = value_t::string ;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->string_, toml::string(std::move(s)));
+        return *this;
+    }
+    basic_value(std::string s, string_t kind)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(std::move(s), kind));
+    }
+    basic_value(std::string s, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(std::move(s)));
+    }
+    basic_value(std::string s, string_t kind, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(std::move(s), kind));
+    }
+
+    basic_value(const char* s)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(std::string(s)));
+    }
+    basic_value& operator=(const char* s)
+    {
+        this->cleanup();
+        this->type_ = value_t::string ;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->string_, toml::string(std::string(s)));
+        return *this;
+    }
+    basic_value(const char* s, string_t kind)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(std::string(s), kind));
+    }
+    basic_value(const char* s, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(std::string(s)));
+    }
+    basic_value(const char* s, string_t kind, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(std::string(s), kind));
+    }
+
+#if defined(TOML11_USING_STRING_VIEW) && TOML11_USING_STRING_VIEW>0
+    basic_value(std::string_view s)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(s));
+    }
+    basic_value& operator=(std::string_view s)
+    {
+        this->cleanup();
+        this->type_ = value_t::string ;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->string_, toml::string(s));
+        return *this;
+    }
+    basic_value(std::string_view s, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(s));
+    }
+    basic_value(std::string_view s, string_t kind)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->string_, toml::string(s, kind));
+    }
+    basic_value(std::string_view s, string_t kind, std::vector<std::string> com)
+        : type_(value_t::string),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->string_, toml::string(s, kind));
+    }
+#endif
+
+    // local date ===========================================================
+
+    basic_value(const local_date& ld)
+        : type_(value_t::local_date),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->local_date_, ld);
+    }
+    basic_value& operator=(const local_date& ld)
+    {
+        this->cleanup();
+        this->type_ = value_t::local_date;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->local_date_, ld);
+        return *this;
+    }
+    basic_value(const local_date& ld, std::vector<std::string> com)
+        : type_(value_t::local_date),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->local_date_, ld);
+    }
+
+    // local time ===========================================================
+
+    basic_value(const local_time& lt)
+        : type_(value_t::local_time),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->local_time_, lt);
+    }
+    basic_value(const local_time& lt, std::vector<std::string> com)
+        : type_(value_t::local_time),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->local_time_, lt);
+    }
+    basic_value& operator=(const local_time& lt)
+    {
+        this->cleanup();
+        this->type_ = value_t::local_time;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->local_time_, lt);
+        return *this;
+    }
+
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& dur)
+        : type_(value_t::local_time),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->local_time_, local_time(dur));
+    }
+    template<typename Rep, typename Period>
+    basic_value(const std::chrono::duration<Rep, Period>& dur,
+                std::vector<std::string> com)
+        : type_(value_t::local_time),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->local_time_, local_time(dur));
+    }
+    template<typename Rep, typename Period>
+    basic_value& operator=(const std::chrono::duration<Rep, Period>& dur)
+    {
+        this->cleanup();
+        this->type_ = value_t::local_time;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->local_time_, local_time(dur));
+        return *this;
+    }
+
+    // local datetime =======================================================
+
+    basic_value(const local_datetime& ldt)
+        : type_(value_t::local_datetime),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->local_datetime_, ldt);
+    }
+    basic_value(const local_datetime& ldt, std::vector<std::string> com)
+        : type_(value_t::local_datetime),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->local_datetime_, ldt);
+    }
+    basic_value& operator=(const local_datetime& ldt)
+    {
+        this->cleanup();
+        this->type_ = value_t::local_datetime;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->local_datetime_, ldt);
+        return *this;
+    }
+
+    // offset datetime ======================================================
+
+    basic_value(const offset_datetime& odt)
+        : type_(value_t::offset_datetime),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->offset_datetime_, odt);
+    }
+    basic_value(const offset_datetime& odt, std::vector<std::string> com)
+        : type_(value_t::offset_datetime),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->offset_datetime_, odt);
+    }
+    basic_value& operator=(const offset_datetime& odt)
+    {
+        this->cleanup();
+        this->type_ = value_t::offset_datetime;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->offset_datetime_, odt);
+        return *this;
+    }
+    basic_value(const std::chrono::system_clock::time_point& tp)
+        : type_(value_t::offset_datetime),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->offset_datetime_, offset_datetime(tp));
+    }
+    basic_value(const std::chrono::system_clock::time_point& tp,
+                std::vector<std::string> com)
+        : type_(value_t::offset_datetime),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->offset_datetime_, offset_datetime(tp));
+    }
+    basic_value& operator=(const std::chrono::system_clock::time_point& tp)
+    {
+        this->cleanup();
+        this->type_ = value_t::offset_datetime;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->offset_datetime_, offset_datetime(tp));
+        return *this;
+    }
+
+    // array ================================================================
+
+    basic_value(const array_type& ary)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->array_, ary);
+    }
+    basic_value(const array_type& ary, std::vector<std::string> com)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->array_, ary);
+    }
+    basic_value& operator=(const array_type& ary)
+    {
+        this->cleanup();
+        this->type_ = value_t::array ;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->array_, ary);
+        return *this;
+    }
+
+    // array (initializer_list) ----------------------------------------------
+
+    template<typename T, typename std::enable_if<
+            std::is_convertible<T, value_type>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value(std::initializer_list<T> list)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        array_type ary(list.begin(), list.end());
+        assigner(this->array_, std::move(ary));
