On Tue, Jun 30, 2026 at 3:18 PM Tomasz Kaminski <[email protected]> wrote:
> > > On Tue, Jun 30, 2026 at 2:50 PM Anlai Lu <[email protected]> wrote: > >> Add __detail::__chrono_write which formats a chrono object into a >> 128-byte stack buffer via std::format_to_n with _S_empty_fs(), then >> writes through __ostream_insert. All chrono operator<< overloads >> now use this helper instead of hand-rolled format strings or >> basic_stringstream construction. >> >> libstdc++-v3/ChangeLog: >> >> * include/bits/chrono_io.h (__formatter_chrono::_S_empty_fs): >> Make public. >> (__detail::__chrono_write): New function template. >> (operator<<): Use __chrono_write consistently for all chrono >> types. >> >> Signed-off-by: Anlai Lu <[email protected]> >> --- >> libstdc++-v3/include/bits/chrono_io.h | 227 ++++++-------------------- >> 1 file changed, 49 insertions(+), 178 deletions(-) >> > This looks good to me, I only have a small stylistic suggestion and think > local info > It can also be modified in same way. > I have convinced myself that changing local_info would not give us any performance benefits, so please keep it as is. > > Note, I do not have approval rights, so you will need one from Jonathan. > > >> >> diff --git a/libstdc++-v3/include/bits/chrono_io.h >> b/libstdc++-v3/include/bits/chrono_io.h >> index f8bb485d1..a07543bab 100644 >> --- a/libstdc++-v3/include/bits/chrono_io.h >> +++ b/libstdc++-v3/include/bits/chrono_io.h >> @@ -884,11 +884,13 @@ namespace __format >> static constexpr const _CharT* _S_minus_empty_spec = _S_chars + 17; >> static constexpr const _CharT* _S_empty_spec = _S_chars + 18; >> >> + public: >> [[__gnu__::__always_inline__]] >> static _Dynamic_format_string<_CharT> >> _S_empty_fs() >> { return _Dynamic_format_string<_CharT>(_S_empty_spec); } >> >> + protected: >> static constexpr const _CharT* _S_weekdays[] >> { >> _GLIBCXX_WIDEN("Sunday"), >> @@ -3607,6 +3609,28 @@ namespace __detail >> } >> } >> >> + // Format into a stack buffer via std::format_to_n with the empty >> + // chrono-spec (_S_empty_fs), then write through __ostream_insert. >> + // The empty spec lets each formatter use its __defSpec, producing >> + // output equivalent to the corresponding operator<<. >> + template<typename _CharT, typename _Traits, typename _Arg, >> + typename... _OptLocale> >> + inline basic_ostream<_CharT, _Traits>& >> + __chrono_write(basic_ostream<_CharT, _Traits>& __os, >> + const _Arg& __arg, const _OptLocale&... __loc) >> + { >> + static_assert(sizeof...(_OptLocale) <= 1); >> + using _Fmt = __format::__formatter_chrono<_CharT>; >> > I would save __fs here, instad of alias, something like: > constexpr auto __fs = > __format::__formatter_chrono<_CharT>::_S_empty_fs(); > > + constexpr size_t __bufsize = 128; >> + _CharT __buf[__bufsize]; >> + auto __res = std::format_to_n(__buf, __bufsize, __loc..., >> + _Fmt::_S_empty_fs(), __arg); >> + if (static_cast<size_t>(__res.size) <= __bufsize) [[likely]] >> + return std::__ostream_insert(__os, __buf, __res.size); >> + auto __s = std::format(__loc..., _Fmt::_S_empty_fs(), __arg); >> + return std::__ostream_insert(__os, __s.data(), __s.size()); >> + } >> + >> } // namespace __detail >> /// @endcond >> >> @@ -3629,14 +3653,7 @@ namespace __detail >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const day& __d) >> { >> - using _Ctx = __format::__format_context<_CharT>; >> - using _Str = basic_string_view<_CharT>; >> - _Str __s = _GLIBCXX_WIDEN("{:02d} is not a valid day"); >> - if (__d.ok()) >> - __s = __s.substr(0, 6); >> - auto __u = (unsigned)__d; >> - __os << std::vformat(__s, make_format_args<_Ctx>(__u)); >> - return __os; >> + return __detail::__chrono_write(__os, __d); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3657,18 +3674,7 @@ namespace __detail >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const month& __m) >> { >> - using _Ctx = __format::__format_context<_CharT>; >> - using _Str = basic_string_view<_CharT>; >> - _Str __s = _GLIBCXX_WIDEN("{:L%b}{} is not a valid month"); >> - if (__m.ok()) >> - __os << std::vformat(__os.getloc(), __s.substr(0, 6), >> - make_format_args<_Ctx>(__m)); >> - else >> - { >> - auto __u = (unsigned)__m; >> - __os << std::vformat(__s.substr(6), >> make_format_args<_Ctx>(__u)); >> - } >> - return __os; >> + return __detail::__chrono_write(__os, __m, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3689,18 +3695,7 @@ namespace __detail >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const year& __y) >> { >> - using _Ctx = __format::__format_context<_CharT>; >> - using _Str = basic_string_view<_CharT>; >> - _Str __s = _GLIBCXX_WIDEN("-{:04d} is not a valid year"); >> - if (__y.ok()) >> - __s = __s.substr(0, 7); >> - int __i = (int)__y; >> - if (__i >= 0) [[likely]] >> - __s.remove_prefix(1); >> - else >> - __i = -__i; >> - __os << std::vformat(__s, make_format_args<_Ctx>(__i)); >> - return __os; >> + return __detail::__chrono_write(__os, __y); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3721,18 +3716,7 @@ namespace __detail >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const weekday& __wd) >> { >> - using _Ctx = __format::__format_context<_CharT>; >> - using _Str = basic_string_view<_CharT>; >> - _Str __s = _GLIBCXX_WIDEN("{:L%a}{} is not a valid weekday"); >> - if (__wd.ok()) >> - __os << std::vformat(__os.getloc(), __s.substr(0, 6), >> - make_format_args<_Ctx>(__wd)); >> - else >> - { >> - auto __c = __wd.c_encoding(); >> - __os << std::vformat(__s.substr(6), >> make_format_args<_Ctx>(__c)); >> - } >> - return __os; >> + return __detail::__chrono_write(__os, __wd, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3754,23 +3738,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const weekday_indexed& __wdi) >> { >> - // The standard says to format wdi.weekday() and wdi.index() using >> - // either "{:L}[{}]" or "{:L}[{} is not a valid index]". The {:L} >> spec >> - // means to format the weekday using ostringstream, so just do >> that. >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __wdi.weekday(); >> - const auto __i = __wdi.index(); >> - basic_string_view<_CharT> __s >> - = _GLIBCXX_WIDEN("[ is not a valid index]"); >> - __os2 << __s[0]; >> - __os2 << std::format(_GLIBCXX_WIDEN("{}"), __i); >> - if (__i >= 1 && __i <= 5) >> - __os2 << __s.back(); >> - else >> - __os2 << __s.substr(1); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __wdi, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> @@ -3778,29 +3746,14 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const weekday_last& __wdl) >> { >> - // As above, just write straight to a stringstream, as if by >> "{:L}[last]" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __wdl.weekday() << _GLIBCXX_WIDEN("[last]"); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __wdl, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const month_day& >> __md) >> { >> - // As above, just write straight to a stringstream, as if by >> "{:L}/{}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __md.month(); >> - if constexpr (is_same_v<_CharT, char>) >> - __os2 << '/'; >> - else >> - __os2 << L'/'; >> - __os2 << __md.day(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __md, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3824,12 +3777,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const month_day_last& __mdl) >> { >> - // As above, just write straight to a stringstream, as if by >> "{:L}/last" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __mdl.month() << _GLIBCXX_WIDEN("/last"); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __mdl, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> @@ -3837,17 +3785,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const month_weekday& __mwd) >> { >> - // As above, just write straight to a stringstream, as if by >> "{:L}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __mwd.month(); >> - if constexpr (is_same_v<_CharT, char>) >> - __os2 << '/'; >> - else >> - __os2 << L'/'; >> - __os2 << __mwd.weekday_indexed(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __mwd, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> @@ -3855,34 +3793,14 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const month_weekday_last& __mwdl) >> { >> - // As above, just write straight to a stringstream, as if by >> "{:L}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __mwdl.month(); >> - if constexpr (is_same_v<_CharT, char>) >> - __os2 << '/'; >> - else >> - __os2 << L'/'; >> - __os2 << __mwdl.weekday_last(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __mwdl, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const year_month& >> __ym) >> { >> - // As above, just write straight to a stringstream, as if by >> "{}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __ym.year(); >> - if constexpr (is_same_v<_CharT, char>) >> - __os2 << '/'; >> - else >> - __os2 << L'/'; >> - __os2 << __ym.month(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __ym, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3906,12 +3824,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const year_month_day& __ymd) >> { >> - using _Ctx = __format::__format_context<_CharT>; >> - using _Str = basic_string_view<_CharT>; >> - _Str __s = _GLIBCXX_WIDEN("{:%F} is not a valid date"); >> - __os << std::vformat(__ymd.ok() ? __s.substr(0, 5) : __s, >> - make_format_args<_Ctx>(__ymd)); >> - return __os; >> + return __detail::__chrono_write(__os, __ymd); >> } >> >> template<typename _CharT, typename _Traits, >> @@ -3936,17 +3849,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const year_month_day_last& __ymdl) >> { >> - // As above, just