[libcxx-commits] [libcxx] [libc++] P0792R14: `std::function_ref` (PR #94894)

[via libcxx-commits]

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@@ -0,0 +1,186 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// This header is unguarded on purpose. This header is an implementation detail of function_ref.h
+// and generates multiple versions of std::function_ref
+
+#include <__assert>
+#include <__config>
+#include <__functional/invoke.h>
+#include <__memory/addressof.h>
+#include <__type_traits/conditional.h>
+#include <__type_traits/invoke.h>
+#include <__type_traits/is_const.h>
+#include <__type_traits/is_function.h>
+#include <__type_traits/is_member_pointer.h>
+#include <__type_traits/is_object.h>
+#include <__type_traits/is_pointer.h>
+#include <__type_traits/is_reference.h>
+#include <__type_traits/is_same.h>
+#include <__type_traits/is_void.h>
+#include <__type_traits/remove_cvref.h>
+#include <__type_traits/remove_pointer.h>
+#include <__type_traits/remove_reference.h>
+#include <__utility/constant_arg.h>
+#include <__utility/forward.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#ifndef _LIBCPP___FUNCTIONAL_FUNCTION_REF_H
+#  error This header should only be included from function_ref.h
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 26
+
+template <class...>
+class function_ref;
+
+template <class _Rp, class... _ArgTypes, bool __is_noexcept>
+class function_ref<_Rp(_ArgTypes...) _LIBCPP_FUNCTION_REF_CV noexcept(__is_noexcept)> {
+private:
+  template <class... _Tp>
+  static constexpr bool __is_invocable_using =
+      _If<__is_noexcept, is_nothrow_invocable_r<_Rp, _Tp..., _ArgTypes...>, is_invocable_r<_Rp, _Tp..., _ArgTypes...>>::
+          value;
+
+  // use a union instead of a plain `void*` to avoid dropping const qualifiers and casting function pointers to data
+  // pointers
+  // todo: libstdc++ does not support volatile objects. shall we support it? the standard does not say it should not be
+  // supported
+  union __storage_t {
+    void* __obj_ptr_;
+    void const* __obj_const_ptr_;
+
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __storage_t() noexcept : __obj_ptr_(nullptr) {}
+
+    template <class _Tp>
+    _LIBCPP_HIDE_FROM_ABI constexpr explicit __storage_t(_Tp* __ptr) noexcept {
+      if constexpr (is_object_v<_Tp>) {
+        if constexpr (is_const_v<_Tp>) {
+          __obj_const_ptr_ = __ptr;
+        } else {
+          __obj_ptr_ = __ptr;
+        }
+      } else {
+        static_assert(is_function_v<_Tp>);
+        __obj_ptr_ = reinterpret_cast<void*>(__ptr);
+      }
+    }
+  };
+
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __get(__storage_t __storage) {
+    if constexpr (is_object_v<_Tp>) {
+      if constexpr (is_const_v<_Tp>) {
+        return static_cast<_Tp*>(__storage.__obj_const_ptr_);
+      } else {
+        return static_cast<_Tp*>(__storage.__obj_ptr_);
+      }
+    } else {
+      static_assert(is_function_v<_Tp>);
+      return reinterpret_cast<_Tp*>(__storage.__obj_ptr_);
+    }
+  }
+
+  __storage_t __storage_;
+
+  using __call_t _LIBCPP_NODEBUG = _Rp (*)(__storage_t, _ArgTypes&&...) noexcept(__is_noexcept);
+  __call_t __call_;
+
+public:
+  template <class _Fp>
+    requires is_function_v<_Fp> && __is_invocable_using<_Fp>
+  _LIBCPP_HIDE_FROM_ABI function_ref(_Fp* __fn_ptr) noexcept
+      : __storage_(__fn_ptr),
+        __call_([](__storage_t __storage, _ArgTypes&&... __args) static noexcept(__is_noexcept) -> _Rp {
+          return __get<_Fp>(__storage)(std::forward<_ArgTypes>(__args)...);
+        }) {
+    _LIBCPP_ASSERT_NON_NULL(__fn_ptr != nullptr, "the function pointer should not be a nullptr");
+  }
+
+  template <class _Fn, class _Tp = remove_reference_t<_Fn>>
+    requires(!is_same_v<remove_cvref_t<_Fn>, function_ref> && !is_member_pointer_v<_Tp> &&
+             __is_invocable_using<_LIBCPP_FUNCTION_REF_CV _Tp&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr function_ref(_Fn&& __obj) noexcept
----------------
huixie90 wrote:

TODO:
If `__obj` is another `function_ref`, we should unwrap it and copy the pointers

https://github.com/llvm/llvm-project/pull/94894