[committed 2/2] libstdc++: Implement P2918R0 "Runtime format strings II" for C++26

[Jonathan Wakely]

On Wed, 10 Jan 2024 at 18:33, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
>
> Am Mo., 8. Jan. 2024 um 03:25 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
> >
> > Tested x86_64-linux and aarch64-linux. Pushed to trunk.
> >
> > -- >8 --
> >
> > This adds std::runtime_format for C++26. These new overloaded functions
> > enhance the std::format API so that it isn't necessary to use the less
> > ergonomic std::vformat and std::make_format_args (which are meant to be
> > implementation details). This was approved in Kona 2023 for C++26.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/std/format (__format::_Runtime_format_string): Define
> >         new class template.
> >         (basic_format_string): Add non-consteval constructor for runtime
> >         format strings.
> >         (runtime_format): Define new function for C++26.
> >         * testsuite/std/format/runtime_format.cc: New test.
> > ---
> >  libstdc++-v3/include/std/format               | 22 +++++++++++
> >  .../testsuite/std/format/runtime_format.cc    | 37 +++++++++++++++++++
> >  2 files changed, 59 insertions(+)
> >  create mode 100644 libstdc++-v3/testsuite/std/format/runtime_format.cc
> >
> > diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
> > index 160efa5155c..b3b5a0bbdbc 100644
> > --- a/libstdc++-v3/include/std/format
> > +++ b/libstdc++-v3/include/std/format
> > @@ -81,6 +81,9 @@ namespace __format
> >
> >    template<typename _CharT>
> >      using __format_context = basic_format_context<_Sink_iter<_CharT>, _CharT>;
> > +
> > +  template<typename _CharT>
> > +    struct _Runtime_format_string { basic_string_view<_CharT> _M_str; };
> >  } // namespace __format
> >  /// @endcond
> >
> > @@ -115,6 +118,11 @@ namespace __format
> >         consteval
> >         basic_format_string(const _Tp& __s);
> >
> > +      [[__gnu__::__always_inline__]]
> > +      basic_format_string(__format::_Runtime_format_string<_CharT>&& __s)
> > +      : _M_str(__s._M_str)
> > +      { }
> > +
>
> My understanding is that this constructor should be noexcept according to N4971.
>
> >        [[__gnu__::__always_inline__]]
> >        constexpr basic_string_view<_CharT>
> >        get() const noexcept
> > @@ -133,6 +141,20 @@ namespace __format
> >        = basic_format_string<wchar_t, type_identity_t<_Args>...>;
> >  #endif
> >
> > +#if __cplusplus > 202302L
> > +  [[__gnu__::__always_inline__]]
> > +  inline __format::_Runtime_format_string<char>
> > +  runtime_format(string_view __fmt)
> > +  { return {__fmt}; }
> > +
> > +#ifdef _GLIBCXX_USE_WCHAR_T
> > +  [[__gnu__::__always_inline__]]
> > +  inline __format::_Runtime_format_string<wchar_t>
> > +  runtime_format(wstring_view __fmt)
> > +  { return {__fmt}; }
> > +#endif
> > +#endif // C++26
> > +
>
> These runtime_format overloads should also be noexcept.

Good catch. Looks like I implemented it from the P2918R0 proposal, not
the final R2. Oops.

I'll apply the changes, thanks!