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