Should std::error_code::message respect the locale set by the user?

In our program, we use std::setlocale(LC_ALL, ".UTF8"); to support UTF-8 encoding, as noted in the Microsoft documentation. However, when certain C++ STL functions fail, their message() outputs are not in UTF-8.

After investigating the source code, I found that the error messages are always formatted according to the system's default locale, as shown in this STL implementation:

STL/stl/src/syserror_import_lib.cpp

Lines 38 to 54 in e36ee6c

    
           [[nodiscard]] size_t __CLRCALL_PURE_OR_STDCALL __std_system_error_allocate_message( 
        
               const unsigned long _Message_id, char** const _Ptr_str) noexcept { 
        
               // convert to name of Windows error, return 0 for failure, otherwise return number of chars in buffer 
        
               // __std_system_error_deallocate_message should be called even if 0 is returned 
        
               // pre: *_Ptr_str == nullptr 
        
               DWORD _Lang_id; 
        
               const int _Ret = GetLocaleInfoEx(LOCALE_NAME_SYSTEM_DEFAULT, LOCALE_ILANGUAGE | LOCALE_RETURN_NUMBER, 
        
                   reinterpret_cast<LPWSTR>(&_Lang_id), sizeof(_Lang_id) / sizeof(wchar_t)); 
        
               if (_Ret == 0) { 
        
                   _Lang_id = 0; 
        
               } 
        
               const unsigned long _Chars = 
        
                   FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 
        
                       nullptr, _Message_id, _Lang_id, reinterpret_cast<char*>(_Ptr_str), 0, nullptr); 
        
               return _CSTD __std_get_string_size_without_trailing_whitespace(*_Ptr_str, _Chars); 
        
           }

While it would be acceptable if the system locale used UTF-8 as the codepage, allowing the messages to be correctly printed to log files, this is not the case. There is a "Beta: Use Unicode UTF-8 for worldwide language support" setting in the Region settings, which does the job correctly, but toggling that setting requires a reboot, which is not acceptable for our requirements.

Question

Should the std::error_code::message function respect the locale set by the user (e.g., via std::setlocale), or should it continue to use the system's default locale? If the former, is there a plan to implement this behavior in the Microsoft STL implementation?

Related Information

I learned that the locale is by default set across the whole program, unless specified by configthreadlocale, from the Microsoft documentation.
The issue of system_error not honoring the current thread's locale is mentioned in this issue.

Please let me know if you need any additional information or clarification.

This is basically a duplicate of the #3254.
Also see the #3260.

I have the same question, when I get filesystem_error, it's message often uses the system locale, and I have to change the message to utf8

jovibor

jovibor Yep, agreed. Marking as "bug" but not resolving as a duplicate, until we review the pending PR.

The standard is unclear what encoding error_category::what should use - I've filed an LWG issue about it: https://cplusplus.github.io/LWG/issue4156. The current proposed resolution there is a bit of a strawman and likely to change. I would recommend waiting until this LWG issue is resolved.

Stephan T. Lavavej (@StephanTLavavej)

FYI SG16 just voted in favor of specifying that the string returned by error_category::what() is in the C locale (aka execution) encoding as part of P3395. The formatter specialization for error_code was made more conservative and left the transcoding decision to the implementation if the literal encoding is not UTF-8. Stephan T. Lavavej (@StephanTLavavej), please let me know if you have any concerns regarding these decisions.

The standard says that the encoding of the execution character set is locale-specific ([character.seq.general]/(1.2)):

The encodings of the execution character sets and the sets of additional elements (if any) are locale-specific.

It's unclear which locale should be used. Possible choices are

The "C" locale with UTF-8 encoding (setlocale(LC_CTYPE, "C.UTF-8") if supported)
The "C" locale (setlocale(LC_CTYPE, "C"))
The currently-installed global locale, whatever it is (setlocale(LC_CTYPE, nullptr))
Something else (like setlocale(LC_CTYPE, ""))

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Should `std::error_code::message` respect the locale set by the user? #4711

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	[[nodiscard]] size_t __CLRCALL_PURE_OR_STDCALL __std_system_error_allocate_message(
	const unsigned long _Message_id, char** const _Ptr_str) noexcept {
	// convert to name of Windows error, return 0 for failure, otherwise return number of chars in buffer
	// __std_system_error_deallocate_message should be called even if 0 is returned
	// pre: *_Ptr_str == nullptr
	DWORD _Lang_id;
	const int _Ret = GetLocaleInfoEx(LOCALE_NAME_SYSTEM_DEFAULT, LOCALE_ILANGUAGE \| LOCALE_RETURN_NUMBER,
	reinterpret_cast<LPWSTR>(&_Lang_id), sizeof(_Lang_id) / sizeof(wchar_t));
	if (_Ret == 0) {
	_Lang_id = 0;
	}
	const unsigned long _Chars =
	FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER \| FORMAT_MESSAGE_FROM_SYSTEM \| FORMAT_MESSAGE_IGNORE_INSERTS,
	nullptr, _Message_id, _Lang_id, reinterpret_cast<char*>(_Ptr_str), 0, nullptr);

	return _CSTD __std_get_string_size_without_trailing_whitespace(*_Ptr_str, _Chars);
	}

Should std::error_code::message respect the locale set by the user? #4711

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jovibor commented on Jun 4, 2024

miyanyan commented on Jun 5, 2024

StephanTLavavej commented on Jun 12, 2024

vitaut commented on Sep 28, 2024

vitaut commented on Mar 12, 2025

cpplearner commented on Mar 13, 2025

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Should `std::error_code::message` respect the locale set by the user? #4711