On Thu, 12 Jun 2025, 16:56 Patrick Palka, <ppa...@redhat.com> wrote:
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
> I'm sure if introducing a new overload is preferable to
> introducing a constexpr if branch in the existing overload.
>
I think a constexpr if branch in the existing functions is cheaper. We need
to check the traits either way, but we can avoid doing overload resolution.
> -- >8 --
>
> When composing a comparator/predicate and projection function, if both
> are stateless (and default constructible) then we don't need to capture
> them by reference; we can just return a captureless lambda and materialize
> them on the spot.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_algo.h (__detail::__make_comp_proj): New
> more specialized overload returning a stateless lambda if both
> the comparator/predicate and projection are stateless.
> (__detail::__make_pred_proj): Likewise.
> ---
> libstdc++-v3/include/bits/ranges_algo.h | 27 +++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h
> b/libstdc++-v3/include/bits/ranges_algo.h
> index 819316fa79c3..7dc84d639fbd 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -61,6 +61,21 @@ namespace ranges
> };
> }
>
> + template<typename _Comp, typename _Proj>
> + requires is_empty_v<_Comp> && is_default_constructible_v<_Comp>
> + && is_empty_v<_Proj> && is_default_constructible_v<_Proj>
> + constexpr auto
> + __make_comp_proj(_Comp& __comp, _Proj& __proj)
> + {
> + return [] (auto&& __lhs, auto&& __rhs) -> bool {
> + using _TL = decltype(__lhs);
> + using _TR = decltype(__rhs);
> + return std::__invoke(_Comp{},
> + std::__invoke(_Proj{},
> std::forward<_TL>(__lhs)),
> + std::__invoke(_Proj{},
> std::forward<_TR>(__rhs)));
> + };
> + }
> +
> template<typename _Pred, typename _Proj>
> constexpr auto
> __make_pred_proj(_Pred& __pred, _Proj& __proj)
> @@ -70,6 +85,18 @@ namespace ranges
> std::__invoke(__proj,
> std::forward<_Tp>(__arg)));
> };
> }
> +
> + template<typename _Pred, typename _Proj>
> + requires is_empty_v<_Pred> && is_default_constructible_v<_Pred>
> + && is_empty_v<_Proj> && is_default_constructible_v<_Pred>
> + constexpr auto
> + __make_pred_proj(_Pred& __pred, _Proj& __proj)
> + {
> + return [] <typename _Tp> (_Tp&& __arg) -> bool {
> + return std::__invoke(_Pred{},
> + std::__invoke(_Proj{},
> std::forward<_Tp>(__arg)));
> + };
> + }
> } // namespace __detail
>
> struct __all_of_fn
> --
> 2.50.0.rc2
>
>
