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/logging.h" 43 #include "base/memory/aligned_memory.h" 44 #include "base/threading/thread_restrictions.h" 45 46 // LazyInstance uses its own struct initializer-list style static 47 // initialization, as base's LINKER_INITIALIZED requires a constructor and on 48 // some compilers (notably gcc 4.4) this still ends up needing runtime 49 // initialization. 50 #ifdef __clang__ 51 #define LAZY_INSTANCE_INITIALIZER {} 52 #else 53 #define LAZY_INSTANCE_INITIALIZER {0, 0} 54 #endif 55 56 namespace base { 57 58 template <typename Type> 59 struct DefaultLazyInstanceTraits { 60 static const bool kRegisterOnExit = true; 61 #ifndef NDEBUG 62 static const bool kAllowedToAccessOnNonjoinableThread = false; 63 #endif 64 65 static Type* New(void* instance) { 66 DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u) 67 << ": Bad boy, the buffer passed to placement new is not aligned!\n" 68 "This may break some stuff like SSE-based optimizations assuming the " 69 "<Type> objects are word aligned."; 70 // Use placement new to initialize our instance in our preallocated space. 71 // The parenthesis is very important here to force POD type initialization. 72 return new (instance) Type(); 73 } 74 static void Delete(Type* instance) { 75 // Explicitly call the destructor. 76 instance->~Type(); 77 } 78 }; 79 80 // We pull out some of the functionality into non-templated functions, so we 81 // can implement the more complicated pieces out of line in the .cc file. 82 namespace internal { 83 84 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.: 85 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance; 86 // instead of: 87 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> > 88 // my_leaky_lazy_instance; 89 // (especially when T is MyLongTypeNameImplClientHolderFactory). 90 // Only use this internal::-qualified verbose form to extend this traits class 91 // (depending on its implementation details). 92 template <typename Type> 93 struct LeakyLazyInstanceTraits { 94 static const bool kRegisterOnExit = false; 95 #ifndef NDEBUG 96 static const bool kAllowedToAccessOnNonjoinableThread = true; 97 #endif 98 99 static Type* New(void* instance) { 100 return DefaultLazyInstanceTraits<Type>::New(instance); 101 } 102 static void Delete(Type* /* instance */) { 103 } 104 }; 105 106 // Our AtomicWord doubles as a spinlock, where a value of 107 // kBeingCreatedMarker means the spinlock is being held for creation. 108 static const subtle::AtomicWord kLazyInstanceStateCreating = 1; 109 110 // Check if instance needs to be created. If so return true otherwise 111 // if another thread has beat us, wait for instance to be created and 112 // return false. 113 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state); 114 115 // After creating an instance, call this to register the dtor to be called 116 // at program exit and to update the atomic state to hold the |new_instance| 117 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state, 118 subtle::AtomicWord new_instance, 119 void* lazy_instance, 120 void (*dtor)(void*)); 121 122 } // namespace internal 123 124 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> > 125 class LazyInstance { 126 public: 127 // Do not define a destructor, as doing so makes LazyInstance a 128 // non-POD-struct. We don't want that because then a static initializer will 129 // be created to register the (empty) destructor with atexit() under MSVC, for 130 // example. We handle destruction of the contained Type class explicitly via 131 // the OnExit member function, where needed. 132 // ~LazyInstance() {} 133 134 // Convenience typedef to avoid having to repeat Type for leaky lazy 135 // instances. 136 typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky; 137 138 Type& Get() { 139 return *Pointer(); 140 } 141 142 Type* Pointer() { 143 #ifndef NDEBUG 144 // Avoid making TLS lookup on release builds. 145 if (!Traits::kAllowedToAccessOnNonjoinableThread) 146 ThreadRestrictions::AssertSingletonAllowed(); 147 #endif 148 // If any bit in the created mask is true, the instance has already been 149 // fully constructed. 150 static const subtle::AtomicWord kLazyInstanceCreatedMask = 151 ~internal::kLazyInstanceStateCreating; 152 153 // We will hopefully have fast access when the instance is already created. 154 // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating 155 // at most once, the load is taken out of NeedsInstance() as a fast-path. 156 // The load has acquire memory ordering as a thread which sees 157 // private_instance_ > creating needs to acquire visibility over 158 // the associated data (private_buf_). Pairing Release_Store is in 159 // CompleteLazyInstance(). 160 subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_); 161 if (!(value & kLazyInstanceCreatedMask) && 162 internal::NeedsLazyInstance(&private_instance_)) { 163 // Create the instance in the space provided by |private_buf_|. 164 value = reinterpret_cast<subtle::AtomicWord>( 165 Traits::New(private_buf_.void_data())); 166 internal::CompleteLazyInstance(&private_instance_, value, this, 167 Traits::kRegisterOnExit ? OnExit : NULL); 168 } 169 return instance(); 170 } 171 172 bool operator==(Type* p) { 173 switch (subtle::NoBarrier_Load(&private_instance_)) { 174 case 0: 175 return p == NULL; 176 case internal::kLazyInstanceStateCreating: 177 return static_cast<void*>(p) == private_buf_.void_data(); 178 default: 179 return p == instance(); 180 } 181 } 182 183 // Effectively private: member data is only public to allow the linker to 184 // statically initialize it and to maintain a POD class. DO NOT USE FROM 185 // OUTSIDE THIS CLASS. 186 187 subtle::AtomicWord private_instance_; 188 // Preallocated space for the Type instance. 189 base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_; 190 191 private: 192 Type* instance() { 193 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_)); 194 } 195 196 // Adapter function for use with AtExit. This should be called single 197 // threaded, so don't synchronize across threads. 198 // Calling OnExit while the instance is in use by other threads is a mistake. 199 static void OnExit(void* lazy_instance) { 200 LazyInstance<Type, Traits>* me = 201 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance); 202 Traits::Delete(me->instance()); 203 subtle::NoBarrier_Store(&me->private_instance_, 0); 204 } 205 }; 206 207 } // namespace base 208 209 #endif // BASE_LAZY_INSTANCE_H_ 210
