1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
|
// -*- 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_PUSH_MACROS
#include <__undef_macros>
_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
_LIBCPP_POP_MACROS
#endif // _LIBCPP___RANGES_MOVABLE_BOX_H
|