+    }
+    template<typename T, typename std::enable_if<
+            std::is_convertible<T, value_type>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value(std::initializer_list<T> list, std::vector<std::string> com)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        array_type ary(list.begin(), list.end());
+        assigner(this->array_, std::move(ary));
+    }
+    template<typename T, typename std::enable_if<
+            std::is_convertible<T, value_type>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value& operator=(std::initializer_list<T> list)
+    {
+        this->cleanup();
+        this->type_ = value_t::array;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+
+        array_type ary(list.begin(), list.end());
+        assigner(this->array_, std::move(ary));
+        return *this;
+    }
+
+    // array (STL Containers) ------------------------------------------------
+
+    template<typename T, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<T, array_type>>,
+            detail::is_container<T>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value(const T& list)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
+            "elements of a container should be convertible to toml::value");
+
+        array_type ary(list.size());
+        std::copy(list.begin(), list.end(), ary.begin());
+        assigner(this->array_, std::move(ary));
+    }
+    template<typename T, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<T, array_type>>,
+            detail::is_container<T>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value(const T& list, std::vector<std::string> com)
+        : type_(value_t::array),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
+            "elements of a container should be convertible to toml::value");
+
+        array_type ary(list.size());
+        std::copy(list.begin(), list.end(), ary.begin());
+        assigner(this->array_, std::move(ary));
+    }
+    template<typename T, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<T, array_type>>,
+            detail::is_container<T>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value& operator=(const T& list)
+    {
+        static_assert(std::is_convertible<typename T::value_type, value_type>::value,
+            "elements of a container should be convertible to toml::value");
+
+        this->cleanup();
+        this->type_ = value_t::array;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+
+        array_type ary(list.size());
+        std::copy(list.begin(), list.end(), ary.begin());
+        assigner(this->array_, std::move(ary));
+        return *this;
+    }
+
+    // table ================================================================
+
+    basic_value(const table_type& tab)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        assigner(this->table_, tab);
+    }
+    basic_value(const table_type& tab, std::vector<std::string> com)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        assigner(this->table_, tab);
+    }
+    basic_value& operator=(const table_type& tab)
+    {
+        this->cleanup();
+        this->type_ = value_t::table;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+        assigner(this->table_, tab);
+        return *this;
+    }
+
+    // initializer-list ------------------------------------------------------
+
+    basic_value(std::initializer_list<std::pair<key, basic_value>> list)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        table_type tab;
+        for(const auto& elem : list) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+    }
+
+    basic_value(std::initializer_list<std::pair<key, basic_value>> list,
+                std::vector<std::string> com)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        table_type tab;
+        for(const auto& elem : list) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+    }
+    basic_value& operator=(std::initializer_list<std::pair<key, basic_value>> list)
+    {
+        this->cleanup();
+        this->type_ = value_t::table;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+
+        table_type tab;
+        for(const auto& elem : list) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+        return *this;
+    }
+
+    // other table-like -----------------------------------------------------
+
+    template<typename Map, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<Map, table_type>>,
+            detail::is_map<Map>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value(const Map& mp)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{}))
+    {
+        table_type tab;
+        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+    }
+    template<typename Map, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<Map, table_type>>,
+            detail::is_map<Map>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value(const Map& mp, std::vector<std::string> com)
+        : type_(value_t::table),
+          region_info_(std::make_shared<region_base>(region_base{})),
+          comments_(std::move(com))
+    {
+        table_type tab;
+        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+    }
+    template<typename Map, typename std::enable_if<detail::conjunction<
+            detail::negation<std::is_same<Map, table_type>>,
+            detail::is_map<Map>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value& operator=(const Map& mp)
+    {
+        this->cleanup();
+        this->type_ = value_t::table;
+        this->region_info_ = std::make_shared<region_base>(region_base{});
+
+        table_type tab;
+        for(const auto& elem : mp) {tab[elem.first] = elem.second;}
+        assigner(this->table_, std::move(tab));
+        return *this;
+    }
+
+    // user-defined =========================================================
+
+    // convert using into_toml() method -------------------------------------
+
+    template<typename T, typename std::enable_if<
+        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value(const T& ud): basic_value(ud.into_toml()) {}
+
+    template<typename T, typename std::enable_if<
+        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value(const T& ud, std::vector<std::string> com)
+        : basic_value(ud.into_toml(), std::move(com))
+    {}
+    template<typename T, typename std::enable_if<
+        detail::has_into_toml_method<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value& operator=(const T& ud)
+    {
+        *this = ud.into_toml();
+        return *this;
+    }
+
+    // convert using into<T> struct -----------------------------------------
+
+    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
+    basic_value(const T& ud): basic_value(::toml::into<T>::into_toml(ud)) {}
+    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
+    basic_value(const T& ud, std::vector<std::string> com)
+        : basic_value(::toml::into<T>::into_toml(ud), std::move(com))
+    {}
+    template<typename T, std::size_t S = sizeof(::toml::into<T>)>
+    basic_value& operator=(const T& ud)
+    {
+        *this = ::toml::into<T>::into_toml(ud);
+        return *this;
+    }
+
+    // for internal use ------------------------------------------------------
+    //
+    // Those constructors take detail::region that contains parse result.
