1// Copyright (c) 2012 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4//
5// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! DEPRECATED !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
6// Please don't introduce new instances of LazyInstance<T>. Use a function-local
7// static of type base::NoDestructor<T> instead:
8//
9// Factory& Factory::GetInstance() {
10// static base::NoDestructor<Factory> instance;
11// return *instance;
12// }
13// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
14//
15// The LazyInstance<Type, Traits> class manages a single instance of Type,
16// which will be lazily created on the first time it's accessed. This class is
17// useful for places you would normally use a function-level static, but you
18// need to have guaranteed thread-safety. The Type constructor will only ever
19// be called once, even if two threads are racing to create the object. Get()
20// and Pointer() will always return the same, completely initialized instance.
21// When the instance is constructed it is registered with AtExitManager. The
22// destructor will be called on program exit.
23//
24// LazyInstance is completely thread safe, assuming that you create it safely.
25// The class was designed to be POD initialized, so it shouldn't require a
26// static constructor. It really only makes sense to declare a LazyInstance as
27// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
28//
29// LazyInstance is similar to Singleton, except it does not have the singleton
30// property. You can have multiple LazyInstance's of the same type, and each
31// will manage a unique instance. It also preallocates the space for Type, as
32// to avoid allocating the Type instance on the heap. This may help with the
33// performance of creating the instance, and reducing heap fragmentation. This
34// requires that Type be a complete type so we can determine the size.
35//
36// Example usage:
37// static LazyInstance<MyClass>::Leaky inst = LAZY_INSTANCE_INITIALIZER;
38// void SomeMethod() {
39// inst.Get().SomeMethod(); // MyClass::SomeMethod()
40//
41// MyClass* ptr = inst.Pointer();
42// ptr->DoDoDo(); // MyClass::DoDoDo
43// }
44
45#ifndef BASE_LAZY_INSTANCE_H_
46#define BASE_LAZY_INSTANCE_H_
47
48#include <new> // For placement new.
49
50#include "base/atomicops.h"
51#include "base/debug/leak_annotations.h"
52#include "base/lazy_instance_helpers.h"
53#include "base/logging.h"
54#include "base/threading/thread_restrictions.h"
55
56// LazyInstance uses its own struct initializer-list style static
57// initialization, which does not require a constructor.
58#define LAZY_INSTANCE_INITIALIZER {}
59
60namespace base {
61
62template <typename Type>
63struct LazyInstanceTraitsBase {
64 static Type* New(void* instance) {
65 DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (alignof(Type) - 1), 0u);
66 // Use placement new to initialize our instance in our preallocated space.
67 // The parenthesis is very important here to force POD type initialization.
68 return new (instance) Type();
69 }
70
71 static void CallDestructor(Type* instance) {
72 // Explicitly call the destructor.
73 instance->~Type();
74 }
75};
76
77// We pull out some of the functionality into non-templated functions, so we
78// can implement the more complicated pieces out of line in the .cc file.
79namespace internal {
80
81// This traits class causes destruction the contained Type at process exit via
82// AtExitManager. This is probably generally not what you want. Instead, prefer
83// Leaky below.
84template <typename Type>
85struct DestructorAtExitLazyInstanceTraits {
86 static const bool kRegisterOnExit = true;
87#if DCHECK_IS_ON()
88 static const bool kAllowedToAccessOnNonjoinableThread = false;
89#endif
90
91 static Type* New(void* instance) {
92 return LazyInstanceTraitsBase<Type>::New(instance);
93 }
94
95 static void Delete(Type* instance) {
96 LazyInstanceTraitsBase<Type>::CallDestructor(instance);
97 }
98};
99
100// Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
101// base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
102// instead of:
103// base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
104// my_leaky_lazy_instance;
105// (especially when T is MyLongTypeNameImplClientHolderFactory).
106// Only use this internal::-qualified verbose form to extend this traits class
107// (depending on its implementation details).
108template <typename Type>
109struct LeakyLazyInstanceTraits {
110 static const bool kRegisterOnExit = false;
111#if DCHECK_IS_ON()
112 static const bool kAllowedToAccessOnNonjoinableThread = true;
113#endif
114
115 static Type* New(void* instance) {
116 ANNOTATE_SCOPED_MEMORY_LEAK;
117 return LazyInstanceTraitsBase<Type>::New(instance);
118 }
119 static void Delete(Type* instance) {
120 }
121};
122
123template <typename Type>
124struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};
125
126} // namespace internal
127
128template <
129 typename Type,
130 typename Traits =
131 internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<Type>>
132class LazyInstance {
133 public:
134 // Do not define a destructor, as doing so makes LazyInstance a
135 // non-POD-struct. We don't want that because then a static initializer will
136 // be created to register the (empty) destructor with atexit() under MSVC, for
137 // example. We handle destruction of the contained Type class explicitly via
138 // the OnExit member function, where needed.
139 // ~LazyInstance() {}
140
141 // Convenience typedef to avoid having to repeat Type for leaky lazy
142 // instances.
143 typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type>> Leaky;
144 typedef LazyInstance<Type, internal::DestructorAtExitLazyInstanceTraits<Type>>
145 DestructorAtExit;
146
147 Type& Get() {
148 return *Pointer();
149 }
150
151 Type* Pointer() {
152#if DCHECK_IS_ON()
153 if (!Traits::kAllowedToAccessOnNonjoinableThread)
154 ThreadRestrictions::AssertSingletonAllowed();
155#endif
156
157 return subtle::GetOrCreateLazyPointer(
158 &private_instance_, &Traits::New, private_buf_,
159 Traits::kRegisterOnExit ? OnExit : nullptr, this);
160 }
161
162 // Returns true if the lazy instance has been created. Unlike Get() and
163 // Pointer(), calling IsCreated() will not instantiate the object of Type.
164 bool IsCreated() {
165 // Return true (i.e. "created") if |private_instance_| is either being
166 // created right now (i.e. |private_instance_| has value of
167 // internal::kLazyInstanceStateCreating) or was already created (i.e.
168 // |private_instance_| has any other non-zero value).
169 return 0 != subtle::NoBarrier_Load(&private_instance_);
170 }
171
172 // MSVC gives a warning that the alignment expands the size of the
173 // LazyInstance struct to make the size a multiple of the alignment. This
174 // is expected in this case.
175#if defined(OS_WIN)
176#pragma warning(push)
177#pragma warning(disable: 4324)
178#endif
179
180 // Effectively private: member data is only public to allow the linker to
181 // statically initialize it and to maintain a POD class. DO NOT USE FROM
182 // OUTSIDE THIS CLASS.
183 subtle::AtomicWord private_instance_;
184
185 // Preallocated space for the Type instance.
186 alignas(Type) char private_buf_[sizeof(Type)];
187
188#if defined(OS_WIN)
189#pragma warning(pop)
190#endif
191
192 private:
193 Type* instance() {
194 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
195 }
196
197 // Adapter function for use with AtExit. This should be called single
198 // threaded, so don't synchronize across threads.
199 // Calling OnExit while the instance is in use by other threads is a mistake.
200 static void OnExit(void* lazy_instance) {
201 LazyInstance<Type, Traits>* me =
202 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
203 Traits::Delete(me->instance());
204 subtle::NoBarrier_Store(&me->private_instance_, 0);
205 }
206};
207
208} // namespace base
209
210#endif // BASE_LAZY_INSTANCE_H_
211