[PATCH v2] libstdc++: Implement formatters for pair and tuple [PR109162]

[Jonathan Wakely]

On Wed, 16 Apr 2025 at 11:10, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 16/04/25 11:45 +0200, Tomasz Kamiński wrote:
> >This patch implements formatter specializations for pair and tuple form
> >P2286R8. In addition using 'm` and range_format::map (from P2585R1) for
> >ranges are now supported.
> >
> >The formatters for pairs and tuples whose corresponding elements are the same
> >(after applying remove_cvref_t) derive from the same __tuple_formatter class.
> >This reduce the code duplication, as most of the parsing and formatting is the
> >same in such cases. We use a custom reduced implementation of the tuple
> >(__formatters_storage) to store the elements formatters.
> >
> >Handling of the padding (width and fill) options, is extracted to
> >__format::__format_padded function, that is used both by __tuple_formatter and
> >range_formatter. To reduce number of instantations range_formatter::format
> >triggers, we cast incoming range to __format::__maybe_const_range<_Rg, _CharT>&,
> >before formatting it.
> >
> >As in the case of previous commits, the signatures of the user-facing parse
> >and format methods of the provided formatters deviate from the standard by
> >constraining types of parameters:
> >* _CharT is constrained __formatter::__char
> >* basic_format_parse_context<_CharT> for parse argument
> >* basic_format_context<_Out, _CharT> for format second argument
> >The standard specifies last three of above as unconstrained types.
> >
> >Finally, test for tuple-like std::array and std::ranges::subrange,
> >that illustrate that they remain formatted as ranges.
> >
> >       PR libstdc++/PR109162
> >
> >libstdc++-v3/ChangeLog:
> >
> >       * include/std/format (__formatter_int::_M_format_character_escaped)
> >       (__formatter_str::format): Use __sink.out() to produce _Sink_iter.
> >       (__format::__const_formattable_range): Moved closer to range_formatter.
> >       (__format::__maybe_const_range): Use `__conditional_t` and moved closer
> >       to range_formatter.
> >       (__format::__format_padded, __format::maybe_const)
> >       (__format::__indexed_formatter_storage, __format::__tuple_formatter)
> >       (std::formatter<pair<_Fp, _Sp>, _CharT>>)
> >       (std::formatter<tuple<_Tps...>, _CharT): Define.
> >       (std::formatter<_Rg, _CharT>::format): Cast incoming range to
> >       __format::__maybe_const_range<_Rg, _CharT>&.
> >       (std::formatter<_Rg, _CharT>::_M_format): Extracted from format,
> >       and use __format_padded.
> >       (std::formatter<_Rg, _CharT>::_M_format_no_padding): Rename...
> >       (std::formatter<_Rg, _CharT>::_M_format_elems): ...to this.
> >       (std::formatter<_Rg, _CharT>::_M_format_with_padding): Extracted as
> >       __format_padded.
> >       * testsuite/util/testsuite_iterators.h (test_input_range_nocopy):
> >       Define.
> >       * testsuite/std/format/ranges/formatter.cc: Tests for `m` specifier.
> >       * testsuite/std/format/ranges/sequence.cc: Tests for array and subrange.
> >       * testsuite/std/format/ranges/map.cc: New test.
> >       * testsuite/std/format/tuple.cc: New test.
> >---
> >I doged the which is safe, static_cast or const_cast discussion by
> >calling:
> >+         using __maybe_const_range
> >+           = __format::__maybe_const_range<_Rg, _CharT>;
> >+         return _M_format<__maybe_const_range>(__rg, __fc);
>
> OK.
>
> >I have also applied the `__conditional_t` to __maybe_const_range,
> >and moved this two helpers closer to range_formatter.
>
> Thanks.
>
> And addressof is qualified, and trigger is renamed to _M_trigger.
>
> >OK for trunk?
>
> OK, thanks.

(With the base class removed from __indexed_formatter_storage of course)