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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 
60 namespace base {
61 
62 template <typename Type>
63 struct LazyInstanceTraitsBase {
NewLazyInstanceTraitsBase64   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 
CallDestructorLazyInstanceTraitsBase71   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.
79 namespace 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.
84 template <typename Type>
85 struct DestructorAtExitLazyInstanceTraits {
86   static const bool kRegisterOnExit = true;
87 #if DCHECK_IS_ON()
88   static const bool kAllowedToAccessOnNonjoinableThread = false;
89 #endif
90 
NewDestructorAtExitLazyInstanceTraits91   static Type* New(void* instance) {
92     return LazyInstanceTraitsBase<Type>::New(instance);
93   }
94 
DeleteDestructorAtExitLazyInstanceTraits95   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).
108 template <typename Type>
109 struct LeakyLazyInstanceTraits {
110   static const bool kRegisterOnExit = false;
111 #if DCHECK_IS_ON()
112   static const bool kAllowedToAccessOnNonjoinableThread = true;
113 #endif
114 
NewLeakyLazyInstanceTraits115   static Type* New(void* instance) {
116     ANNOTATE_SCOPED_MEMORY_LEAK;
117     return LazyInstanceTraitsBase<Type>::New(instance);
118   }
DeleteLeakyLazyInstanceTraits119   static void Delete(Type* instance) {
120   }
121 };
122 
123 template <typename Type>
124 struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};
125 
126 }  // namespace internal
127 
128 template <
129     typename Type,
130     typename Traits =
131         internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<Type>>
132 class 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 
Get()147   Type& Get() {
148     return *Pointer();
149   }
150 
Pointer()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.
IsCreated()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:
instance()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.
OnExit(void * lazy_instance)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