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.
5 // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! DEPRECATED !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
6 // Please don't introduce new instances of LazyInstance<T>. Use a function-local
7 // static of type base::NoDestructor<T> instead:
9 // Factory& Factory::GetInstance() {
10 // static base::NoDestructor<Factory> instance;
13 // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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.
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.
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.
37 // static LazyInstance<MyClass>::Leaky inst = LAZY_INSTANCE_INITIALIZER;
38 // void SomeMethod() {
39 // inst.Get().SomeMethod(); // MyClass::SomeMethod()
41 // MyClass* ptr = inst.Pointer();
42 // ptr->DoDoDo(); // MyClass::DoDoDo
45 #ifndef BASE_LAZY_INSTANCE_H_
46 #define BASE_LAZY_INSTANCE_H_
48 #include <new> // For placement new.
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"
56 // LazyInstance uses its own struct initializer-list style static
57 // initialization, which does not require a constructor.
58 #define LAZY_INSTANCE_INITIALIZER {}
62 template <typename Type>
63 struct 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();
71 static void CallDestructor(Type* instance) {
72 // Explicitly call the destructor.
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.
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
84 template <typename Type>
85 struct DestructorAtExitLazyInstanceTraits {
86 static const bool kRegisterOnExit = true;
88 static const bool kAllowedToAccessOnNonjoinableThread = false;
91 static Type* New(void* instance) {
92 return LazyInstanceTraitsBase<Type>::New(instance);
95 static void Delete(Type* instance) {
96 LazyInstanceTraitsBase<Type>::CallDestructor(instance);
100 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
101 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
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).
108 template <typename Type>
109 struct LeakyLazyInstanceTraits {
110 static const bool kRegisterOnExit = false;
112 static const bool kAllowedToAccessOnNonjoinableThread = true;
115 static Type* New(void* instance) {
116 ANNOTATE_SCOPED_MEMORY_LEAK;
117 return LazyInstanceTraitsBase<Type>::New(instance);
119 static void Delete(Type* instance) {
123 template <typename Type>
124 struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};
126 } // namespace internal
131 internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<Type>>
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() {}
141 // Convenience typedef to avoid having to repeat Type for leaky lazy
143 typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type>> Leaky;
144 typedef LazyInstance<Type, internal::DestructorAtExitLazyInstanceTraits<Type>>
153 if (!Traits::kAllowedToAccessOnNonjoinableThread)
154 ThreadRestrictions::AssertSingletonAllowed();
157 return subtle::GetOrCreateLazyPointer(
158 &private_instance_, &Traits::New, private_buf_,
159 Traits::kRegisterOnExit ? OnExit : nullptr, this);
162 // Returns true if the lazy instance has been created. Unlike Get() and
163 // Pointer(), calling IsCreated() will not instantiate the object of Type.
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_);
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.
176 #pragma warning(push)
177 #pragma warning(disable: 4324)
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_;
185 // Preallocated space for the Type instance.
186 alignas(Type) char private_buf_[sizeof(Type)];
194 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
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);
210 #endif // BASE_LAZY_INSTANCE_H_