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.
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
>
>
>