+
+    basic_value(boolean b, detail::region reg, std::vector<std::string> cm)
+        : type_(value_t::boolean),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->boolean_, b);
+    }
+    template<typename T, typename std::enable_if<
+        detail::conjunction<
+            std::is_integral<T>, detail::negation<std::is_same<T, boolean>>
+        >::value, std::nullptr_t>::type = nullptr>
+    basic_value(T i, detail::region reg, std::vector<std::string> cm)
+        : type_(value_t::integer),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->integer_, static_cast<integer>(i));
+    }
+    template<typename T, typename std::enable_if<
+        std::is_floating_point<T>::value, std::nullptr_t>::type = nullptr>
+    basic_value(T f, detail::region reg, std::vector<std::string> cm)
+        : type_(value_t::floating),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->floating_, static_cast<floating>(f));
+    }
+    basic_value(toml::string s, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::string),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->string_, std::move(s));
+    }
+    basic_value(const local_date& ld, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::local_date),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->local_date_, ld);
+    }
+    basic_value(const local_time& lt, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::local_time),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->local_time_, lt);
+    }
+    basic_value(const local_datetime& ldt, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::local_datetime),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->local_datetime_, ldt);
+    }
+    basic_value(const offset_datetime& odt, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::offset_datetime),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->offset_datetime_, odt);
+    }
+    basic_value(const array_type& ary, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::array),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->array_, ary);
+    }
+    basic_value(const table_type& tab, detail::region reg,
+                std::vector<std::string> cm)
+        : type_(value_t::table),
+          region_info_(std::make_shared<detail::region>(std::move(reg))),
+          comments_(std::move(cm))
+    {
+        assigner(this->table_, tab);
+    }
+
+    template<typename T, typename std::enable_if<
+        detail::is_exact_toml_type<T, value_type>::value,
+        std::nullptr_t>::type = nullptr>
+    basic_value(std::pair<T, detail::region> parse_result, std::vector<std::string> com)
+        : basic_value(std::move(parse_result.first),
+                      std::move(parse_result.second),
+                      std::move(com))
+    {}
+
+    // type checking and casting ============================================
+
+    template<typename T, typename std::enable_if<
+        detail::is_exact_toml_type<T, value_type>::value,
+        std::nullptr_t>::type = nullptr>
+    bool is() const noexcept
+    {
+        return detail::type_to_enum<T, value_type>::value == this->type_;
+    }
+    bool is(value_t t) const noexcept {return t == this->type_;}
+
+    bool is_uninitialized()   const noexcept {return this->is(value_t::empty          );}
+    bool is_boolean()         const noexcept {return this->is(value_t::boolean        );}
+    bool is_integer()         const noexcept {return this->is(value_t::integer        );}
+    bool is_floating()        const noexcept {return this->is(value_t::floating       );}
+    bool is_string()          const noexcept {return this->is(value_t::string         );}
+    bool is_offset_datetime() const noexcept {return this->is(value_t::offset_datetime);}
+    bool is_local_datetime()  const noexcept {return this->is(value_t::local_datetime );}
+    bool is_local_date()      const noexcept {return this->is(value_t::local_date     );}
+    bool is_local_time()      const noexcept {return this->is(value_t::local_time     );}
+    bool is_array()           const noexcept {return this->is(value_t::array          );}
+    bool is_table()           const noexcept {return this->is(value_t::table          );}
+
+    value_t type() const noexcept {return type_;}
+
+    template<value_t T>
+    typename detail::enum_to_type<T, value_type>::type&       cast() &
+    {
+        if(this->type_ != T)
+        {
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
+        }
+        return detail::switch_cast<T>::invoke(*this);
+    }
+    template<value_t T>
+    typename detail::enum_to_type<T, value_type>::type const& cast() const&
+    {
+        if(this->type_ != T)
+        {
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
+        }
+        return detail::switch_cast<T>::invoke(*this);
+    }
+    template<value_t T>
+    typename detail::enum_to_type<T, value_type>::type&&      cast() &&
+    {
+        if(this->type_ != T)
+        {
+            detail::throw_bad_cast<T>("toml::value::cast: ", this->type_, *this);
+        }
+        return detail::switch_cast<T>::invoke(std::move(*this));
+    }
+
+    // ------------------------------------------------------------------------
+    // nothrow version
+
+    boolean         const& as_boolean        (const std::nothrow_t&) const& noexcept {return this->boolean_;}
+    integer         const& as_integer        (const std::nothrow_t&) const& noexcept {return this->integer_;}
+    floating        const& as_floating       (const std::nothrow_t&) const& noexcept {return this->floating_;}
