On Wed, Aug 20, 2025 at 12:09 PM Tomasz Kamiński <tkami...@redhat.com>
wrote:
> This patch adjusts the implementation of bind_front(f) and bind_back(f)
> (with zero bound arguments), so it returns an auto(f) (a copy of the
> functor), rather than a specialization of Bind_front/Bind_back. This change
> is mostly unobservable, as standard does not specify return type of
> above functions, but may result in elision of move/copy operations when
> by-value parameters are initialized with prvalues. See the update to
> testMaterialization.
>
> This behavior is technically not allowed by C++23 [func.require] p3,
> which requires that rvalue arguments are delivered as rvalue references,
> forcing their materialization. I have reported a LWG issue to make this
> conforming.
>
For future reference, the issue number is LWG4319. I will update the
description,
with review comments.
>
> The _Bind_front_t/_Bind_back_t aliases were removed, as the aliased types
> are now spelled only once.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/functional (_Bind_front_t, _Bind_back_t): Remove.
> (std::bind_front, std::bind_back): Return copy of __fn for
> zero bounds args.
> (_Bind_front, _Bind_back): Remove check for move constructor
> of _Fd, that is now part of bind_xxx functors.
> * testsuite/20_util/function_objects/bind_back/1.cc: Updated
> testMaterialization.
> * testsuite/20_util/function_objects/bind_front/1.cc: Likewise.
>
> Signed-off-by: Tomasz Kamiński <tkami...@redhat.com>
> ---
> This is implemented on top of 20250820081809.50754-1-tkami...@redhat.com.
> Do we think that this is OK for trunk? As mentioned technically not
> conforming, I do not think user will care about extra move.
>
> The goal here, would be able to say that bind_front<f>(args...) is
> bind_front(nttp<f>, args...).
>
> Tested on x86_64-linux.
>
> libstdc++-v3/include/std/functional | 37 ++++++++++---------
> .../20_util/function_objects/bind_back/1.cc | 5 ++-
> .../20_util/function_objects/bind_front/1.cc | 5 ++-
> 3 files changed, 26 insertions(+), 21 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/functional
> b/libstdc++-v3/include/std/functional
> index b1cda87929d..c8699ac849f 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -973,7 +973,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _Fd, typename... _BoundArgs>
> struct _Bind_front
> {
> - static_assert(is_move_constructible_v<_Fd>);
> static_assert((is_move_constructible_v<_BoundArgs> && ...));
>
> // First parameter is to ensure this constructor is never used
> @@ -1081,9 +1080,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> [[no_unique_address]] _BoundArgsStorage _M_bound_args;
> };
>
> - template<typename _Fn, typename... _Args>
> - using _Bind_front_t = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
> -
> /** Create call wrapper by partial application of arguments to function.
> *
> * The result of `std::bind_front(f, args...)` is a function object that
> @@ -1094,13 +1090,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> * @since C++20
> */
> template<typename _Fn, typename... _Args>
> - constexpr _Bind_front_t<_Fn, _Args...>
> + constexpr auto
> bind_front(_Fn&& __fn, _Args&&... __args)
> - noexcept(is_nothrow_constructible_v<_Bind_front_t<_Fn, _Args...>,
> - int, _Fn, _Args...>)
> + noexcept(__and_<is_nothrow_constructible<decay_t<_Fn>, _Fn>,
> + is_nothrow_constructible<decay_t<_Args>,
> _Args>...>::value)
> {
> - return _Bind_front_t<_Fn, _Args...>(0, std::forward<_Fn>(__fn),
> - std::forward<_Args>(__args)...);
> + using _Fd = decay_t<_Fn>;
> + static_assert(is_move_constructible_v<_Fd>);
> + if constexpr (sizeof...(_Args) == 0)
> + return _Fd(std::forward<_Fn>(__fn));
> + else
> + return _Bind_front<_Fd, decay_t<_Args>...>(0,
> std::forward<_Fn>(__fn),
> +
> std::forward<_Args>(__args)...);
> }
> #endif // __cpp_lib_bind_front
>
> @@ -1158,9 +1159,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> [[no_unique_address]] _BoundArgsStorage _M_bound_args;
> };
>
> - template<typename _Fn, typename... _Args>
> - using _Bind_back_t = _Bind_back<decay_t<_Fn>, decay_t<_Args>...>;
> -
> /** Create call wrapper by partial application of arguments to function.
> *
> * The result of `std::bind_back(f, args...)` is a function object that
> @@ -1171,13 +1169,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> * @since C++23
> */
> template<typename _Fn, typename... _Args>
> - constexpr _Bind_back_t<_Fn, _Args...>
> + constexpr auto
> bind_back(_Fn&& __fn, _Args&&... __args)
> - noexcept(is_nothrow_constructible_v<_Bind_back_t<_Fn, _Args...>,
> - int, _Fn, _Args...>)
> + noexcept(__and_<is_nothrow_constructible<decay_t<_Fn>, _Fn>,
> + is_nothrow_constructible<decay_t<_Args>,
> _Args>...>::value)
> {
> - return _Bind_back_t<_Fn, _Args...>(0, std::forward<_Fn>(__fn),
> - std::forward<_Args>(__args)...);
> + using _Fd = decay_t<_Fn>;
> + static_assert(is_move_constructible_v<_Fd>);
> + if constexpr (sizeof...(_Args) == 0)
> + return _Fd(std::forward<_Fn>(__fn));
> + else
> + return _Bind_back<_Fd, decay_t<_Args>...>(0,
> std::forward<_Fn>(__fn),
> +
> std::forward<_Args>(__args)...);
> }
> #endif // __cpp_lib_bind_back
>
> diff --git
> a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/1.cc
> b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/1.cc
> index a31528fc755..ef7531f0045 100644
> --- a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/1.cc
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/1.cc
> @@ -290,10 +290,11 @@ testMaterialization()
> { return arg.counter; };
> };
>
> - // CountedArg is bound to rvalue-reference thus moved
> + // copy of F is returned
> auto f0 = std::bind_back(F{});
> - VERIFY( f0(CountedArg(), 10) == 1 );
> + VERIFY( f0(CountedArg(), 10) == 0 );
>
> + // CountedArg is bound to rvalue-reference thus moved
> auto f1 = std::bind_back(F{}, 10);
> VERIFY( f1(CountedArg()) == 1 );
> }
> diff --git
> a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/1.cc
> b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/1.cc
> index ef28de8321b..5b0ae3772ab 100644
> --- a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/1.cc
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/1.cc
> @@ -288,10 +288,11 @@ testMaterialization()
> { return arg.counter; };
> };
>
> - // CountedArg is bound to rvalue-reference thus moved
> + // copy of F is returned
> auto f0 = std::bind_front(F{});
> - VERIFY( f0(10, CountedArg()) == 1 );
> + VERIFY( f0(10, CountedArg()) == 0 );
>
> + // CountedArg is bound to rvalue-reference thus moved
> auto f1 = std::bind_front(F{}, 10);
> VERIFY( f1(CountedArg()) == 1 );
> }
> --
> 2.50.1
>
>
