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
|
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
* A class storing one of two optional value types that supports
* in-place lazy construction.
*/
#ifndef mozilla_MaybeOneOf_h
#define mozilla_MaybeOneOf_h
#include "mozilla/Assertions.h"
#include "mozilla/Move.h"
#include "mozilla/TemplateLib.h"
#include <new> // for placement new
#include <stddef.h> // for size_t
namespace mozilla {
/*
* MaybeOneOf<T1, T2> is like Maybe, but it supports constructing either T1
* or T2. When a MaybeOneOf<T1, T2> is constructed, it is |empty()|, i.e.,
* no value has been constructed and no destructor will be called when the
* MaybeOneOf<T1, T2> is destroyed. Upon calling |construct<T1>()| or
* |construct<T2>()|, a T1 or T2 object will be constructed with the given
* arguments and that object will be destroyed when the owning MaybeOneOf is
* destroyed.
*
* Because MaybeOneOf must be aligned suitable to hold any value stored within
* it, and because |alignas| requirements don't affect platform ABI with respect
* to how parameters are laid out in memory, MaybeOneOf can't be used as the
* type of a function parameter. Pass MaybeOneOf to functions by pointer or
* reference instead.
*/
template<class T1, class T2>
class MOZ_NON_PARAM MaybeOneOf
{
static constexpr size_t StorageAlignment =
tl::Max<alignof(T1), alignof(T2)>::value;
static constexpr size_t StorageSize =
tl::Max<sizeof(T1), sizeof(T2)>::value;
alignas(StorageAlignment) unsigned char storage_[StorageSize];
void* storage() { return storage_; }
const void* storage() const { return storage_; }
enum State { None, SomeT1, SomeT2 } state;
template <class T, class Ignored = void> struct Type2State {};
template <class T>
T& as()
{
MOZ_ASSERT(state == Type2State<T>::result);
return *static_cast<T*>(storage());
}
template <class T>
const T& as() const
{
MOZ_ASSERT(state == Type2State<T>::result);
return *static_cast<const T*>(storage());
}
public:
MaybeOneOf() : state(None) {}
~MaybeOneOf() { destroyIfConstructed(); }
MaybeOneOf(MaybeOneOf&& rhs)
: state(None)
{
if (!rhs.empty()) {
if (rhs.constructed<T1>()) {
construct<T1>(Move(rhs.as<T1>()));
rhs.as<T1>().~T1();
} else {
construct<T2>(Move(rhs.as<T2>()));
rhs.as<T2>().~T2();
}
rhs.state = None;
}
}
MaybeOneOf& operator=(MaybeOneOf&& rhs)
{
MOZ_ASSERT(this != &rhs, "Self-move is prohibited");
this->~MaybeOneOf();
new(this) MaybeOneOf(Move(rhs));
return *this;
}
bool empty() const { return state == None; }
template <class T>
bool constructed() const { return state == Type2State<T>::result; }
template <class T, class... Args>
void construct(Args&&... aArgs)
{
MOZ_ASSERT(state == None);
state = Type2State<T>::result;
::new (storage()) T(Forward<Args>(aArgs)...);
}
template <class T>
T& ref()
{
return as<T>();
}
template <class T>
const T& ref() const
{
return as<T>();
}
void destroy()
{
MOZ_ASSERT(state == SomeT1 || state == SomeT2);
if (state == SomeT1) {
as<T1>().~T1();
} else if (state == SomeT2) {
as<T2>().~T2();
}
state = None;
}
void destroyIfConstructed()
{
if (!empty()) {
destroy();
}
}
private:
MaybeOneOf(const MaybeOneOf& aOther) = delete;
const MaybeOneOf& operator=(const MaybeOneOf& aOther) = delete;
};
template <class T1, class T2>
template <class Ignored>
struct MaybeOneOf<T1, T2>::Type2State<T1, Ignored>
{
typedef MaybeOneOf<T1, T2> Enclosing;
static const typename Enclosing::State result = Enclosing::SomeT1;
};
template <class T1, class T2>
template <class Ignored>
struct MaybeOneOf<T1, T2>::Type2State<T2, Ignored>
{
typedef MaybeOneOf<T1, T2> Enclosing;
static const typename Enclosing::State result = Enclosing::SomeT2;
};
} // namespace mozilla
#endif /* mozilla_MaybeOneOf_h */
|
