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// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP___RANGES_MOVABLE_BOX_H
#define _LIBCPP___RANGES_MOVABLE_BOX_H
#include <__concepts/constructible.h>
#include <__concepts/copyable.h>
#include <__concepts/movable.h>
#include <__config>
#include <__memory/addressof.h>
#include <__memory/construct_at.h>
#include <__type_traits/is_nothrow_constructible.h>
#include <__type_traits/is_nothrow_copy_constructible.h>
#include <__type_traits/is_nothrow_default_constructible.h>
#include <__utility/move.h>
#include <optional>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
#if _LIBCPP_STD_VER >= 20
// __movable_box allows turning a type that is move-constructible (but maybe not move-assignable) into
// a type that is both move-constructible and move-assignable. It does that by introducing an empty state
// and basically doing destroy-then-copy-construct in the assignment operator. The empty state is necessary
// to handle the case where the copy construction fails after destroying the object.
//
// In some cases, we can completely avoid the use of an empty state; we provide a specialization of
// __movable_box that does this, see below for the details.
// until C++23, `__movable_box` was named `__copyable_box` and required the stored type to be copy-constructible, not
// just move-constructible; we preserve the old behavior in pre-C++23 modes.
template <class _Tp>
concept __movable_box_object =
# if _LIBCPP_STD_VER >= 23
move_constructible<_Tp>
# else
copy_constructible<_Tp>
# endif
&& is_object_v<_Tp>;
namespace ranges {
// Primary template - uses std::optional and introduces an empty state in case assignment fails.
template <__movable_box_object _Tp>
class __movable_box {
_LIBCPP_NO_UNIQUE_ADDRESS optional<_Tp> __val_;
public:
template <class... _Args>
requires is_constructible_v<_Tp, _Args...>
_LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box(in_place_t, _Args&&... __args) noexcept(
is_nothrow_constructible_v<_Tp, _Args...>)
: __val_(in_place, std::forward<_Args>(__args)...) {}
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box() noexcept(is_nothrow_default_constructible_v<_Tp>)
requires default_initializable<_Tp>
: __val_(in_place) {}
_LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box const&) = default;
_LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box&&) = default;
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box&
operator=(__movable_box const& __other) noexcept(is_nothrow_copy_constructible_v<_Tp>)
# if _LIBCPP_STD_VER >= 23
requires copy_constructible<_Tp>
# endif
{
if (this != std::addressof(__other)) {
if (__other.__has_value())
__val_.emplace(*__other);
else
__val_.reset();
}
return *this;
}
_LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box&&)
requires movable<_Tp>
= default;
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box&
operator=(__movable_box&& __other) noexcept(is_nothrow_move_constructible_v<_Tp>) {
if (this != std::addressof(__other)) {
if (__other.__has_value())
__val_.emplace(std::move(*__other));
else
__val_.reset();
}
return *this;
}
_LIBCPP_HIDE_FROM_ABI constexpr _Tp const& operator*() const noexcept { return *__val_; }
_LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() noexcept { return *__val_; }
_LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept { return __val_.operator->(); }
_LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept { return __val_.operator->(); }
_LIBCPP_HIDE_FROM_ABI constexpr bool __has_value() const noexcept { return __val_.has_value(); }
};
// This partial specialization implements an optimization for when we know we don't need to store
// an empty state to represent failure to perform an assignment. For copy-assignment, this happens:
//
// 1. If the type is copyable (which includes copy-assignment), we can use the type's own assignment operator
// directly and avoid using std::optional.
// 2. If the type is not copyable, but it is nothrow-copy-constructible, then we can implement assignment as
// destroy-and-then-construct and we know it will never fail, so we don't need an empty state.
//
// The exact same reasoning can be applied for move-assignment, with copyable replaced by movable and
// nothrow-copy-constructible replaced by nothrow-move-constructible. This specialization is enabled
// whenever we can apply any of these optimizations for both the copy assignment and the move assignment
// operator.
# if _LIBCPP_STD_VER >= 23
template <class _Tp>
concept __doesnt_need_empty_state =
(copy_constructible<_Tp>
// 1. If copy_constructible<T> is true, movable-box<T> should store only a T if either T models
// copyable, or is_nothrow_move_constructible_v<T> && is_nothrow_copy_constructible_v<T> is true.
? copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Tp>)
// 2. Otherwise, movable-box<T> should store only a T if either T models movable or
// is_nothrow_move_constructible_v<T> is true.
: movable<_Tp> || is_nothrow_move_constructible_v<_Tp>);
# else
template <class _Tp>
concept __doesnt_need_empty_state_for_copy = copyable<_Tp> || is_nothrow_copy_constructible_v<_Tp>;
template <class _Tp>
concept __doesnt_need_empty_state_for_move = movable<_Tp> || is_nothrow_move_constructible_v<_Tp>;
template <class _Tp>
concept __doesnt_need_empty_state = __doesnt_need_empty_state_for_copy<_Tp> && __doesnt_need_empty_state_for_move<_Tp>;
# endif
template <__movable_box_object _Tp>
requires __doesnt_need_empty_state<_Tp>
class __movable_box<_Tp> {
_LIBCPP_NO_UNIQUE_ADDRESS _Tp __val_;
public:
template <class... _Args>
requires is_constructible_v<_Tp, _Args...>
_LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box(in_place_t, _Args&&... __args) noexcept(
is_nothrow_constructible_v<_Tp, _Args...>)
: __val_(std::forward<_Args>(__args)...) {}
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box() noexcept(is_nothrow_default_constructible_v<_Tp>)
requires default_initializable<_Tp>
: __val_() {}
_LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box const&) = default;
_LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box&&) = default;
// Implementation of assignment operators in case we perform optimization (1)
_LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box const&)
requires copyable<_Tp>
= default;
_LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box&&)
requires movable<_Tp>
= default;
// Implementation of assignment operators in case we perform optimization (2)
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box& operator=(__movable_box const& __other) noexcept {
static_assert(is_nothrow_copy_constructible_v<_Tp>);
if (this != std::addressof(__other)) {
std::destroy_at(std::addressof(__val_));
std::construct_at(std::addressof(__val_), __other.__val_);
}
return *this;
}
_LIBCPP_HIDE_FROM_ABI constexpr __movable_box& operator=(__movable_box&& __other) noexcept {
static_assert(is_nothrow_move_constructible_v<_Tp>);
if (this != std::addressof(__other)) {
std::destroy_at(std::addressof(__val_));
std::construct_at(std::addressof(__val_), std::move(__other.__val_));
}
return *this;
}
_LIBCPP_HIDE_FROM_ABI constexpr _Tp const& operator*() const noexcept { return __val_; }
_LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() noexcept { return __val_; }
_LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept { return std::addressof(__val_); }
_LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept { return std::addressof(__val_); }
_LIBCPP_HIDE_FROM_ABI constexpr bool __has_value() const noexcept { return true; }
};
} // namespace ranges
#endif // _LIBCPP_STD_VER >= 20
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___RANGES_MOVABLE_BOX_H
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