On Wed, 3 Sep 2025, Tomasz Kamiński wrote:
> The _Bind_front and _Bind_back class templates are now merged into a single
> _Binder implementation that accepts _Back as a template parameter. This makes
> the bind_back implementation available in C++20 mode, allowing it to be used
> for range adaptor closures.
>
> With zero bound arguments, bind_back and bind_front have equivalent
> functionality. Consequently, _Bind_back_t now produces the same type as
> bind_front (_Binder<false, _Fd>). A simple copy of the functor cannot be
> returned in this case, as it would visibly affect overload resolution
> (see included test cases).
>
> libstdc++-v3/ChangeLog:
>
> * include/std/functional: (__Bound_arg_storage::_S_apply_front)
> (__Bound_arg_storage::_S_apply_front): Merged into _S_apply.
> (__Bound_arg_storage::_S_apply): Merged above, add _Back
> template parameter.
> (std::_Bind_front): Renamed to std::_Binder and add _Back
> template parameter.
> (std::_Binder): Renamed from std::_Bind_front.
> (_Binder::_Result_t, _Binder::_S_noexcept_invoke): Define.
> (_Binder::operator()): Use _Result_t and _S_noexcept_invoke.
> (_Binder::_S_call): Handle zero args specially.
> (std::_Bind_front_t, std::_Bind_back_t): Defined in terms
> of _Binder.
> (std::_Bind_back): Merged into _Binder.
> * testsuite/20_util/function_objects/bind_back/1.cc: New tests.
> * testsuite/20_util/function_objects/bind_back/111327.cc: Updated
> error messages.
> * testsuite/20_util/function_objects/bind_front/1.cc: New tests.
> * testsuite/20_util/function_objects/bind_front/111327.cc: Updated
> error messages.
> ---
> Tested on x86_64-linux locally. OK for trunk?
>
> libstdc++-v3/include/std/functional | 172 +++++++-----------
> .../20_util/function_objects/bind_back/1.cc | 16 +-
> .../function_objects/bind_back/111327.cc | 3 +-
> .../20_util/function_objects/bind_front/1.cc | 15 ++
> .../function_objects/bind_front/111327.cc | 2 +-
> 5 files changed, 103 insertions(+), 105 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/functional
> b/libstdc++-v3/include/std/functional
> index b1cda87929d..4122e8f9988 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -921,9 +921,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> std::forward<_BoundArgs>(__args)...);
> }
>
> -#ifdef __cpp_lib_bind_front // C++ >= 20
> +#if __cplusplus >= 202002L
> template<size_t, typename _Tp>
> - struct _Indexed_bound_arg
> + struct _Indexed_bound_arg
Unintended whitespace change?
> {
> [[no_unique_address]] _Tp _M_val;
> };
> @@ -931,24 +931,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename... _IndexedArgs>
> struct _Bound_arg_storage : _IndexedArgs...
> {
> - template<typename _Fd, typename _Self, typename... _CallArgs>
> + template<bool _Back, typename _Fd, typename _Self, typename...
> _CallArgs>
> static constexpr
> decltype(auto)
> - _S_apply_front(_Fd&& __fd, _Self&& __self, _CallArgs&&... __call_args)
> + _S_apply(_Fd&& __fd, _Self&& __self, _CallArgs&&... __call_args)
> {
> - return std::invoke(std::forward<_Fd>(__fd),
> - __like_t<_Self, _IndexedArgs>(__self)._M_val...,
> - std::forward<_CallArgs>(__call_args)...);
It occurs to me that we should use std::__invoke throughout, since
std::invoke is just a simple wrapper for it.
LGTM besides that (which could be done in a separate patch).
