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[libc++][functional] Implement std::bind_front<NTTP> #165096

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[libc++][functional] Implement std::bind_front<NTTP> #165096

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44e4880
[libc++][functional] Implement `bind_front<NTTP>`
JMazurkiewicz Oct 15, 2025
0a0f2e2
[libc++][functional] Test `bind_front<NTTP>`
JMazurkiewicz Oct 15, 2025
6d965f9
Remove unnecessary `{}`
JMazurkiewicz Oct 27, 2025
cb45215
Fix `bind_front.nttp.verify.cpp` test
JMazurkiewicz Oct 27, 2025
4cfab17
Use explicit `this` parameter instead of `__perfect_forward` base class
JMazurkiewicz Oct 27, 2025
328e808
Return different* function object when `sizeof...(_BoundArgs) == 0`
JMazurkiewicz Oct 27, 2025
4f8d61f
Merge remote-tracking branch 'upstream/main' into libcxx/bind_front_nttp
JMazurkiewicz Oct 28, 2025
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34 changes: 34 additions & 0 deletions libcxx/include/__functional/bind_front.h
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,8 @@
#include <__type_traits/decay.h>
#include <__type_traits/enable_if.h>
#include <__type_traits/is_constructible.h>
#include <__type_traits/is_member_pointer.h>
#include <__type_traits/is_pointer.h>
#include <__utility/forward.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
Expand Down Expand Up @@ -49,6 +51,38 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Fn&& __f, _Args&&... __args) {

#endif // _LIBCPP_STD_VER >= 20

#if _LIBCPP_STD_VER >= 26

template <auto _Fn>
struct __nttp_bind_front_op {
template <class... _Args>
_LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Args&&... __args) const
noexcept(noexcept(std::invoke(_Fn, std::forward<_Args>(__args)...)))
-> decltype(std::invoke(_Fn, std::forward<_Args>(__args)...)) {
return std::invoke(_Fn, std::forward<_Args>(__args)...);
}
};

template <auto _Fn, class... _BoundArgs>
struct __nttp_bind_front_t : __perfect_forward<__nttp_bind_front_op<_Fn>, _BoundArgs...> {
using __perfect_forward<__nttp_bind_front_op<_Fn>, _BoundArgs...>::__perfect_forward;
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@frederick-vs-ja frederick-vs-ja Oct 26, 2025

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We should be able to use explicit this parameters.

};

template <auto _Fn, class... _Args>
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Args&&... __args) {
static_assert((is_constructible_v<decay_t<_Args>, _Args> && ...),
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@huixie90 huixie90 Nov 2, 2025

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can we also test these static_assert in a verify test?

"bind_front requires all decay_t<Args> to be constructible from respective Args");
static_assert((is_move_constructible_v<decay_t<_Args>> && ...),
"bind_front requires all decay_t<Args> to be move constructible");
if constexpr (using _Ty = decltype(_Fn); is_pointer_v<_Ty> || is_member_pointer_v<_Ty>) {
static_assert(_Fn != nullptr, "f cannot be equal to nullptr");
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@huixie90 huixie90 Nov 2, 2025

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Perhaps mention bind_front in the message as you did in other messages?
Could we have a verify test where we pass a nullptr and this static assertion triigered?

}

return __nttp_bind_front_t<_Fn, decay_t<_Args>...>(std::forward<_Args>(__args)...);
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@frederick-vs-ja frederick-vs-ja Oct 26, 2025

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When sizeof...(_Args) == 0, it seems better to return a functor with static operator().

}

#endif // _LIBCPP_STD_VER >= 26

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___FUNCTIONAL_BIND_FRONT_H
2 changes: 2 additions & 0 deletions libcxx/include/functional
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Original file line number Diff line number Diff line change
Expand Up @@ -221,6 +221,8 @@ template <auto f>
// [func.bind.partial], function templates bind_front and bind_back
template<class F, class... Args>
constexpr unspecified bind_front(F&&, Args&&...); // C++20
template<auto f, class... Args>
constexpr unspecified bind_front(Args&&...); // C++26
template<class F, class... Args>
constexpr unspecified bind_back(F&&, Args&&...); // C++23

Expand Down
19 changes: 19 additions & 0 deletions .../libcxx/utilities/function.objects/func.bind.partial/bind_front.nttp.nodiscard.verify.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// REQUIRES: std-at-least-c++26

// <functional>

// Test the libc++ extension that std::bind_front<NTTP> is marked as [[nodiscard]].

#include <functional>

void test() {
std::bind_front<test>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
}
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