On Wed, 3 Sep 2025, Tomasz Kamiński wrote:
> This patch refactors ranges::_Partial to be implemented using _Bind_back_t.
> This allows it to benefit from the changes in r16-3398-g250dd5b5604fbc,
> specifically making the closure trivially copyable. Since _Bind_back_t
> already provides an optimized implementation for a single bound argument,
> specializations for _Partial with a single argument are now removed.
>
> We still preserve a specialization of _Partial for trivially
> copy-constructible
> arguments that define only a const overload of operator(). To avoid
> re-checking invocability constraints, this specialization calls the
> now-public,
> unconstrained _Binder::_S_call static method instead of the constrained
> _Binder::operator().
>
> The primary specialization of _Partial retains its operator(), which
> uses a simpler __adaptor_invocable constraint that does not consider
> member pointers, as they are not relevant here. This implementation also
> calls _Binder::_S_call to avoid re-performing overload resolution and
> invocability checks for _Binder::operator().
Thanks for preserving/working around these _Partial optimizatinos! Note that
the primary template (in C++20 mode) also just defines 3 operator()
overloads instead of the full 8, sacrificing const&& invocability which
presumably isn't useful in practice. (The _Pipe closure object does
similar tricks.)
The simpler __adaptor_invocable constraint was added not just because
it's cheaper than is_invocable, but also because it made overload
resolution failure diagnostics actually explain why the adaptor wasn't
invocable. But now that we have builtins for is_invocable, and
satisfaction failure replays __is_invocable failures (after Nathaniel's
r16-2652-gbfb8615031a8c2), we might be able to get away with using
is_invocable_v directly without sacrificing compile time performance
and diagnostics. We could look into that as a follow-up.
LGTM, the second patch in the series LGTM too.
>
> Finally, the _M_binder member (_Bind_back_t) is now marked
> [[no_unique_address]]. This is beneficial as ranges::_Partial is used with
> ranges::to, which commonly has zero or empty bound arguments (e.g., stateless
> allocators, comparators, or hash functions).
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/binders.h (_Binder::_S_call): Make public.
> * include/std/ranges (ranges::_Partial<_Adaptor, _Args...>):
> Replace tuple<_Args...> with _Bind_back_t<_Adaptor, _Args...>.
> (ranges::_Partial<_Adaptor, _Arg>): Remove.
> ---
> Tested on x86_64-linux locally. OK for trunk?
>
> libstdc++-v3/include/bits/binders.h | 10 +--
> libstdc++-v3/include/std/ranges | 104 ++++------------------------
> 2 files changed, 17 insertions(+), 97 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/binders.h
> b/libstdc++-v3/include/bits/binders.h
> index 08a330da6fe..f274389e5b4 100644
> --- a/libstdc++-v3/include/bits/binders.h
> +++ b/libstdc++-v3/include/bits/binders.h
> @@ -186,11 +186,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> void operator()(_CallArgs&&...) const && = delete;
> #endif
>
> - private:
> - using _BoundArgsStorage
> - // _BoundArgs are required to be move-constructible, so this is valid.
> - =
> decltype(__make_bound_args<_BoundArgs...>(std::declval<_BoundArgs>()...));
> -
> template<typename _Tp, typename... _CallArgs>
> static constexpr
> decltype(auto)
> @@ -214,6 +209,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> std::forward<_CallArgs>(__call_args)...);
> }
>
> + private:
> + using _BoundArgsStorage
> + // _BoundArgs are required to be move-constructible, so this is valid.
> + =
> decltype(__make_bound_args<_BoundArgs...>(std::declval<_BoundArgs>()...));
> +
> [[no_unique_address]] _Fd _M_fd;
> [[no_unique_address]] _BoundArgsStorage _M_bound_args;
> };
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 2970b2cbe00..98154a02e9e 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -51,6 +51,7 @@
> #include <utility>
> #include <variant>
> #endif
> +#include <bits/binders.h>
> #include <bits/ranges_util.h>
> #include <bits/refwrap.h>
>
> @@ -1047,14 +1048,15 @@ namespace views::__adaptor
> template<typename _Adaptor, typename... _Args>
> struct _Partial : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> {
> - tuple<_Args...> _M_args;
> + using _Binder = _Bind_back_t<_Adaptor, _Args...>;
> + [[no_unique_address]] _Binder _M_binder;
>
> // First parameter is to ensure this constructor is never used
> // instead of the copy/move constructor.
> template<typename... _Ts>
> constexpr
> _Partial(int, _Ts&&... __args)
> - : _M_args(std::forward<_Ts>(__args)...)
> + : _M_binder(0, _Adaptor(), std::forward<_Ts>(__args)...)
