1// Copyright (c) 2011 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// PLEASE READ: Do you really need a singleton? 6// 7// Singletons make it hard to determine the lifetime of an object, which can 8// lead to buggy code and spurious crashes. 9// 10// Instead of adding another singleton into the mix, try to identify either: 11// a) An existing singleton that can manage your object's lifetime 12// b) Locations where you can deterministically create the object and pass 13// into other objects 14// 15// If you absolutely need a singleton, please keep them as trivial as possible 16// and ideally a leaf dependency. Singletons get problematic when they attempt 17// to do too much in their destructor or have circular dependencies. 18 19#ifndef BASE_MEMORY_SINGLETON_H_ 20#define BASE_MEMORY_SINGLETON_H_ 21 22#include "base/at_exit.h" 23#include "base/atomicops.h" 24#include "base/base_export.h" 25#include "base/memory/aligned_memory.h" 26#include "base/third_party/dynamic_annotations/dynamic_annotations.h" 27#include "base/threading/thread_restrictions.h" 28 29namespace base { 30namespace internal { 31 32// Our AtomicWord doubles as a spinlock, where a value of 33// kBeingCreatedMarker means the spinlock is being held for creation. 34static const subtle::AtomicWord kBeingCreatedMarker = 1; 35 36// We pull out some of the functionality into a non-templated function, so that 37// we can implement the more complicated pieces out of line in the .cc file. 38BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance); 39 40} // namespace internal 41} // namespace base 42 43// TODO(joth): Move more of this file into namespace base 44 45// Default traits for Singleton<Type>. Calls operator new and operator delete on 46// the object. Registers automatic deletion at process exit. 47// Overload if you need arguments or another memory allocation function. 48template<typename Type> 49struct DefaultSingletonTraits { 50 // Allocates the object. 51 static Type* New() { 52 // The parenthesis is very important here; it forces POD type 53 // initialization. 54 return new Type(); 55 } 56 57 // Destroys the object. 58 static void Delete(Type* x) { 59 delete x; 60 } 61 62 // Set to true to automatically register deletion of the object on process 63 // exit. See below for the required call that makes this happen. 64 static const bool kRegisterAtExit = true; 65 66 // Set to false to disallow access on a non-joinable thread. This is 67 // different from kRegisterAtExit because StaticMemorySingletonTraits allows 68 // access on non-joinable threads, and gracefully handles this. 69 static const bool kAllowedToAccessOnNonjoinableThread = false; 70}; 71 72 73// Alternate traits for use with the Singleton<Type>. Identical to 74// DefaultSingletonTraits except that the Singleton will not be cleaned up 75// at exit. 76template<typename Type> 77struct LeakySingletonTraits : public DefaultSingletonTraits<Type> { 78 static const bool kRegisterAtExit = false; 79 static const bool kAllowedToAccessOnNonjoinableThread = true; 80}; 81 82 83// Alternate traits for use with the Singleton<Type>. Allocates memory 84// for the singleton instance from a static buffer. The singleton will 85// be cleaned up at exit, but can't be revived after destruction unless 86// the Resurrect() method is called. 87// 88// This is useful for a certain category of things, notably logging and 89// tracing, where the singleton instance is of a type carefully constructed to 90// be safe to access post-destruction. 91// In logging and tracing you'll typically get stray calls at odd times, like 92// during static destruction, thread teardown and the like, and there's a 93// termination race on the heap-based singleton - e.g. if one thread calls 94// get(), but then another thread initiates AtExit processing, the first thread 95// may call into an object residing in unallocated memory. If the instance is 96// allocated from the data segment, then this is survivable. 97// 98// The destructor is to deallocate system resources, in this case to unregister 99// a callback the system will invoke when logging levels change. Note that 100// this is also used in e.g. Chrome Frame, where you have to allow for the 101// possibility of loading briefly into someone else's process space, and 102// so leaking is not an option, as that would sabotage the state of your host 103// process once you've unloaded. 