> - }
> -
> - template<typename _Fd, typename _Self, typename... _CallArgs>
> - static constexpr
> - decltype(auto)
> - _S_apply_back(_Fd&& __fd, _Self&& __self, _CallArgs&&... __call_args)
> - {
> - return std::invoke(std::forward<_Fd>(__fd),
> - std::forward<_CallArgs>(__call_args)...,
> - __like_t<_Self, _IndexedArgs>(__self)._M_val...);
> + if constexpr (_Back)
> + return std::invoke(std::forward<_Fd>(__fd),
> + std::forward<_CallArgs>(__call_args)...,
> + __like_t<_Self, _IndexedArgs>(__self)._M_val...);
> + else
> + return std::invoke(std::forward<_Fd>(__fd),
> + __like_t<_Self, _IndexedArgs>(__self)._M_val...,
> + std::forward<_CallArgs>(__call_args)...);
> }
> };
>
> @@ -970,9 +965,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> }
>
> - template<typename _Fd, typename... _BoundArgs>
> - struct _Bind_front
> + template<bool _Back, typename _Fd, typename... _BoundArgs>
> + class _Binder
> {
> + template<typename _Self, typename... _CallArgs>
> + using _Result_t = __conditional_t<
> + _Back,
> + invoke_result<__like_t<_Self, _Fd>,
> + _CallArgs..., __like_t<_Self, _BoundArgs>...>,
> + invoke_result<__like_t<_Self, _Fd>,
> + __like_t<_Self, _BoundArgs>..., _CallArgs...>>::type;
> +
> + template<typename _Self, typename... _CallArgs>
> + static consteval bool
> + _S_noexcept_invocable()
> + {
> + if constexpr (_Back)
> + return is_nothrow_invocable_v< __like_t<_Self, _Fd>,
> + _CallArgs..., __like_t<_Self, _BoundArgs>...>;
> + else
> + return is_nothrow_invocable_v<__like_t<_Self, _Fd>,
> + __like_t<_Self, _BoundArgs>..., _CallArgs...>;
> + }
> +
> + public:
> static_assert(is_move_constructible_v<_Fd>);
> static_assert((is_move_constructible_v<_BoundArgs> && ...));
>
> @@ -980,7 +996,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // instead of the copy/move constructor.
> template<typename _Fn, typename... _Args>
> explicit constexpr
> - _Bind_front(int, _Fn&& __fn, _Args&&... __args)
> + _Binder(int, _Fn&& __fn, _Args&&... __args)
> noexcept(__and_<is_nothrow_constructible<_Fd, _Fn>,
> is_nothrow_constructible<_BoundArgs, _Args>...>::value)
> : _M_fd(std::forward<_Fn>(__fn)),
> @@ -989,43 +1005,37 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> #if __cpp_explicit_this_parameter
> template<typename _Self, typename... _CallArgs>
> - constexpr
> - invoke_result_t<__like_t<_Self, _Fd>, __like_t<_Self, _BoundArgs>...,
> _CallArgs...>
> + constexpr _Result_t<_Self, _CallArgs...>
> operator()(this _Self&& __self, _CallArgs&&... __call_args)
> - noexcept(is_nothrow_invocable_v<__like_t<_Self, _Fd>,
> - __like_t<_Self, _BoundArgs>...,
> _CallArgs...>)
> + noexcept(_S_noexcept_invocable<_Self, _CallArgs...>())
> {
> - return _S_call(__like_t<_Self, _Bind_front>(__self),
> + return _S_call(__like_t<_Self, _Binder>(__self),
> std::forward<_CallArgs>(__call_args)...);
> }
> #else
> template<typename... _CallArgs>
> requires true
> - constexpr
> - invoke_result_t<_Fd&, _BoundArgs&..., _CallArgs...>
> + constexpr _Result_t<_Binder&, _CallArgs...>
> operator()(_CallArgs&&... __call_args) &
> - noexcept(is_nothrow_invocable_v<_Fd&, _BoundArgs&..., _CallArgs...>)
> + noexcept(_S_noexcept_invocable<_Binder&, _CallArgs...>())
> {
> return _S_call(*this, std::forward<_CallArgs>(__call_args)...);
> }
>
> template<typename... _CallArgs>
> requires true
> - constexpr
> - invoke_result_t<const _Fd&, const _BoundArgs&..., _CallArgs...>
> + constexpr _Result_t<const _Binder&, _CallArgs...>
> operator()(_CallArgs&&... __call_args) const &
> - noexcept(is_nothrow_invocable_v<const _Fd&, const _BoundArgs&...,
> - _CallArgs...>)
> + noexcept(_S_noexcept_invocable<const _Binder&, _CallArgs...>())
> {
> return _S_call(*this, std::forward<_CallArgs>(__call_args)...);
> }
>
