1 // Copyright (c) 2011 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 // The LazyInstance<Type, Traits> class manages a single instance of Type, 6 // which will be lazily created on the first time it's accessed. This class is 7 // useful for places you would normally use a function-level static, but you 8 // need to have guaranteed thread-safety. The Type constructor will only ever 9 // be called once, even if two threads are racing to create the object. Get() 10 // and Pointer() will always return the same, completely initialized instance. 11 // When the instance is constructed it is registered with AtExitManager. The 12 // destructor will be called on program exit. 13 // 14 // LazyInstance is completely thread safe, assuming that you create it safely. 15 // The class was designed to be POD initialized, so it shouldn't require a 16 // static constructor. It really only makes sense to declare a LazyInstance as 17 // a global variable using the base::LinkerInitialized constructor. 18 // 19 // LazyInstance is similar to Singleton, except it does not have the singleton 20 // property. You can have multiple LazyInstance's of the same type, and each 21 // will manage a unique instance. It also preallocates the space for Type, as 22 // to avoid allocating the Type instance on the heap. This may help with the 23 // performance of creating the instance, and reducing heap fragmentation. This 24 // requires that Type be a complete type so we can determine the size. 25 // 26 // Example usage: 27 // static LazyInstance<MyClass> my_instance(base::LINKER_INITIALIZED); 28 // void SomeMethod() { 29 // my_instance.Get().SomeMethod(); // MyClass::SomeMethod() 30 // 31 // MyClass* ptr = my_instance.Pointer(); 32 // ptr->DoDoDo(); // MyClass::DoDoDo 33 // } 34 35 #ifndef BASE_LAZY_INSTANCE_H_ 36 #define BASE_LAZY_INSTANCE_H_ 37 #pragma once 38 39 #include <new> // For placement new. 40 41 #include "base/atomicops.h" 42 #include "base/base_api.h" 43 #include "base/basictypes.h" 44 #include "base/third_party/dynamic_annotations/dynamic_annotations.h" 45 #include "base/threading/thread_restrictions.h" 46 47 namespace base { 48 49 template <typename Type> 50 struct DefaultLazyInstanceTraits { 51 static const bool kAllowedToAccessOnNonjoinableThread = false; 52 53 static Type* New(void* instance) { 54 // Use placement new to initialize our instance in our preallocated space. 55 // The parenthesis is very important here to force POD type initialization. 56 return new (instance) Type(); 57 } 58 static void Delete(void* instance) { 59 // Explicitly call the destructor. 60 reinterpret_cast<Type*>(instance)->~Type(); 61 } 62 }; 63 64 template <typename Type> 65 struct LeakyLazyInstanceTraits { 66 static const bool kAllowedToAccessOnNonjoinableThread = true; 67 68 static Type* New(void* instance) { 69 return DefaultLazyInstanceTraits<Type>::New(instance); 70 } 71 // Rather than define an empty Delete function, we make Delete itself 72 // a null pointer. This allows us to completely sidestep registering 73 // this object with an AtExitManager, which allows you to use 74 // LeakyLazyInstanceTraits in contexts where you don't have an 75 // AtExitManager. 76 static void (*Delete)(void* instance); 77 }; 78 79 template <typename Type> 80 void (*LeakyLazyInstanceTraits<Type>::Delete)(void* instance) = NULL; 81 82 // We pull out some of the functionality into a non-templated base, so that we 83 // can implement the more complicated pieces out of line in the .cc file. 84 class BASE_API LazyInstanceHelper { 85 protected: 86 enum { 87 STATE_EMPTY = 0, 88 STATE_CREATING = 1, 89 STATE_CREATED = 2 90 }; 91 92 explicit LazyInstanceHelper(LinkerInitialized /*unused*/) {/* state_ is 0 */} 93 // Declaring a destructor (even if it's empty) will cause MSVC to register a 94 // static initializer to register the empty destructor with atexit(). 95 96 // Check if instance needs to be created. If so return true otherwise 97 // if another thread has beat us, wait for instance to be created and 98 // return false. 99 bool NeedsInstance(); 100 101 // After creating an instance, call this to register the dtor to be called 102 // at program exit and to update the state to STATE_CREATED. 103 void CompleteInstance(void* instance, void (*dtor)(void*)); 104 105 base::subtle::Atomic32 state_; 106 107 private: 108 DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper); 109 }; 110 111 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> > 112 class LazyInstance : public LazyInstanceHelper { 113 public: 114 explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { } 115 // Declaring a destructor (even if it's empty) will cause MSVC to register a 116 // static initializer to register the empty destructor with atexit(). 117 118 Type& Get() { 119 return *Pointer(); 120 } 121 122 Type* Pointer() { 123 if (!Traits::kAllowedToAccessOnNonjoinableThread) 124 base::ThreadRestrictions::AssertSingletonAllowed(); 125 126 // We will hopefully have fast access when the instance is already created. 127 if ((base::subtle::NoBarrier_Load(&state_) != STATE_CREATED) && 128 NeedsInstance()) { 129 // Create the instance in the space provided by |buf_|. 130 instance_ = Traits::New(buf_); 131 // Traits::Delete will be null for LeakyLazyInstanceTraits 132 void (*dtor)(void*) = Traits::Delete; 133 CompleteInstance(this, (dtor == NULL) ? NULL : OnExit); 134 } 135 136 // This annotation helps race detectors recognize correct lock-less 137 // synchronization between different threads calling Pointer(). 138 // We suggest dynamic race detection tool that "Traits::New" above 139 // and CompleteInstance(...) happens before "return instance_" below. 140 // See the corresponding HAPPENS_BEFORE in CompleteInstance(...). 141 ANNOTATE_HAPPENS_AFTER(&state_); 142 return instance_; 143 } 144 145 bool operator==(Type* p) { 146 switch (base::subtle::NoBarrier_Load(&state_)) { 147 case STATE_EMPTY: 148 return p == NULL; 149 case STATE_CREATING: 150 return static_cast<int8*>(static_cast<void*>(p)) == buf_; 151 case STATE_CREATED: 152 return p == instance_; 153 default: 154 return false; 155 } 156 } 157 158 private: 159 // Adapter function for use with AtExit. This should be called single 160 // threaded, so don't use atomic operations. 161 // Calling OnExit while the instance is in use by other threads is a mistake. 162 static void OnExit(void* lazy_instance) { 163 LazyInstance<Type, Traits>* me = 164 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance); 165 Traits::Delete(me->instance_); 166 me->instance_ = NULL; 167 base::subtle::Release_Store(&me->state_, STATE_EMPTY); 168 } 169 170 int8 buf_[sizeof(Type)]; // Preallocate the space for the Type instance. 171 Type *instance_; 172 173 DISALLOW_COPY_AND_ASSIGN(LazyInstance); 174 }; 175 176 } // namespace base 177 178 #endif // BASE_LAZY_INSTANCE_H_ 179
