lazy_instance.h revision 1320f92c476a1ad9d19dba2a48c72b75566198e9
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 54namespace base { 55 56template <typename Type> 57struct DefaultLazyInstanceTraits { 58 static const bool kRegisterOnExit = true; 59#ifndef NDEBUG 60 static const bool kAllowedToAccessOnNonjoinableThread = false; 61#endif 62 63 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 } 72 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. 80namespace 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). 90template <typename Type> 91struct LeakyLazyInstanceTraits { 92 static const bool kRegisterOnExit = false; 93#ifndef NDEBUG 94 static const bool kAllowedToAccessOnNonjoinableThread = true; 95#endif 96 97 static Type* New(void* instance) { 98 ANNOTATE_SCOPED_MEMORY_LEAK; 99 return DefaultLazyInstanceTraits<Type>::New(instance); 100 } 101 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. 107static 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. 112BASE_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| 116BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state, 117 subtle::AtomicWord new_instance, 118 void* lazy_instance, 119 void (*dtor)(void*)); 120 121} // namespace internal 122 123template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> > 124class 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; 136 137 Type& Get() { 138 return *Pointer(); 139 } 140 141 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: 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. 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