+    string          const& as_string         (const std::nothrow_t&) const& noexcept {return this->string_;}
+    offset_datetime const& as_offset_datetime(const std::nothrow_t&) const& noexcept {return this->offset_datetime_;}
+    local_datetime  const& as_local_datetime (const std::nothrow_t&) const& noexcept {return this->local_datetime_;}
+    local_date      const& as_local_date     (const std::nothrow_t&) const& noexcept {return this->local_date_;}
+    local_time      const& as_local_time     (const std::nothrow_t&) const& noexcept {return this->local_time_;}
+    array_type      const& as_array          (const std::nothrow_t&) const& noexcept {return this->array_.value();}
+    table_type      const& as_table          (const std::nothrow_t&) const& noexcept {return this->table_.value();}
+
+    boolean        & as_boolean        (const std::nothrow_t&) & noexcept {return this->boolean_;}
+    integer        & as_integer        (const std::nothrow_t&) & noexcept {return this->integer_;}
+    floating       & as_floating       (const std::nothrow_t&) & noexcept {return this->floating_;}
+    string         & as_string         (const std::nothrow_t&) & noexcept {return this->string_;}
+    offset_datetime& as_offset_datetime(const std::nothrow_t&) & noexcept {return this->offset_datetime_;}
+    local_datetime & as_local_datetime (const std::nothrow_t&) & noexcept {return this->local_datetime_;}
+    local_date     & as_local_date     (const std::nothrow_t&) & noexcept {return this->local_date_;}
+    local_time     & as_local_time     (const std::nothrow_t&) & noexcept {return this->local_time_;}
+    array_type     & as_array          (const std::nothrow_t&) & noexcept {return this->array_.value();}
+    table_type     & as_table          (const std::nothrow_t&) & noexcept {return this->table_.value();}
+
+    boolean        && as_boolean        (const std::nothrow_t&) && noexcept {return std::move(this->boolean_);}
+    integer        && as_integer        (const std::nothrow_t&) && noexcept {return std::move(this->integer_);}
+    floating       && as_floating       (const std::nothrow_t&) && noexcept {return std::move(this->floating_);}
+    string         && as_string         (const std::nothrow_t&) && noexcept {return std::move(this->string_);}
+    offset_datetime&& as_offset_datetime(const std::nothrow_t&) && noexcept {return std::move(this->offset_datetime_);}
+    local_datetime && as_local_datetime (const std::nothrow_t&) && noexcept {return std::move(this->local_datetime_);}
+    local_date     && as_local_date     (const std::nothrow_t&) && noexcept {return std::move(this->local_date_);}
+    local_time     && as_local_time     (const std::nothrow_t&) && noexcept {return std::move(this->local_time_);}
+    array_type     && as_array          (const std::nothrow_t&) && noexcept {return std::move(this->array_.value());}
+    table_type     && as_table          (const std::nothrow_t&) && noexcept {return std::move(this->table_.value());}
+
+    // ========================================================================
+    // throw version
+    // ------------------------------------------------------------------------
+    // const reference {{{
+
+    boolean const& as_boolean() const&
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
+        }
+        return this->boolean_;
+    }
+    integer const& as_integer() const&
+    {
+        if(this->type_ != value_t::integer)
+        {
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
+        }
+        return this->integer_;
+    }
+    floating const& as_floating() const&
+    {
+        if(this->type_ != value_t::floating)
+        {
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
+        }
+        return this->floating_;
+    }
+    string const& as_string() const&
+    {
+        if(this->type_ != value_t::string)
+        {
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
+        }
+        return this->string_;
+    }
+    offset_datetime const& as_offset_datetime() const&
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
+        }
+        return this->offset_datetime_;
+    }
+    local_datetime const& as_local_datetime() const&
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
+        }
+        return this->local_datetime_;
+    }
+    local_date const& as_local_date() const&
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
+        }
+        return this->local_date_;
+    }
+    local_time const& as_local_time() const&
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
+        }
+        return this->local_time_;
+    }
+    array_type const& as_array() const&
+    {
+        if(this->type_ != value_t::array)
+        {
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
+        }
+        return this->array_.value();
+    }
+    table_type const& as_table() const&
+    {
+        if(this->type_ != value_t::table)
+        {
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
+        }
+        return this->table_.value();
+    }
+    // }}}
+    // ------------------------------------------------------------------------
+    // nonconst reference {{{
+
+    boolean & as_boolean() &
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
+        }
+        return this->boolean_;
+    }
+    integer & as_integer() &
+    {
+        if(this->type_ != value_t::integer)
+        {
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
+        }
+        return this->integer_;
+    }
+    floating & as_floating() &
+    {