> { }
>
> // Invoke _Adaptor with arguments __r, _M_args... according to the
> @@ -1065,75 +1067,21 @@ namespace views::__adaptor
> constexpr auto
> operator()(this _Self&& __self, _Range&& __r)
> {
> - auto __forwarder = [&__r] (auto&&... __args) {
> - return _Adaptor{}(std::forward<_Range>(__r),
> - std::forward<decltype(__args)>(__args)...);
> - };
> - return std::apply(__forwarder, __like_t<_Self,
> _Partial>(__self)._M_args);
> + return _Binder::_S_call(__like_t<_Self, _Partial>(__self)._M_binder,
> + std::forward<_Range>(__r));
> }
> #else
> template<typename _Range>
> requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
> constexpr auto
> operator()(_Range&& __r) const &
> - {
> - auto __forwarder = [&__r] (const auto&... __args) {
> - return _Adaptor{}(std::forward<_Range>(__r), __args...);
> - };
> - return std::apply(__forwarder, _M_args);
> - }
> + { return _Binder::_S_call(_M_binder, std::forward<_Range>(__r)); }
>
> template<typename _Range>
> requires __adaptor_invocable<_Adaptor, _Range, _Args...>
> constexpr auto
> operator()(_Range&& __r) &&
> - {
> - auto __forwarder = [&__r] (auto&... __args) {
> - return _Adaptor{}(std::forward<_Range>(__r), std::move(__args)...);
> - };
> - return std::apply(__forwarder, _M_args);
> - }
> -
> - template<typename _Range>
> - constexpr auto
> - operator()(_Range&& __r) const && = delete;
> -#endif
> - };
> -
> - // A lightweight specialization of the above primary template for
> - // the common case where _Adaptor accepts a single extra argument.
> - template<typename _Adaptor, typename _Arg>
> - struct _Partial<_Adaptor, _Arg> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> - {
> - _Arg _M_arg;
> -
> - template<typename _Tp>
> - constexpr
> - _Partial(int, _Tp&& __arg)
> - : _M_arg(std::forward<_Tp>(__arg))
> - { }
> -
> -#if __cpp_explicit_this_parameter
> - template<typename _Self, typename _Range>
> - requires __adaptor_invocable<_Adaptor, _Range, __like_t<_Self, _Arg>>
> - constexpr auto
> - operator()(this _Self&& __self, _Range&& __r)
> - {
> - return _Adaptor{}(std::forward<_Range>(__r),
> - __like_t<_Self, _Partial>(__self)._M_arg);
> - }
> -#else
> - template<typename _Range>
> - requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
> - constexpr auto
> - operator()(_Range&& __r) const &
> - { return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
> -
> - template<typename _Range>
> - requires __adaptor_invocable<_Adaptor, _Range, _Arg>
> - constexpr auto
> - operator()(_Range&& __r) &&
> - { return _Adaptor{}(std::forward<_Range>(__r), std::move(_M_arg)); }
> + { return _Binder::_S_call(std::move(_M_binder),
> std::forward<_Range>(__r)); }
>
> template<typename _Range>
> constexpr auto
> @@ -1150,12 +1098,13 @@ namespace views::__adaptor
> && (is_trivially_copy_constructible_v<_Args> && ...)
> struct _Partial<_Adaptor, _Args...> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> {
> - tuple<_Args...> _M_args;
> + using _Binder = _Bind_back_t<_Adaptor, _Args...>;
> + [[no_unique_address]] _Binder _M_binder;
>
> template<typename... _Ts>
> constexpr
> _Partial(int, _Ts&&... __args)
> - : _M_args(std::forward<_Ts>(__args)...)
> + : _M_binder(0, _Adaptor(), std::forward<_Ts>(__args)...)
> { }
>
> // Invoke _Adaptor with arguments __r, const _M_args&... regardless
> @@ -1164,36 +1113,7 @@ namespace views::__adaptor
> requires __adaptor_invocable<_Adaptor, _Range, const _Args&...>
> constexpr auto
> operator()(_Range&& __r) const
> - {
> - auto __forwarder = [&__r] (const auto&... __args) {
> - return _Adaptor{}(std::forward<_Range>(__r), __args...);
> - };
> - return std::apply(__forwarder, _M_args);
> - }
> -
> - static constexpr bool _S_has_simple_call_op = true;
> - };
> -
> - // A lightweight specialization of the above template for the common case
> - // where _Adaptor accepts a single extra argument.
> - template<typename _Adaptor, typename _Arg>
> - requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
> - && is_trivially_copy_constructible_v<_Arg>
> - struct _Partial<_Adaptor, _Arg> :
> _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> - {
> - _Arg _M_arg;
> -
> - template<typename _Tp>
> - constexpr
> - _Partial(int, _Tp&& __arg)
> - : _M_arg(std::forward<_Tp>(__arg))
> - { }
> -
> - template<typename _Range>
> - requires __adaptor_invocable<_Adaptor, _Range, const _Arg&>
> - constexpr auto
> - operator()(_Range&& __r) const
> - { return _Adaptor{}(std::forward<_Range>(__r), _M_arg); }
> + { return _Binder::_S_call(_M_binder, std::forward<_Range>(__r)); }
>
> static constexpr bool _S_has_simple_call_op = true;
> };
> --
> 2.51.0
>
>
