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
|
// Copyright (c) 2009 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SINGLETON_H_
#define BASE_SINGLETON_H_
#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/dynamic_annotations.h"
#include "base/platform_thread.h"
// Default traits for Singleton<Type>. Calls operator new and operator delete on
// the object. Registers automatic deletion at process exit.
// Overload if you need arguments or another memory allocation function.
template<typename Type>
struct DefaultSingletonTraits {
// Allocates the object.
static Type* New() {
// The parenthesis is very important here; it forces POD type
// initialization.
return new Type();
}
// Destroys the object.
static void Delete(Type* x) {
delete x;
}
// Set to true to automatically register deletion of the object on process
// exit. See below for the required call that makes this happen.
static const bool kRegisterAtExit = true;
};
// Alternate traits for use with the Singleton<Type>. Identical to
// DefaultSingletonTraits except that the Singleton will not be cleaned up
// at exit.
template<typename Type>
struct LeakySingletonTraits : public DefaultSingletonTraits<Type> {
static const bool kRegisterAtExit = false;
};
// The Singleton<Type, Traits, DifferentiatingType> class manages a single
// instance of Type which will be created on first use and will be destroyed at
// normal process exit). The Trait::Delete function will not be called on
// abnormal process exit.
//
// DifferentiatingType is used as a key to differentiate two different
// singletons having the same memory allocation functions but serving a
// different purpose. This is mainly used for Locks serving different purposes.
//
// Example usages: (none are preferred, they all result in the same code)
// 1. FooClass* ptr = Singleton<FooClass>::get();
// ptr->Bar();
// 2. Singleton<FooClass>()->Bar();
// 3. Singleton<FooClass>::get()->Bar();
//
// Singleton<> has no non-static members and doesn't need to actually be
// instantiated. It does no harm to instantiate it and use it as a class member
// or at global level since it is acting as a POD type.
//
// This class is itself thread-safe. The underlying Type must of course be
// thread-safe if you want to use it concurrently. Two parameters may be tuned
// depending on the user's requirements.
//
// Glossary:
// RAE = kRegisterAtExit
//
// On every platform, if Traits::RAE is true, the singleton will be destroyed at
// process exit. More precisely it uses base::AtExitManager which requires an
// object of this type to be instantiated. AtExitManager mimics the semantics
// of atexit() such as LIFO order but under Windows is safer to call. For more
// information see at_exit.h.
//
// If Traits::RAE is false, the singleton will not be freed at process exit,
// thus the singleton will be leaked if it is ever accessed. Traits::RAE
// shouldn't be false unless absolutely necessary. Remember that the heap where
// the object is allocated may be destroyed by the CRT anyway.
//
// If you want to ensure that your class can only exist as a singleton, make
// its constructors private, and make DefaultSingletonTraits<> a friend:
//
// #include "base/singleton.h"
// class FooClass {
// public:
// void Bar() { ... }
// private:
// FooClass() { ... }
// friend struct DefaultSingletonTraits<FooClass>;
//
// DISALLOW_EVIL_CONSTRUCTORS(FooClass);
// };
//
// Caveats:
// (a) Every call to get(), operator->() and operator*() incurs some overhead
// (16ns on my P4/2.8GHz) to check whether the object has already been
// initialized. You may wish to cache the result of get(); it will not
// change.
//
// (b) Your factory function must never throw an exception. This class is not
// exception-safe.
//
template <typename Type,
typename Traits = DefaultSingletonTraits<Type>,
typename DifferentiatingType = Type>
class Singleton {
public:
// This class is safe to be constructed and copy-constructed since it has no
// member.
// Return a pointer to the one true instance of the class.
static Type* get() {
// Our AtomicWord doubles as a spinlock, where a value of
// kBeingCreatedMarker means the spinlock is being held for creation.
static const base::subtle::AtomicWord kBeingCreatedMarker = 1;
base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_);
if (value != 0 && value != kBeingCreatedMarker) {
// See the corresponding HAPPENS_BEFORE below.
ANNOTATE_HAPPENS_AFTER(&instance_);
return reinterpret_cast<Type*>(value);
}
// Object isn't created yet, maybe we will get to create it, let's try...
if (base::subtle::Acquire_CompareAndSwap(&instance_,
0,
kBeingCreatedMarker) == 0) {
// instance_ was NULL and is now kBeingCreatedMarker. Only one thread
// will ever get here. Threads might be spinning on us, and they will
// stop right after we do this store.
Type* newval = Traits::New();
// This annotation helps race detectors recognize correct lock-less
// synchronization between different threads calling get().
// See the corresponding HAPPENS_AFTER below and above.
ANNOTATE_HAPPENS_BEFORE(&instance_);
base::subtle::Release_Store(
&instance_, reinterpret_cast<base::subtle::AtomicWord>(newval));
if (Traits::kRegisterAtExit)
base::AtExitManager::RegisterCallback(OnExit, NULL);
return newval;
}
// We hit a race. Another thread beat us and either:
// - Has the object in BeingCreated state
// - Already has the object created...
// We know value != NULL. It could be kBeingCreatedMarker, or a valid ptr.
// Unless your constructor can be very time consuming, it is very unlikely
// to hit this race. When it does, we just spin and yield the thread until
// the object has been created.
while (true) {
value = base::subtle::NoBarrier_Load(&instance_);
if (value != kBeingCreatedMarker)
break;
PlatformThread::YieldCurrentThread();
}
// See the corresponding HAPPENS_BEFORE above.
ANNOTATE_HAPPENS_AFTER(&instance_);
return reinterpret_cast<Type*>(value);
}
// Shortcuts.
Type& operator*() {
return *get();
}
Type* operator->() {
return get();
}
private:
// Adapter function for use with AtExit(). This should be called single
// threaded, but we might as well take the precautions anyway.
static void OnExit(void* unused) {
// AtExit should only ever be register after the singleton instance was
// created. We should only ever get here with a valid instance_ pointer.
Traits::Delete(reinterpret_cast<Type*>(
base::subtle::NoBarrier_AtomicExchange(&instance_, 0)));
}
static base::subtle::AtomicWord instance_;
};
template <typename Type, typename Traits, typename DifferentiatingType>
base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
instance_ = 0;
#endif // BASE_SINGLETON_H_
|