+        if(this->type_ != value_t::floating)
+        {
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
+        }
+        return this->floating_;
+    }
+    string & as_string() &
+    {
+        if(this->type_ != value_t::string)
+        {
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
+        }
+        return this->string_;
+    }
+    offset_datetime & as_offset_datetime() &
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
+        }
+        return this->offset_datetime_;
+    }
+    local_datetime & as_local_datetime() &
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
+        }
+        return this->local_datetime_;
+    }
+    local_date & as_local_date() &
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
+        }
+        return this->local_date_;
+    }
+    local_time & as_local_time() &
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
+        }
+        return this->local_time_;
+    }
+    array_type & as_array() &
+    {
+        if(this->type_ != value_t::array)
+        {
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
+        }
+        return this->array_.value();
+    }
+    table_type & as_table() &
+    {
+        if(this->type_ != value_t::table)
+        {
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
+        }
+        return this->table_.value();
+    }
+
+    // }}}
+    // ------------------------------------------------------------------------
+    // rvalue reference {{{
+
+    boolean && as_boolean() &&
+    {
+        if(this->type_ != value_t::boolean)
+        {
+            detail::throw_bad_cast<value_t::boolean>(
+                    "toml::value::as_boolean(): ", this->type_, *this);
+        }
+        return std::move(this->boolean_);
+    }
+    integer && as_integer() &&
+    {
+        if(this->type_ != value_t::integer)
+        {
+            detail::throw_bad_cast<value_t::integer>(
+                    "toml::value::as_integer(): ", this->type_, *this);
+        }
+        return std::move(this->integer_);
+    }
+    floating && as_floating() &&
+    {
+        if(this->type_ != value_t::floating)
+        {
+            detail::throw_bad_cast<value_t::floating>(
+                    "toml::value::as_floating(): ", this->type_, *this);
+        }
+        return std::move(this->floating_);
+    }
+    string && as_string() &&
+    {
+        if(this->type_ != value_t::string)
+        {
+            detail::throw_bad_cast<value_t::string>(
+                    "toml::value::as_string(): ", this->type_, *this);
+        }
+        return std::move(this->string_);
+    }
+    offset_datetime && as_offset_datetime() &&
+    {
+        if(this->type_ != value_t::offset_datetime)
+        {
+            detail::throw_bad_cast<value_t::offset_datetime>(
+                    "toml::value::as_offset_datetime(): ", this->type_, *this);
+        }
+        return std::move(this->offset_datetime_);
+    }
+    local_datetime && as_local_datetime() &&
+    {
+        if(this->type_ != value_t::local_datetime)
+        {
+            detail::throw_bad_cast<value_t::local_datetime>(
+                    "toml::value::as_local_datetime(): ", this->type_, *this);
+        }
+        return std::move(this->local_datetime_);
+    }
+    local_date && as_local_date() &&
+    {
+        if(this->type_ != value_t::local_date)
+        {
+            detail::throw_bad_cast<value_t::local_date>(
+                    "toml::value::as_local_date(): ", this->type_, *this);
+        }
+        return std::move(this->local_date_);
+    }
+    local_time && as_local_time() &&
+    {
+        if(this->type_ != value_t::local_time)
+        {
+            detail::throw_bad_cast<value_t::local_time>(
+                    "toml::value::as_local_time(): ", this->type_, *this);
+        }
+        return std::move(this->local_time_);
+    }
+    array_type && as_array() &&
+    {
+        if(this->type_ != value_t::array)
+        {
+            detail::throw_bad_cast<value_t::array>(
+                    "toml::value::as_array(): ", this->type_, *this);
+        }
+        return std::move(this->array_.value());
+    }
+    table_type && as_table() &&
+    {
+        if(this->type_ != value_t::table)
+        {
+            detail::throw_bad_cast<value_t::table>(
+                    "toml::value::as_table(): ", this->type_, *this);
+        }
+        return std::move(this->table_.value());
+    }
+    // }}}
+
+    // accessors =============================================================
+    //
+    // may throw type_error or out_of_range
+    //
+    value_type&       at(const key& k)
+    {
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::at(key): ", this->type_, *this);
+        }
+        if(this->as_table(std::nothrow).count(k) == 0)
+        {
+            detail::throw_key_not_found_error(*this, k);
+        }
+        return this->as_table(std::nothrow).at(k);
+    }
+    value_type const& at(const key& k) const
+    {
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::at(key): ", this->type_, *this);
+        }
+        if(this->as_table(std::nothrow).count(k) == 0)
+        {
+            detail::throw_key_not_found_error(*this, k);
+        }
+        return this->as_table(std::nothrow).at(k);
+    }
+    value_type&       operator[](const key& k)
+    {
+        if(this->is_uninitialized())
+        {
+            *this = table_type{};
+        }
+        else if(!this->is_table()) // initialized, but not a table
