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 // 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 LAZY_INSTANCE_INITIALIZER initializer. 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 = LAZY_INSTANCE_INITIALIZER; 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 38 #include <new> // For placement new. 39 40 #include "base/atomicops.h" 41 #include "base/base_export.h" 42 #include "base/basictypes.h" 43 #include "base/debug/leak_annotations.h" 44 #include "base/logging.h" 45 #include "base/memory/aligned_memory.h" 46 #include "base/threading/thread_restrictions.h" 47 48 // LazyInstance uses its own struct initializer-list style static 49 // initialization, as base's LINKER_INITIALIZED requires a constructor and on 50 // some compilers (notably gcc 4.4) this still ends up needing runtime 51 // initialization. 52 #define LAZY_INSTANCE_INITIALIZER {0} 53 54 namespace base { 55 56 template <typename Type> 57 struct DefaultLazyInstanceTraits { 58 static const bool kRegisterOnExit = true; 59 #ifndef NDEBUG 60 static const bool kAllowedToAccessOnNonjoinableThread = false; 61 #endif 62 NewDefaultLazyInstanceTraits63 static Type* New(void* instance) { 64 DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u) 65 << ": Bad boy, the buffer passed to placement new is not aligned!\n" 66 "This may break some stuff like SSE-based optimizations assuming the " 67 "<Type> objects are word aligned."; 68 // Use placement new to initialize our instance in our preallocated space. 69 // The parenthesis is very important here to force POD type initialization. 70 return new (instance) Type(); 71 } DeleteDefaultLazyInstanceTraits72 static void Delete(Type* instance) { 73 // Explicitly call the destructor. 74 instance->~Type(); 75 } 76 }; 77 78 // We pull out some of the functionality into non-templated functions, so we 79 // can implement the more complicated pieces out of line in the .cc file. 80 namespace internal { 81 82 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.: 83 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance; 84 // instead of: 85 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> > 86 // my_leaky_lazy_instance; 87 // (especially when T is MyLongTypeNameImplClientHolderFactory). 88 // Only use this internal::-qualified verbose form to extend this traits class 89 // (depending on its implementation details). 90 template <typename Type> 91 struct LeakyLazyInstanceTraits { 92 static const bool kRegisterOnExit = false; 93 #ifndef NDEBUG 94 static const bool kAllowedToAccessOnNonjoinableThread = true; 95 #endif 96 NewLeakyLazyInstanceTraits97 static Type* New(void* instance) { 98 ANNOTATE_SCOPED_MEMORY_LEAK; 99 return DefaultLazyInstanceTraits<Type>::New(instance); 100 } DeleteLeakyLazyInstanceTraits101 static void Delete(Type* instance) { 102 } 103 }; 104 105 // Our AtomicWord doubles as a spinlock, where a value of 106 // kBeingCreatedMarker means the spinlock is being held for creation. 107 static const subtle::AtomicWord kLazyInstanceStateCreating = 1; 108 109 // Check if instance needs to be created. If so return true otherwise 110 // if another thread has beat us, wait for instance to be created and 111 // return false. 112 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state); 113 114 // After creating an instance, call this to register the dtor to be called 115 // at program exit and to update the atomic state to hold the |new_instance| 116 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state, 117 subtle::AtomicWord new_instance, 118 void* lazy_instance, 119 void (*dtor)(void*)); 120 121 } // namespace internal 122 123 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> > 124 class LazyInstance { 125 public: 126 // Do not define a destructor, as doing so makes LazyInstance a 127 // non-POD-struct. We don't want that because then a static initializer will 128 // be created to register the (empty) destructor with atexit() under MSVC, for 129 // example. We handle destruction of the contained Type class explicitly via 130 // the OnExit member function, where needed. 131 // ~LazyInstance() {} 132 133 // Convenience typedef to avoid having to repeat Type for leaky lazy 134 // instances. 135 typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky; 136 Get()137 Type& Get() { 138 return *Pointer(); 139 } 140 Pointer()141 Type* Pointer() { 142 #ifndef NDEBUG 143 // Avoid making TLS lookup on release builds. 144 if (!Traits::kAllowedToAccessOnNonjoinableThread) 145 ThreadRestrictions::AssertSingletonAllowed(); 146 #endif 147 // If any bit in the created mask is true, the instance has already been 148 // fully constructed. 149 static const subtle::AtomicWord kLazyInstanceCreatedMask = 150 ~internal::kLazyInstanceStateCreating; 151 152 // We will hopefully have fast access when the instance is already created. 153 // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating 154 // at most once, the load is taken out of NeedsInstance() as a fast-path. 155 // The load has acquire memory ordering as a thread which sees 156 // private_instance_ > creating needs to acquire visibility over 157 // the associated data (private_buf_). Pairing Release_Store is in 158 // CompleteLazyInstance(). 159 subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_); 160 if (!(value & kLazyInstanceCreatedMask) && 161 internal::NeedsLazyInstance(&private_instance_)) { 162 // Create the instance in the space provided by |private_buf_|. 163 value = reinterpret_cast<subtle::AtomicWord>( 164 Traits::New(private_buf_.void_data())); 165 internal::CompleteLazyInstance(&private_instance_, value, this, 166 Traits::kRegisterOnExit ? OnExit : NULL); 167 } 168 return instance(); 169 } 170 171 bool operator==(Type* p) { 172 switch (subtle::NoBarrier_Load(&private_instance_)) { 173 case 0: 174 return p == NULL; 175 case internal::kLazyInstanceStateCreating: 176 return static_cast<void*>(p) == private_buf_.void_data(); 177 default: 178 return p == instance(); 179 } 180 } 181 182 // Effectively private: member data is only public to allow the linker to 183 // statically initialize it and to maintain a POD class. DO NOT USE FROM 184 // OUTSIDE THIS CLASS. 185 186 subtle::AtomicWord private_instance_; 187 // Preallocated space for the Type instance. 188 base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_; 189 190 private: instance()191 Type* instance() { 192 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_)); 193 } 194 195 // Adapter function for use with AtExit. This should be called single 196 // threaded, so don't synchronize across threads. 197 // Calling OnExit while the instance is in use by other threads is a mistake. OnExit(void * lazy_instance)198 static void OnExit(void* lazy_instance) { 199 LazyInstance<Type, Traits>* me = 200 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance); 201 Traits::Delete(me->instance()); 202 subtle::NoBarrier_Store(&me->private_instance_, 0); 203 } 204 }; 205 206 } // namespace base 207 208 #endif // BASE_LAZY_INSTANCE_H_ 209