> template<typename... _CallArgs>
> requires true
> - constexpr
> - invoke_result_t<_Fd, _BoundArgs..., _CallArgs...>
> + constexpr _Result_t<_Binder&&, _CallArgs...>
> operator()(_CallArgs&&... __call_args) &&
> - noexcept(is_nothrow_invocable_v<_Fd, _BoundArgs..., _CallArgs...>)
> + noexcept(_S_noexcept_invocable<_Binder&&, _CallArgs...>())
> {
> return _S_call(std::move(*this),
> std::forward<_CallArgs>(__call_args)...);
> @@ -1033,11 +1043,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<typename... _CallArgs>
> requires true
> - constexpr
> - invoke_result_t<const _Fd, const _BoundArgs..., _CallArgs...>
> + constexpr _Result_t<const _Binder&&, _CallArgs...>
> operator()(_CallArgs&&... __call_args) const &&
> - noexcept(is_nothrow_invocable_v<const _Fd, const _BoundArgs...,
> - _CallArgs...>)
> + noexcept(_S_noexcept_invocable<const _Binder&&, _CallArgs...>())
> {
> return _S_call(std::move(*this),
> std::forward<_CallArgs>(__call_args)...);
> @@ -1066,15 +1074,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> decltype(auto)
> _S_call(_Tp&& __g, _CallArgs&&... __call_args)
> {
> - if constexpr (sizeof...(_BoundArgs) == 1)
> - return std::invoke(std::forward<_Tp>(__g)._M_fd,
> - std::forward<_Tp>(__g)._M_bound_args,
> - std::forward<_CallArgs>(__call_args)...);
> - else
> - return _BoundArgsStorage::_S_apply_front(
> + if constexpr (sizeof...(_BoundArgs) > 1)
> + return _BoundArgsStorage::template _S_apply<_Back>(
> std::forward<_Tp>(__g)._M_fd,
> std::forward<_Tp>(__g)._M_bound_args,
> std::forward<_CallArgs>(__call_args)...);
> + else if constexpr (sizeof...(_BoundArgs) == 0)
> + return std::invoke(std::forward<_Tp>(__g)._M_fd,
> + std::forward<_CallArgs>(__call_args)...);
> + else if constexpr (_Back) // sizeof...(_BoundArgs) == 1
> + return std::invoke(std::forward<_Tp>(__g)._M_fd,
> + std::forward<_CallArgs>(__call_args)...,
> + std::forward<_Tp>(__g)._M_bound_args);
> + else // !_Back && sizeof...(_BoundArgs) == 1
> + return std::invoke(std::forward<_Tp>(__g)._M_fd,
> + std::forward<_Tp>(__g)._M_bound_args,
> + std::forward<_CallArgs>(__call_args)...);
> +
> }
>
> [[no_unique_address]] _Fd _M_fd;
> @@ -1082,8 +1098,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> };
>
> template<typename _Fn, typename... _Args>
> - using _Bind_front_t = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
> + using _Bind_front_t = _Binder<false, decay_t<_Fn>, decay_t<_Args>...>;
> +
> + // for zero bounds args behavior of bind_front and bind_back is the same,
> + // so reuse _Bind_front_t, i.e. _Binder<false, ...>
> + template<typename _Fn, typename... _Args>
> + using _Bind_back_t
> + = _Binder<(sizeof...(_Args) > 0), decay_t<_Fn>, decay_t<_Args>...>;
> +#endif // __cplusplus >= 202002L
>
> +#ifdef __cpp_lib_bind_front // C++ >= 20
> /** Create call wrapper by partial application of arguments to function.
> *
> * The result of `std::bind_front(f, args...)` is a function object that
> @@ -1105,62 +1129,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif // __cpp_lib_bind_front
>
> #ifdef __cpp_lib_bind_back // C++ >= 23
> - template<typename _Fd, typename... _BoundArgs>
> - struct _Bind_back
> - {
> - static_assert(is_move_constructible_v<_Fd>);
> - static_assert((is_move_constructible_v<_BoundArgs> && ...));
> -
> - // First parameter is to ensure this constructor is never used
> - // instead of the copy/move constructor.
> - template<typename _Fn, typename... _Args>
> - explicit constexpr
> - _Bind_back(int, _Fn&& __fn, _Args&&... __args)
> - noexcept(__and_<is_nothrow_constructible<_Fd, _Fn>,
> - is_nothrow_constructible<_BoundArgs, _Args>...>::value)
> - : _M_fd(std::forward<_Fn>(__fn)),
> -
> _M_bound_args(__make_bound_args<_BoundArgs...>(std::forward<_Args>(__args)...))