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::operator[](key): ", this->type_, *this);
+        }
+        return this->as_table(std::nothrow)[k];
+    }
+
+    value_type&       at(const std::size_t idx)
+    {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::at(idx): ", this->type_, *this);
+        }
+        if(this->as_array(std::nothrow).size() <= idx)
+        {
+            throw std::out_of_range(detail::format_underline(
+                "toml::value::at(idx): no element corresponding to the index", {
+                    {this->location(), concat_to_string("the length is ",
+                        this->as_array(std::nothrow).size(),
+                        ", and the specified index is ", idx)}
+                }));
+        }
+        return this->as_array().at(idx);
+    }
+    value_type const& at(const std::size_t idx) const
+    {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::at(idx): ", this->type_, *this);
+        }
+        if(this->as_array(std::nothrow).size() <= idx)
+        {
+            throw std::out_of_range(detail::format_underline(
+                "toml::value::at(idx): no element corresponding to the index", {
+                    {this->location(), concat_to_string("the length is ",
+                        this->as_array(std::nothrow).size(),
+                        ", and the specified index is ", idx)}
+                }));
+        }
+        return this->as_array(std::nothrow).at(idx);
+    }
+
+    value_type&       operator[](const std::size_t idx) noexcept
+    {
+        // no check...
+        return this->as_array(std::nothrow)[idx];
+    }
+    value_type const& operator[](const std::size_t idx) const noexcept
+    {
+        // no check...
+        return this->as_array(std::nothrow)[idx];
+    }
+
+    void push_back(const value_type& x)
+    {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::push_back(value): ", this->type_, *this);
+        }
+        this->as_array(std::nothrow).push_back(x);
+        return;
+    }
+    void push_back(value_type&& x)
+    {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::push_back(value): ", this->type_, *this);
+        }
+        this->as_array(std::nothrow).push_back(std::move(x));
+        return;
+    }
+
+    template<typename ... Ts>
+    value_type& emplace_back(Ts&& ... args)
+    {
+        if(!this->is_array())
+        {
+            detail::throw_bad_cast<value_t::array>(
+                "toml::value::emplace_back(...): ", this->type_, *this);
+        }
+        this->as_array(std::nothrow).emplace_back(std::forward<Ts>(args) ...);
+        return this->as_array(std::nothrow).back();
+    }
+
+    std::size_t size() const
+    {
+        switch(this->type_)
+        {
+            case value_t::array:
+            {
+                return this->as_array(std::nothrow).size();
+            }
+            case value_t::table:
+            {
+                return this->as_table(std::nothrow).size();
+            }
+            case value_t::string:
+            {
+                return this->as_string(std::nothrow).str.size();
+            }
+            default:
+            {
+                throw type_error(detail::format_underline(
+                    "toml::value::size(): bad_cast to container types", {
+                        {this->location(),
+                         concat_to_string("the actual type is ", this->type_)}
+                    }), this->location());
+            }
+        }
+    }
+
+    std::size_t count(const key_type& k) const
+    {
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::count(key): ", this->type_, *this);
+        }
+        return this->as_table(std::nothrow).count(k);
+    }
+
+    bool contains(const key_type& k) const
+    {
+        if(!this->is_table())
+        {
+            detail::throw_bad_cast<value_t::table>(
+                "toml::value::contains(key): ", this->type_, *this);
+        }
+        return (this->as_table(std::nothrow).count(k) != 0);
+    }
+
+    source_location location() const
+    {
+        return source_location(this->region_info_.get());
+    }
+
+    comment_type const& comments() const noexcept {return this->comments_;}
+    comment_type&       comments()       noexcept {return this->comments_;}
+
+  private:
+
+    void cleanup() noexcept
+    {
+        switch(this->type_)
+        {
+            case value_t::string  : {string_.~string();       return;}
+            case value_t::array   : {array_.~array_storage(); return;}
+            case value_t::table   : {table_.~table_storage(); return;}
+            default              : return;
+        }
+    }
+
+    // for error messages
+    template<typename Value>
+    friend region_base const* detail::get_region(const Value& v);
+
+    template<typename Value>
+    friend void detail::change_region(Value& v, detail::region reg);
+
+  private:
+
+    using array_storage = detail::storage<array_type>;
+    using table_storage = detail::storage<table_type>;
+
+    value_t type_;
+    union
+    {
+        boolean         boolean_;
+        integer         integer_;
+        floating        floating_;
+        string          string_;
+        offset_datetime offset_datetime_;
+        local_datetime  local_datetime_;
+        local_date      local_date_;
+        local_time      local_time_;
+        array_storage   array_;
+        table_storage   table_;
+    };
+    std::shared_ptr<region_base> region_info_;
+    comment_type                 comments_;
+};
+
+// default toml::value and default array/table.
