On 01/07/2026 13:33, Tomasz Kaminski wrote:
>
>
> On Wed, Jul 1, 2026 at 2:15 PM Anlai Lu <[email protected] <mailto:[email protected]>>
> wrote:
>
> Add __detail::__chrono_write which formats a chrono object into a
> stack buffer via std::format_to_n with _S_empty_fs(), then writes
> through __ostream_insert. A non-type template parameter _BufSize
> (default 128) allows per-type buffer tuning. Each _BufSize is the
> smallest power of two that accommodates the longest possible formatted
> output for that type, including error cases. All chrono operator<<
> overloads that previously used std::format, std::vformat, or
> basic_stringstream now use this helper.
>
> 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] <mailto:[email protected]>>
> ---
> libstdc++-v3/include/bits/chrono_io.h | 244 ++++++--------------------
> 1 file changed, 52 insertions(+), 192 deletions(-)
>
> LGTM. Thank you for working on this.
>
I think this is causing:
/home/rearnsha/gnusrc/gcc-cross/misc-fixes/libstdc++-v3/src/c++20/tzdb.cc: In st
atic member function ‘static const std::chrono::tzdb& std::chrono::tzdb_list::_N
ode::_S_replace_head(std::shared_ptr<std::chrono::tzdb_list::_Node>, std::shared
_ptr<std::chrono::tzdb_list::_Node>)’:
/home/rearnsha/gnusrc/gcc-cross/misc-fixes/libstdc++-v3/src/c++20/tzdb.cc:1617:22:
error: ‘struct std::chrono::tzdb_list::_Node::NumLeapSeconds’ has no member
named ‘set_locked’
1617 | num_leap_seconds.set_locked(new_head_ptr->db.leap_seconds.size(),
lock);
| ^~~~~~~~~~
make[9]: *** [Makefile:575: tzdb.lo] Error 1
when building an arm-eabi cross with newlib.
>
> diff --git a/libstdc++-v3/include/bits/chrono_io.h
> b/libstdc++-v3/include/bits/chrono_io.h
> index f8bb485d1..d3740a0e1 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 +3609,30 @@ 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<<.
> + // _BufSize allows per-type tuning of the stack buffer size.
> + template<size_t _BufSize = 128, typename _CharT, typename _Traits,
>
> I am wondering about making buffer size explicit on each call, and
> removing the default. But I will handle that before committing, if Jonathan
> also like that direction.
>
> + 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);
> + constexpr auto __fs
> + = __format::__formatter_chrono<_CharT>::_S_empty_fs;
>
> Nice trick for storing a function pointer. I really like it.
>
>
> + constexpr size_t __bufsize = _BufSize;
> + _CharT __buf[__bufsize];
> + auto __res = std::format_to_n(__buf, __bufsize, __loc...,
> + __fs(), __arg);
> + if (static_cast<size_t>(__res.size) <= __bufsize) [[likely]]
> + return std::__ostream_insert(__os, __buf, __res.size);
> + auto __s = std::format(__loc..., __fs(), __arg);
> + return std::__ostream_insert(__os, __s.data(), __s.size());
> + }
> +
> } // namespace __detail
> /// @endcond
>
> @@ -3629,14 +3655,7 @@ 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<32>(__os, __d);
> }
>
> template<typename _CharT, typename _Traits,
> @@ -3657,18 +3676,7 @@ 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 +3697,7 @@ 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<32>(__os, __y);
> }
>
> template<typename _CharT, typename _Traits,
> @@ -3721,18 +3718,7 @@ 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 +3740,7 @@ 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 +3748,14 @@ 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 +3779,7 @@ 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 +3787,7 @@ 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 +3795,14 @@ 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 +3826,7 @@ 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<64>(__os, __ymd);
> }
>
> template<typename _CharT, typename _Traits,
> @@ -3936,17 +3851,7 @@ 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 +3859,7 @@ 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 +3867,7 @@ 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 +3875,7 @@ 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<64>(__os, __hms, __os.getloc());
> }
>
> #if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
> @@ -4003,7 +3884,7 @@ 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.
> @@ -4011,20 +3892,7 @@ namespace __detail
> basic_ostream<_CharT, _Traits>&
> operator<<(basic_ostream<_CharT, _Traits>& __os, const local_info&
> __li)
> {
> - __os << __format::_Separators<_CharT>::_S_squares()[0];
> - if (__li.result == local_info::unique)
> - __os << __li.first;
> - else
> - {
> - if (__li.result == local_info::nonexistent)
> - __os << _GLIBCXX_WIDEN("nonexistent");
> - else
> - __os << _GLIBCXX_WIDEN("ambiguous");
> - __os << _GLIBCXX_WIDEN(" local time between ") << __li.first;
> - __os << _GLIBCXX_WIDEN(" and ") << __li.second;
> - }
> - __os << __format::_Separators<_CharT>::_S_squares()[1];
> - return __os;
> + return __detail::__chrono_write<256>(__os, __li);
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4033,8 +3901,7 @@ 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 +3912,14 @@ 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<64>(__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<32>(__os, __dp);
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4090,8 +3955,7 @@ 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<64>(__os, __t, __os.getloc());
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4125,8 +3989,7 @@ 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<64>(__os, __t, __os.getloc());
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4164,8 +4027,7 @@ 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<64>(__os, __t, __os.getloc());
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4202,8 +4064,7 @@ 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<64>(__os, __t, __os.getloc());
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> @@ -4228,8 +4089,7 @@ 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<64>(__os, __lt, __os.getloc());
> }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> --
> 2.34.1
>
