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/third_party/dynamic_annotations/dynamic_annotations.h" 47 #include "base/threading/thread_restrictions.h" 48 49 // LazyInstance uses its own struct initializer-list style static 50 // initialization, as base's LINKER_INITIALIZED requires a constructor and on 51 // some compilers (notably gcc 4.4) this still ends up needing runtime 52 // initialization. 53 #define LAZY_INSTANCE_INITIALIZER {0} 54 55 namespace base { 56 57 template <typename Type> 58 struct DefaultLazyInstanceTraits { 59 static const bool kRegisterOnExit = true; 60 #ifndef NDEBUG 61 static const bool kAllowedToAccessOnNonjoinableThread = false; 62 #endif 63 64 static Type* New(void* instance) { 65 DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u) 66 << ": Bad boy, the buffer passed to placement new is not aligned!\n" 67 "This may break some stuff like SSE-based optimizations assuming the " 68 "<Type> objects are word aligned."; 69 // Use placement new to initialize our instance in our preallocated space. 70 // The parenthesis is very important here to force POD type initialization. 71 return new (instance) Type(); 72 } 73 static void Delete(Type* instance) { 74 // Explicitly call the destructor. 75 instance->~Type(); 76 } 77 }; 78 79 // We pull out some of the functionality into non-templated functions, so we 80 // can implement the more complicated pieces out of line in the .cc file. 81 namespace internal { 82 83 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.: 84 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance; 85 // instead of: 86 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> > 87 // my_leaky_lazy_instance; 88 // (especially when T is MyLongTypeNameImplClientHolderFactory). 89 // Only use this internal::-qualified verbose form to extend this traits class 90 // (depending on its implementation details). 91 template <typename Type> 92 struct LeakyLazyInstanceTraits { 93 static const bool kRegisterOnExit = false; 94 #ifndef NDEBUG 95 static const bool kAllowedToAccessOnNonjoinableThread = true; 96 #endif 97 98 static Type* New(void* instance) { 99 ANNOTATE_SCOPED_MEMORY_LEAK; 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 } 169 170 // This annotation helps race detectors recognize correct lock-less 171 // synchronization between different threads calling Pointer(). 172 // We suggest dynamic race detection tool that "Traits::New" above 173 // and CompleteLazyInstance(...) happens before "return instance()" below. 174 // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...). 175 ANNOTATE_HAPPENS_AFTER(&private_instance_); 176 return instance(); 177 } 178 179 bool operator==(Type* p) { 180 switch (subtle::NoBarrier_Load(&private_instance_)) { 181 case 0: 182 return p == NULL; 183 case internal::kLazyInstanceStateCreating: 184 return static_cast<void*>(p) == private_buf_.void_data(); 185 default: 186 return p == instance(); 187 } 188 } 189 190 // Effectively private: member data is only public to allow the linker to 191 // statically initialize it and to maintain a POD class. DO NOT USE FROM 192 // OUTSIDE THIS CLASS. 193 194 subtle::AtomicWord private_instance_; 195 // Preallocated space for the Type instance. 196 base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_; 197 198 private: 199 Type* instance() { 200 return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_)); 201 } 202 203 // Adapter function for use with AtExit. This should be called single 204 // threaded, so don't synchronize across threads. 205 // Calling OnExit while the instance is in use by other threads is a mistake. 206 static void OnExit(void* lazy_instance) { 207 LazyInstance<Type, Traits>* me = 208 reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance); 209 Traits::Delete(me->instance()); 210 subtle::NoBarrier_Store(&me->private_instance_, 0); 211 } 212 }; 213 214 } // namespace base 215 216 #endif // BASE_LAZY_INSTANCE_H_ 217