+// TOML11_DEFAULT_COMMENT_STRATEGY is defined in comments.hpp
+using value = basic_value<TOML11_DEFAULT_COMMENT_STRATEGY, std::unordered_map, std::vector>;
+using array = typename value::array_type;
+using table = typename value::table_type;
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+inline bool
+operator==(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    if(lhs.type()     != rhs.type())     {return false;}
+    if(lhs.comments() != rhs.comments()) {return false;}
+
+    switch(lhs.type())
+    {
+        case value_t::boolean  :
+        {
+            return lhs.as_boolean() == rhs.as_boolean();
+        }
+        case value_t::integer  :
+        {
+            return lhs.as_integer() == rhs.as_integer();
+        }
+        case value_t::floating :
+        {
+            return lhs.as_floating() == rhs.as_floating();
+        }
+        case value_t::string   :
+        {
+            return lhs.as_string() == rhs.as_string();
+        }
+        case value_t::offset_datetime:
+        {
+            return lhs.as_offset_datetime() == rhs.as_offset_datetime();
+        }
+        case value_t::local_datetime:
+        {
+            return lhs.as_local_datetime() == rhs.as_local_datetime();
+        }
+        case value_t::local_date:
+        {
+            return lhs.as_local_date() == rhs.as_local_date();
+        }
+        case value_t::local_time:
+        {
+            return lhs.as_local_time() == rhs.as_local_time();
+        }
+        case value_t::array    :
+        {
+            return lhs.as_array() == rhs.as_array();
+        }
+        case value_t::table    :
+        {
+            return lhs.as_table() == rhs.as_table();
+        }
+        case value_t::empty    : {return true; }
+        default:                 {return false;}
+    }
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+inline bool operator!=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    return !(lhs == rhs);
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+typename std::enable_if<detail::conjunction<
+    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
+    detail::is_comparable<typename basic_value<C, T, A>::table_type>
+    >::value, bool>::type
+operator<(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    if(lhs.type() != rhs.type()){return (lhs.type() < rhs.type());}
+    switch(lhs.type())
+    {
+        case value_t::boolean  :
+        {
+            return lhs.as_boolean() <  rhs.as_boolean() ||
+                  (lhs.as_boolean() == rhs.as_boolean() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::integer  :
+        {
+            return lhs.as_integer() <  rhs.as_integer() ||
+                  (lhs.as_integer() == rhs.as_integer() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::floating :
+        {
+            return lhs.as_floating() <  rhs.as_floating() ||
+                  (lhs.as_floating() == rhs.as_floating() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::string   :
+        {
+            return lhs.as_string() <  rhs.as_string() ||
+                  (lhs.as_string() == rhs.as_string() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::offset_datetime:
+        {
+            return lhs.as_offset_datetime() <  rhs.as_offset_datetime() ||
+                  (lhs.as_offset_datetime() == rhs.as_offset_datetime() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_datetime:
+        {
+            return lhs.as_local_datetime() <  rhs.as_local_datetime() ||
+                  (lhs.as_local_datetime() == rhs.as_local_datetime() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_date:
+        {
+            return lhs.as_local_date() <  rhs.as_local_date() ||
+                  (lhs.as_local_date() == rhs.as_local_date() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::local_time:
+        {
+            return lhs.as_local_time() <  rhs.as_local_time() ||
+                  (lhs.as_local_time() == rhs.as_local_time() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::array    :
+        {
+            return lhs.as_array() <  rhs.as_array() ||
+                  (lhs.as_array() == rhs.as_array() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::table    :
+        {
+            return lhs.as_table() <  rhs.as_table() ||
+                  (lhs.as_table() == rhs.as_table() &&
+                   lhs.comments() < rhs.comments());
+        }
+        case value_t::empty    :
+        {
+            return lhs.comments() < rhs.comments();
+        }
+        default:
+        {
+            return lhs.comments() < rhs.comments();
+        }
+    }
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+typename std::enable_if<detail::conjunction<
+    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
+    detail::is_comparable<typename basic_value<C, T, A>::table_type>
+    >::value, bool>::type
+operator<=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    return (lhs < rhs) || (lhs == rhs);
+}
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+typename std::enable_if<detail::conjunction<
+    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
+    detail::is_comparable<typename basic_value<C, T, A>::table_type>
+    >::value, bool>::type
+operator>(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    return !(lhs <= rhs);
+}
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+typename std::enable_if<detail::conjunction<
