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1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___RANGES_MOVABLE_BOX_H
11#define _LIBCPP___RANGES_MOVABLE_BOX_H
12
13#include <__concepts/constructible.h>
14#include <__concepts/copyable.h>
15#include <__concepts/movable.h>
16#include <__config>
17#include <__memory/addressof.h>
18#include <__memory/construct_at.h>
19#include <__type_traits/is_nothrow_constructible.h>
20#include <__utility/move.h>
21#include <optional>
22
23#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
24# pragma GCC system_header
25#endif
26
27_LIBCPP_PUSH_MACROS
28#include <__undef_macros>
29
30_LIBCPP_BEGIN_NAMESPACE_STD
31
32#if _LIBCPP_STD_VER >= 20
33
34// __movable_box allows turning a type that is move-constructible (but maybe not move-assignable) into
35// a type that is both move-constructible and move-assignable. It does that by introducing an empty state
36// and basically doing destroy-then-copy-construct in the assignment operator. The empty state is necessary
37// to handle the case where the copy construction fails after destroying the object.
38//
39// In some cases, we can completely avoid the use of an empty state; we provide a specialization of
40// __movable_box that does this, see below for the details.
41
42// until C++23, `__movable_box` was named `__copyable_box` and required the stored type to be copy-constructible, not
43// just move-constructible; we preserve the old behavior in pre-C++23 modes.
44template <class _Tp>
45concept __movable_box_object =
46# if _LIBCPP_STD_VER >= 23
47 move_constructible<_Tp>
48# else
49 copy_constructible<_Tp>
50# endif
51 && is_object_v<_Tp>;
52
53namespace ranges {
54// Primary template - uses std::optional and introduces an empty state in case assignment fails.
55template <__movable_box_object _Tp>
56class __movable_box {
57 _LIBCPP_NO_UNIQUE_ADDRESS optional<_Tp> __val_;
58
59public:
60 template <class... _Args>
61 requires is_constructible_v<_Tp, _Args...>
62 _LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box(in_place_t, _Args&&... __args) noexcept(
63 is_nothrow_constructible_v<_Tp, _Args...>)
64 : __val_(in_place, std::forward<_Args>(__args)...) {}
65
66 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box() noexcept(is_nothrow_default_constructible_v<_Tp>)
67 requires default_initializable<_Tp>
68 : __val_(in_place) {}
69
70 _LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box const&) = default;
71 _LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box&&) = default;
72
73 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box&
74 operator=(__movable_box const& __other) noexcept(is_nothrow_copy_constructible_v<_Tp>)
75# if _LIBCPP_STD_VER >= 23
76 requires copy_constructible<_Tp>
77# endif
78 {
79 if (this != std::addressof(__other)) {
80 if (__other.__has_value())
81 __val_.emplace(*__other);
82 else
83 __val_.reset();
84 }
85 return *this;
86 }
87
88 _LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box&&)
89 requires movable<_Tp>
90 = default;
91
92 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box&
93 operator=(__movable_box&& __other) noexcept(is_nothrow_move_constructible_v<_Tp>) {
94 if (this != std::addressof(__other)) {
95 if (__other.__has_value())
96 __val_.emplace(std::move(*__other));
97 else
98 __val_.reset();
99 }
100 return *this;
101 }
102
103 _LIBCPP_HIDE_FROM_ABI constexpr _Tp const& operator*() const noexcept { return *__val_; }
104 _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() noexcept { return *__val_; }
105
106 _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept { return __val_.operator->(); }
107 _LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept { return __val_.operator->(); }
108
109 _LIBCPP_HIDE_FROM_ABI constexpr bool __has_value() const noexcept { return __val_.has_value(); }
110};
111
112// This partial specialization implements an optimization for when we know we don't need to store
113// an empty state to represent failure to perform an assignment. For copy-assignment, this happens:
114//
115// 1. If the type is copyable (which includes copy-assignment), we can use the type's own assignment operator
116// directly and avoid using std::optional.
117// 2. If the type is not copyable, but it is nothrow-copy-constructible, then we can implement assignment as
118// destroy-and-then-construct and we know it will never fail, so we don't need an empty state.
119//
120// The exact same reasoning can be applied for move-assignment, with copyable replaced by movable and
121// nothrow-copy-constructible replaced by nothrow-move-constructible. This specialization is enabled
122// whenever we can apply any of these optimizations for both the copy assignment and the move assignment
123// operator.
124
125# if _LIBCPP_STD_VER >= 23
126template <class _Tp>
127concept __doesnt_need_empty_state =
128 (copy_constructible<_Tp>
129 // 1. If copy_constructible<T> is true, movable-box<T> should store only a T if either T models
130 // copyable, or is_nothrow_move_constructible_v<T> && is_nothrow_copy_constructible_v<T> is true.
131 ? copyable<_Tp> || (is_nothrow_move_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Tp>)
132 // 2. Otherwise, movable-box<T> should store only a T if either T models movable or
133 // is_nothrow_move_constructible_v<T> is true.