write straight to a stringstream, as if by >> "{}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - __os2 << __ymdl.year(); >> - if constexpr (is_same_v<_CharT, char>) >> - __os2 << '/'; >> - else >> - __os2 << L'/'; >> - __os2 << __ymdl.month_day_last(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __ymdl, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> @@ -3954,19 +3857,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const year_month_weekday& __ymwd) >> { >> - // As above, just write straight to a stringstream, as if by >> - // "{}/{:L}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - _CharT __slash; >> - if constexpr (is_same_v<_CharT, char>) >> - __slash = '/'; >> - else >> - __slash = L'/'; >> - __os2 << __ymwd.year() << __slash << __ymwd.month() << __slash >> - << __ymwd.weekday_indexed(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __ymwd, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> @@ -3974,19 +3865,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const year_month_weekday_last& __ymwdl) >> { >> - // As above, just write straight to a stringstream, as if by >> - // "{}/{:L}/{:L}" >> - basic_stringstream<_CharT> __os2; >> - __os2.imbue(__os.getloc()); >> - _CharT __slash; >> - if constexpr (is_same_v<_CharT, char>) >> - __slash = '/'; >> - else >> - __slash = L'/'; >> - __os2 << __ymwdl.year() << __slash << __ymwdl.month() << __slash >> - << __ymwdl.weekday_last(); >> - __os << __os2.view(); >> - return __os; >> + return __detail::__chrono_write(__os, __ymwdl, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration> >> @@ -3994,7 +3873,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const hh_mm_ss<_Duration>& __hms) >> { >> - return __os << format(__os.getloc(), _GLIBCXX_WIDEN("{:L%T}"), >> __hms); >> + return __detail::__chrono_write(__os, __hms, __os.getloc()); >> } >> >> #if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI >> @@ -4003,7 +3882,7 @@ namespace __detail >> basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const sys_info& __i) >> { >> - return __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{}"), >> __i); >> + return __detail::__chrono_write(__os, __i, __os.getloc()); >> } >> >> /// Writes a local_info object to an ostream in an unspecified format. >> @@ -4033,8 +3912,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const zoned_time<_Duration, _TimeZonePtr>& __t) >> { >> - __os << format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T %Z}"), __t); >> - return __os; >> + return __detail::__chrono_write(__os, __t, __os.getloc()); >> } >> #endif >> >> @@ -4045,16 +3923,14 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const sys_time<_Duration>& __tp) >> { >> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"), >> __tp); >> - return __os; >> + return __detail::__chrono_write(__os, __tp, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits> >> inline basic_ostream<_CharT, _Traits>& >> operator<<(basic_ostream<_CharT, _Traits>& __os, const sys_days& >> __dp) >> { >> - __os << year_month_day{__dp}; >> - return __os; >> + return __detail::__chrono_write(__os, __dp); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> @@ -4090,8 +3966,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const utc_time<_Duration>& __t) >> { >> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"), >> __t); >> - return __os; >> + return __detail::__chrono_write(__os, __t, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> @@ -4125,8 +4000,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const tai_time<_Duration>& __t) >> { >> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"), >> __t); >> - return __os; >> + return __detail::__chrono_write(__os, __t, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> @@ -4164,8 +4038,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const gps_time<_Duration>& __t) >> { >> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"), >> __t); >> - return __os; >> + return __detail::__chrono_write(__os, __t, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> @@ -4202,8 +4075,7 @@ namespace __detail >> operator<<(basic_ostream<_CharT, _Traits>& __os, >> const file_time<_Duration>& __t) >> { >> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"), >> __t); >> - return __os; >> + return __detail::__chrono_write(__os, __t, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> @@ -4228,8 +4100,7 @@ namespace __detail >> // 4257. Stream insertion for chrono::local_time should be >> constrained >> requires requires(const sys_time<_Duration>& __st) { __os << __st; } >> { >> - __os << sys_time<_Duration>{__lt.time_since_epoch()}; >> - return __os; >> + return __detail::__chrono_write(__os, __lt, __os.getloc()); >> } >> >> template<typename _CharT, typename _Traits, typename _Duration, >> -- >> 2.34.1 >> >> >>
