lazy_instance.h revision 0d205d712abd16eeed2f5d5b1052a367d23a223f
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 56namespace base { 57 58template <typename Type> 59struct 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. 82namespace 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). 92template <typename Type> 93struct 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. 108static 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. 113BASE_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| 117BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state, 118 subtle::AtomicWord new_instance, 119 void* lazy_instance, 120 void (*dtor)(void*)); 121 122} // namespace internal 123 124template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> > 125class 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