134 : movable<_Tp> || is_nothrow_move_constructible_v<_Tp>);
135
136// When _Tp doesn't have an assignment operator, we must implement __movable_box's assignment operator
137// by doing destroy_at followed by construct_at. However, that implementation strategy leads to UB if the nested
138// _Tp is potentially overlapping, as it is doing a non-transparent replacement of the sub-object, which means that
139// we're not considered "nested" inside the movable-box anymore, and since we're not nested within it, [basic.life]/1.5
140// says that we essentially just reused the storage of the movable-box for a completely unrelated object and ended the
141// movable-box's lifetime.
142// https://github.com/llvm/llvm-project/issues/70494#issuecomment-1845646490
143//
144// Hence, when the _Tp doesn't have an assignment operator, we can't risk making it a potentially-overlapping
145// subobject because of the above, and we don't use [[no_unique_address]] in that case.
146template <class _Tp>
147concept __can_use_no_unique_address = (copy_constructible<_Tp> ? copyable<_Tp> : movable<_Tp>);
148
149# else
150
151template <class _Tp>
152concept __doesnt_need_empty_state_for_copy = copyable<_Tp> || is_nothrow_copy_constructible_v<_Tp>;
153
154template <class _Tp>
155concept __doesnt_need_empty_state_for_move = movable<_Tp> || is_nothrow_move_constructible_v<_Tp>;
156
157template <class _Tp>
158concept __doesnt_need_empty_state = __doesnt_need_empty_state_for_copy<_Tp> && __doesnt_need_empty_state_for_move<_Tp>;
159
160template <class _Tp>
161concept __can_use_no_unique_address = copyable<_Tp>;
162# endif
163
164template <class _Tp>
165struct __movable_box_holder {
166 _Tp __val_;
167
168 template <class... _Args>
169 _LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box_holder(in_place_t, _Args&&... __args)
170 : __val_(std::forward<_Args>(__args)...) {}
171};
172
173template <class _Tp>
174 requires __can_use_no_unique_address<_Tp>
175struct __movable_box_holder<_Tp> {
176 _LIBCPP_NO_UNIQUE_ADDRESS _Tp __val_;
177
178 template <class... _Args>
179 _LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box_holder(in_place_t, _Args&&... __args)
180 : __val_(std::forward<_Args>(__args)...) {}
181};
182
183template <__movable_box_object _Tp>
184 requires __doesnt_need_empty_state<_Tp>
185class __movable_box<_Tp> {
186 _LIBCPP_NO_UNIQUE_ADDRESS __movable_box_holder<_Tp> __holder_;
187
188public:
189 template <class... _Args>
190 requires is_constructible_v<_Tp, _Args...>
191 _LIBCPP_HIDE_FROM_ABI constexpr explicit __movable_box(in_place_t __inplace, _Args&&... __args) noexcept(
192 is_nothrow_constructible_v<_Tp, _Args...>)
193 : __holder_(__inplace, std::forward<_Args>(__args)...) {}
194
195 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box() noexcept(is_nothrow_default_constructible_v<_Tp>)
196 requires default_initializable<_Tp>
197 : __holder_(in_place_t{}) {}
198
199 _LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box const&) = default;
200 _LIBCPP_HIDE_FROM_ABI __movable_box(__movable_box&&) = default;
201
202 // Implementation of assignment operators in case we perform optimization (1)
203 _LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box const&)
204 requires copyable<_Tp>
205 = default;
206 _LIBCPP_HIDE_FROM_ABI __movable_box& operator=(__movable_box&&)
207 requires movable<_Tp>
208 = default;
209
210 // Implementation of assignment operators in case we perform optimization (2)
211 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box& operator=(__movable_box const& __other) noexcept {
212 static_assert(is_nothrow_copy_constructible_v<_Tp>);
213 static_assert(!__can_use_no_unique_address<_Tp>);
214 if (this != std::addressof(__other)) {
215 std::destroy_at(std::addressof(__holder_.__val_));
216 std::construct_at(std::addressof(__holder_.__val_), __other.__holder_.__val_);
217 }
218 return *this;
219 }
220
221 _LIBCPP_HIDE_FROM_ABI constexpr __movable_box& operator=(__movable_box&& __other) noexcept {
222 static_assert(is_nothrow_move_constructible_v<_Tp>);
223 static_assert(!__can_use_no_unique_address<_Tp>);
224 if (this != std::addressof(__other)) {
225 std::destroy_at(std::addressof(__holder_.__val_));
226 std::construct_at(std::addressof(__holder_.__val_), std::move(__other.__holder_.__val_));
227 }
228 return *this;
229 }
230
231 _LIBCPP_HIDE_FROM_ABI constexpr _Tp const& operator*() const noexcept { return __holder_.__val_; }
232 _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() noexcept { return __holder_.__val_; }
233
234 _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept { return std::addressof(__holder_.__val_); }
235 _LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept { return std::addressof(__holder_.__val_); }
236
237 _LIBCPP_HIDE_FROM_ABI constexpr bool __has_value() const noexcept { return true; }
238};
239} // namespace ranges
240
241#endif // _LIBCPP_STD_VER >= 20
242
243_LIBCPP_END_NAMESPACE_STD
244
245_LIBCPP_POP_MACROS
246
247#endif // _LIBCPP___RANGES_MOVABLE_BOX_H