diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 8672edaa8..8e06273ee 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -9,6 +9,10 @@
 
 namespace nix {
 
+Config::Config(StringMap initials)
+    : AbstractConfig(std::move(initials))
+{ }
+
 bool Config::set(const std::string & name, const std::string & value)
 {
     bool append = false;
@@ -29,9 +33,9 @@ bool Config::set(const std::string & name, const std::string & value)
 
 void Config::addSetting(AbstractSetting * setting)
 {
-    _settings.emplace(setting->name, Config::SettingData(false, setting));
+    _settings.emplace(setting->name, Config::SettingData{false, setting});
     for (auto & alias : setting->aliases)
-        _settings.emplace(alias, Config::SettingData(true, setting));
+        _settings.emplace(alias, Config::SettingData{true, setting});
 
     bool set = false;
 
@@ -59,6 +63,10 @@ void Config::addSetting(AbstractSetting * setting)
     }
 }
 
+AbstractConfig::AbstractConfig(StringMap initials)
+    : unknownSettings(std::move(initials))
+{ }
+
 void AbstractConfig::warnUnknownSettings()
 {
     for (auto & s : unknownSettings)
@@ -199,6 +207,13 @@ AbstractSetting::AbstractSetting(
 {
 }
 
+AbstractSetting::~AbstractSetting()
+{
+    // Check against a gcc miscompilation causing our constructor
+    // not to run (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80431).
+    assert(created == 123);
+}
+
 nlohmann::json AbstractSetting::toJSON()
 {
     return nlohmann::json(toJSONObject());
@@ -220,6 +235,9 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 {
 }
 
+
+bool AbstractSetting::isOverridden() const { return overridden; }
+
 template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
 {
     return str;
@@ -385,11 +403,33 @@ static Path parsePath(const AbstractSetting & s, const std::string & str)
         return canonPath(str);
 }
 
+PathSetting::PathSetting(Config * options,
+    const Path & def,
+    const std::string & name,
+    const std::string & description,
+    const std::set<std::string> & aliases)
+    : BaseSetting<Path>(def, true, name, description, aliases)
+{
+    options->addSetting(this);
+}
+
 Path PathSetting::parse(const std::string & str) const
 {
     return parsePath(*this, str);
 }
 
+
+OptionalPathSetting::OptionalPathSetting(Config * options,
+    const std::optional<Path> & def,
+    const std::string & name,
+    const std::string & description,
+    const std::set<std::string> & aliases)
+    : BaseSetting<std::optional<Path>>(def, true, name, description, aliases)
+{
+    options->addSetting(this);
+}
+
+
 std::optional<Path> OptionalPathSetting::parse(const std::string & str) const
 {
     if (str == "")
@@ -398,6 +438,11 @@ std::optional<Path> OptionalPathSetting::parse(const std::string & str) const
         return parsePath(*this, str);
 }
 
+void OptionalPathSetting::operator =(const std::optional<Path> & v)
+{
+    this->assign(v);
+}
+
 bool GlobalConfig::set(const std::string & name, const std::string & value)
 {
     for (auto & config : *configRegistrations)
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index cc8532587..3e6762e54 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -52,9 +52,7 @@ class AbstractConfig
 protected:
     StringMap unknownSettings;
 
-    AbstractConfig(const StringMap & initials = {})
-        : unknownSettings(initials)
-    { }
+    AbstractConfig(StringMap initials = {});
 
 public:
 
@@ -150,9 +148,6 @@ public:
     {
         bool isAlias;
         AbstractSetting * setting;
-        SettingData(bool isAlias, AbstractSetting * setting)
-            : isAlias(isAlias), setting(setting)
-        { }
     };
 
     typedef std::map<std::string, SettingData> Settings;
@@ -163,9 +158,7 @@ private:
 
 public:
 
-    Config(const StringMap & initials = {})
-        : AbstractConfig(initials)
-    { }
+    Config(StringMap initials = {});
 
     bool set(const std::string & name, const std::string & value) override;
 
@@ -206,12 +199,7 @@ protected:
         const std::set<std::string> & aliases,
         std::optional<ExperimentalFeature> experimentalFeature = std::nullopt);
 
-    virtual ~AbstractSetting()
-    {
-        // Check against a gcc miscompilation causing our constructor
-        // not to run (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80431).
-        assert(created == 123);
-    }
+    virtual ~AbstractSetting();
 
     virtual void set(const std::string & value, bool append = false) = 0;
 
@@ -229,7 +217,7 @@ protected:
 
     virtual void convertToArg(Args & args, const std::string & category);
 
-    bool isOverridden() const { return overridden; }
+    bool isOverridden() const;
 };
 
 /**
@@ -324,8 +312,7 @@ public:
 template<typename T>
 std::ostream & operator <<(std::ostream & str, const BaseSetting<T> & opt)
 {
-    str << (const T &) opt;
-    return str;
+    return str << static_cast<const T &>(opt);
 }
 
 template<typename T>
@@ -365,11 +352,7 @@ public:
         const Path & def,
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases = {})
-        : BaseSetting<Path>(def, true, name, description, aliases)
-    {
-        options->addSetting(this);
-    }
+        const std::set<std::string> & aliases = {});
 
     Path parse(const std::string & str) const override;
 
@@ -391,15 +374,11 @@ public:
         const std::optional<Path> & def,
         const std::string & name,
         const std::string & description,
-        const std::set<std::string> & aliases = {})
-        : BaseSetting<std::optional<Path>>(def, true, name, description, aliases)
-    {
-        options->addSetting(this);
-    }
+        const std::set<std::string> & aliases = {});
 
     std::optional<Path> parse(const std::string & str) const override;
 
-    void operator =(const std::optional<Path> & v) { this->assign(v); }
+    void operator =(const std::optional<Path> & v);
 };
 
 struct GlobalConfig : public AbstractConfig