> - { static_assert(sizeof...(_Args) == sizeof...(_BoundArgs)); }
> -
> - template<typename _Self, typename... _CallArgs>
> - constexpr
> - invoke_result_t<__like_t<_Self, _Fd>, _CallArgs..., __like_t<_Self,
> _BoundArgs>...>
> - operator()(this _Self&& __self, _CallArgs&&... __call_args)
> - noexcept(is_nothrow_invocable_v<__like_t<_Self, _Fd>,
> - _CallArgs..., __like_t<_Self,
> _BoundArgs>...>)
> - {
> - return _S_call(__like_t<_Self, _Bind_back>(__self),
> - 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>()...));
> -
> - template<typename _Tp, typename... _CallArgs>
> - static constexpr
> - decltype(auto)
> - _S_call(_Tp&& __g, _CallArgs&&... __call_args)
> - {
> - if constexpr (sizeof...(_BoundArgs) == 1)
> - return std::invoke(std::forward<_Tp>(__g)._M_fd,
> - std::forward<_CallArgs>(__call_args)...,
> - std::forward<_Tp>(__g)._M_bound_args);
> - else
> - return _BoundArgsStorage::_S_apply_back(
> - std::forward<_Tp>(__g)._M_fd,
> - std::forward<_Tp>(__g)._M_bound_args,
> - std::forward<_CallArgs>(__call_args)...);
> - }
> -
> - [[no_unique_address]] _Fd _M_fd;
> - [[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
> 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..7141be282c0 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
> @@ -57,7 +57,6 @@ test01()
> decltype(bind_back(std::declval<const F&>(), std::declval<const
> int&>(), std::declval<const float&>()))
> >);
>
> -
> // Reference wrappers should be handled:
> static_assert(!std::is_same_v<
> decltype(bind_back(std::declval<F>(), std::declval<int&>())),
> @@ -197,6 +196,21 @@ testCallArgs(Args... args)
> VERIFY( q.as_const && q.as_lvalue );
> q = cg(std::move(ci));
> VERIFY( q.as_const && ! q.as_lvalue );
> +
> + struct S
> + {
> + int operator()(long, long, Args...) const { return 1; }
> + int operator()(int, void*, Args...) const { return 2; }
> + };
> +
> + S s;
> + // literal zero can be converted to any pointer, so (int, void*)
> + // is best candidate
> + VERIFY( s(0, 0, args...) == 2 );
> + // both arguments are bound to int&&, and no longer can be
> + // converted to pointer, (long, long) is only candidate
> + VERIFY( bind_back(s)(0, 0, args...) == 1 );
> + VERIFY( bind_back(s, args...)(0, 0) == 1 );
> }
>
> void
> diff --git
> a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/111327.cc
> b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/111327.cc
> index de3ae47e37f..8e7dacf80df 100644
> --- a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/111327.cc
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/111327.cc
> @@ -50,4 +50,5 @@ int main() {
> std::move(std::as_const(g2))();
> }
>
> -// { dg-error "no type named 'type' in 'struct std::invoke_result" "" {
> target c++23 } 0 }
> +// { dg-error "no type named 'type' in 'std::__conditional_t<false,
> std::invoke_result<" "" { target c++23 } 0 }
> +// { dg-error "no type named 'type' in 'std::__conditional_t<true,
> std::invoke_result<" "" { target c++23 } 0 }
> 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..93efb2efea4 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
> @@ -196,6 +196,21 @@ testCallArgs(Args... args)
> VERIFY( q.as_const && q.as_lvalue );
> q = cg(std::move(ci));
> VERIFY( q.as_const && ! q.as_lvalue );
> +
> + struct S
> + {
> + int operator()(Args..., long, long) const { return 1; }
> + int operator()(Args..., int, void*) const { return 2; }
> + };
> +
> + S s;
> + // literal zero can be converted to any pointer, so (int, void*)
> + // is best candidate
> + VERIFY( s(args..., 0, 0) == 2 );
> + // both arguments are bound to int&&, and no longer can be
> + // converted to pointer, (long, long) is only candidate
> + VERIFY( bind_front(s)(args..., 0, 0) == 1 );
> + VERIFY( bind_front(s, args...)(0, 0) == 1 );
> }
>
> void
> diff --git
> a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> index 6694322d67e..58832a61a7e 100644
> --- a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> +++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
> @@ -50,4 +50,4 @@ int main() {
> std::move(std::as_const(g2))();
> }
>
> -// { dg-error "no type named 'type' in 'struct std::invoke_result" "" {
> target c++23 } 0 }
> +// { dg-error "no type named 'type' in 'std::__conditional_t<false,
> std::invoke_result<" "" { target c++23 } 0 }
> --
> 2.51.0
>
>