+    detail::is_comparable<typename basic_value<C, T, A>::array_type>,
+    detail::is_comparable<typename basic_value<C, T, A>::table_type>
+    >::value, bool>::type
+operator>=(const basic_value<C, T, A>& lhs, const basic_value<C, T, A>& rhs)
+{
+    return !(lhs < rhs);
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+inline std::string format_error(const std::string& err_msg,
+        const basic_value<C, T, A>& v, const std::string& comment,
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
+{
+    return detail::format_underline(err_msg, {{v.location(), comment}},
+                                    std::move(hints), colorize);
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+inline std::string format_error(const std::string& err_msg,
+        const toml::basic_value<C, T, A>& v1, const std::string& comment1,
+        const toml::basic_value<C, T, A>& v2, const std::string& comment2,
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
+{
+    return detail::format_underline(err_msg, {
+            {v1.location(), comment1}, {v2.location(), comment2}
+        }, std::move(hints), colorize);
+}
+
+template<typename C, template<typename ...> class T, template<typename ...> class A>
+inline std::string format_error(const std::string& err_msg,
+        const toml::basic_value<C, T, A>& v1, const std::string& comment1,
+        const toml::basic_value<C, T, A>& v2, const std::string& comment2,
+        const toml::basic_value<C, T, A>& v3, const std::string& comment3,
+        std::vector<std::string> hints = {},
+        const bool colorize = TOML11_ERROR_MESSAGE_COLORIZED)
+{
+    return detail::format_underline(err_msg, {{v1.location(), comment1},
+            {v2.location(), comment2}, {v3.location(), comment3}
+        }, std::move(hints), colorize);
+}
+
+template<typename Visitor, typename C,
+         template<typename ...> class T, template<typename ...> class A>
+detail::return_type_of_t<Visitor, const toml::boolean&>
+visit(Visitor&& visitor, const toml::basic_value<C, T, A>& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(v.as_boolean        ());}
+        case value_t::integer        : {return visitor(v.as_integer        ());}
+        case value_t::floating       : {return visitor(v.as_floating       ());}
+        case value_t::string         : {return visitor(v.as_string         ());}
+        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
+        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
+        case value_t::local_date     : {return visitor(v.as_local_date     ());}
+        case value_t::local_time     : {return visitor(v.as_local_time     ());}
+        case value_t::array          : {return visitor(v.as_array          ());}
+        case value_t::table          : {return visitor(v.as_table          ());}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid basic_value.", v, "here"));
+}
+
+template<typename Visitor, typename C,
+         template<typename ...> class T, template<typename ...> class A>
+detail::return_type_of_t<Visitor, toml::boolean&>
+visit(Visitor&& visitor, toml::basic_value<C, T, A>& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(v.as_boolean        ());}
+        case value_t::integer        : {return visitor(v.as_integer        ());}
+        case value_t::floating       : {return visitor(v.as_floating       ());}
+        case value_t::string         : {return visitor(v.as_string         ());}
+        case value_t::offset_datetime: {return visitor(v.as_offset_datetime());}
+        case value_t::local_datetime : {return visitor(v.as_local_datetime ());}
+        case value_t::local_date     : {return visitor(v.as_local_date     ());}
+        case value_t::local_time     : {return visitor(v.as_local_time     ());}
+        case value_t::array          : {return visitor(v.as_array          ());}
+        case value_t::table          : {return visitor(v.as_table          ());}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid basic_value.", v, "here"));
+}
+
+template<typename Visitor, typename C,
+         template<typename ...> class T, template<typename ...> class A>
+detail::return_type_of_t<Visitor, toml::boolean&&>
+visit(Visitor&& visitor, toml::basic_value<C, T, A>&& v)
+{
+    switch(v.type())
+    {
+        case value_t::boolean        : {return visitor(std::move(v.as_boolean        ()));}
+        case value_t::integer        : {return visitor(std::move(v.as_integer        ()));}
+        case value_t::floating       : {return visitor(std::move(v.as_floating       ()));}
+        case value_t::string         : {return visitor(std::move(v.as_string         ()));}
+        case value_t::offset_datetime: {return visitor(std::move(v.as_offset_datetime()));}
+        case value_t::local_datetime : {return visitor(std::move(v.as_local_datetime ()));}
+        case value_t::local_date     : {return visitor(std::move(v.as_local_date     ()));}
+        case value_t::local_time     : {return visitor(std::move(v.as_local_time     ()));}
+        case value_t::array          : {return visitor(std::move(v.as_array          ()));}
+        case value_t::table          : {return visitor(std::move(v.as_table          ()));}
+        case value_t::empty          : break;
+        default: break;
+    }
+    throw std::runtime_error(format_error("[error] toml::visit: toml::basic_value "
+            "does not have any valid basic_value.", v, "here"));
+}
+
+}// toml
+#endif// TOML11_VALUE