pretending to be const

src/error-demo/error-demo.cc

@@ -43,6 +43,22 @@ int main()
         logError(e.info());
     }
 
+    // current exception
+    try {
+        throw DemoError("DemoError handled as a %1%", "std::exception");
+    }
+    catch (...) {
+        const std::exception_ptr &eptr = std::current_exception();
+        try
+        {
+            std::rethrow_exception(eptr);
+        }
+        catch (std::exception& e)
+        {
+            std::cerr << e.what() << std::endl;
+        }
+    }
+
     // For completeness sake, info through vomit levels.
     // But this is maybe a heavy format for those.
     logger->logEI(

src/libutil/error.hh

@@ -82,10 +82,10 @@ class BaseError : public std::exception
 {
 protected:
     string prefix_; // used for location traces etc.
-    ErrorInfo err;
+    mutable ErrorInfo err;
 
-    std::optional<string> what_;
-    const string& calcWhat()
+    mutable std::optional<string> what_;
+    const string& calcWhat() const
     {
         if (what_.has_value())
             return *what_;
@@ -131,12 +131,12 @@ public:
 
 #ifdef EXCEPTION_NEEDS_THROW_SPEC
     ~BaseError() throw () { };
-    const char * what() throw () { return calcWhat().c_str(); }
+    const char * what() const throw () { return calcWhat().c_str(); }
 #else
-    const char * what() noexcept { return calcWhat().c_str(); }
+    const char * what() const noexcept override { return calcWhat().c_str(); }
 #endif
 
-    const string & msg() { return calcWhat(); }
+    const string & msg() const { return calcWhat(); }
     const string & prefix() const { return prefix_; }
     BaseError & addPrefix(const FormatOrString & fs);
 

src/nix-build/nix-build.cc

@@ -368,7 +368,7 @@ static void _main(int argc, char * * argv)
                 shell = drv->queryOutPath() + "/bin/bash";
 
             } catch (Error & e) {
-                // TODO: append error msg
+                // TODO: append error msg; warn()?
                 logError(e.info());
                 printError("warning: %s; will use bash from your environment", e.what());
                 shell = "bash";