Don't construct a locale when formatting year_month_day - #4940
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formatter<year_month_day>::format passes true to get_locale, so every default format constructs a std::locale, and tm_writer then compares it against the classic locale (is_classic_(loc_ == get_classic_locale())). The only thing written is on_iso_date(), which is pure arithmetic and reads neither loc_ nor is_classic_. It is the odd one out among its siblings: formatter<day> and formatter<year> pass false; formatter<weekday> and formatter<month> pass their localized() flag. Only year_month_day hardcodes true, and its parse() never sets localized in the first place. With FMT_USE_LOCALE=0 this costs more than a copy: the loc.get<std::locale>() arm of get_locale is compiled out, so it default-constructs instead, capturing the current global locale in a build that asked for no locale support. No output change: chrono_test.year_month_day already sets a non-classic global locale and expects "2024-01-01". Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
vitaut
approved these changes
Sep 13, 2026
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#4921 subsumes this but since it's not ready to merge let's merge this small change anyway. Thanks. |
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formatter<year_month_day>::formatpassestruetoget_locale, so every default format constructs astd::locale, andtm_writerthen compares it against the classic locale (is_classic_(loc_ == get_classic_locale())). The only thing written ison_iso_date(), which is pure arithmetic and reads neitherloc_noris_classic_.It is the odd one out among its siblings:
formatter<day>andformatter<year>passfalse;formatter<weekday>andformatter<month>pass theirlocalized()flag. Onlyyear_month_dayhardcodestrue, and itsparse()never sets localized in the first place.With
FMT_USE_LOCALE=0this costs more than a copy: theloc.get<std::locale>()arm ofget_localeis compiled out, so it default-constructs instead, which captures the current global locale in a build that asked for no locale support.No output change:
chrono_test.year_month_dayalready sets a non-classic global locale and expects"2024-01-01". Verified 33/33 chrono tests pass (MSVC 19.51,-DCMAKE_CXX_STANDARD=20).