104template <typename Type> 105struct StaticMemorySingletonTraits { 106 // WARNING: User has to deal with get() in the singleton class 107 // this is traits for returning NULL. 108 static Type* New() { 109 // Only constructs once and returns pointer; otherwise returns NULL. 110 if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1)) 111 return NULL; 112 113 return new(buffer_.void_data()) Type(); 114 } 115 116 static void Delete(Type* p) { 117 if (p != NULL) 118 p->Type::~Type(); 119 } 120 121 static const bool kRegisterAtExit = true; 122 static const bool kAllowedToAccessOnNonjoinableThread = true; 123 124 // Exposed for unittesting. 125 static void Resurrect() { 126 base::subtle::NoBarrier_Store(&dead_, 0); 127 } 128 129 private: 130 static base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_; 131 // Signal the object was already deleted, so it is not revived. 132 static base::subtle::Atomic32 dead_; 133}; 134 135template <typename Type> base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> 136 StaticMemorySingletonTraits<Type>::buffer_; 137template <typename Type> base::subtle::Atomic32 138 StaticMemorySingletonTraits<Type>::dead_ = 0; 139 140// The Singleton<Type, Traits, DifferentiatingType> class manages a single 141// instance of Type which will be created on first use and will be destroyed at 142// normal process exit). The Trait::Delete function will not be called on 143// abnormal process exit. 144// 145// DifferentiatingType is used as a key to differentiate two different 146// singletons having the same memory allocation functions but serving a 147// different purpose. This is mainly used for Locks serving different purposes. 148// 149// Example usage: 150// 151// In your header: 152// template <typename T> struct DefaultSingletonTraits; 153// class FooClass { 154// public: 155// static FooClass* GetInstance(); <-- See comment below on this. 156// void Bar() { ... } 157// private: 158// FooClass() { ... } 159// friend struct DefaultSingletonTraits<FooClass>; 160// 161// DISALLOW_COPY_AND_ASSIGN(FooClass); 162// }; 163// 164// In your source file: 165// #include "base/memory/singleton.h" 166// FooClass* FooClass::GetInstance() { 167// return Singleton<FooClass>::get(); 168// } 169// 170// And to call methods on FooClass: 171// FooClass::GetInstance()->Bar(); 172// 173// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance 174// and it is important that FooClass::GetInstance() is not inlined in the 175// header. This makes sure that when source files from multiple targets include 176// this header they don't end up with different copies of the inlined code 177// creating multiple copies of the singleton. 178// 179// Singleton<> has no non-static members and doesn't need to actually be 180// instantiated. 181// 182// This class is itself thread-safe. The underlying Type must of course be 183// thread-safe if you want to use it concurrently. Two parameters may be tuned 184// depending on the user's requirements. 185// 186// Glossary: 187// RAE = kRegisterAtExit 188// 189// On every platform, if Traits::RAE is true, the singleton will be destroyed at 190// process exit. More precisely it uses base::AtExitManager which requires an 191// object of this type to be instantiated. AtExitManager mimics the semantics 192// of atexit() such as LIFO order but under Windows is safer to call. For more 193// information see at_exit.h. 194// 195// If Traits::RAE is false, the singleton will not be freed at process exit, 196// thus the singleton will be leaked if it is ever accessed. Traits::RAE 197// shouldn't be false unless absolutely necessary. Remember that the heap where 198// the object is allocated may be destroyed by the CRT anyway. 199// 200// Caveats: 201// (a) Every call to get(), operator->() and operator*() incurs some overhead 202// (16ns on my P4/2.8GHz) to check whether the object has already been 203// initialized. You may wish to cache the result of get(); it will not 204// change. 205// 206// (b) Your factory function must never throw an exception. This class is not 207// exception-safe. 208// 209template <typename Type, 210 typename Traits = DefaultSingletonTraits<Type>, 211 typename DifferentiatingType = Type> 212class Singleton { 213 private: 214 // Classes using the Singleton<T> pattern should declare a GetInstance() 215 // method and call Singleton::get() from within that. 216 friend Type* Type::GetInstance(); 217 218 // Allow TraceLog tests to test tracing after OnExit. 219 friend class DeleteTraceLogForTesting; 220 221 // This class is safe to be constructed and copy-constructed since it has no 222 // member. 223 224 // Return a pointer to the one true instance of the class. 225 static Type* get() { 226#ifndef NDEBUG 227 // Avoid making TLS lookup on release builds. 228 if (!Traits::kAllowedToAccessOnNonjoinableThread) 229 base::ThreadRestrictions::AssertSingletonAllowed(); 230#endif 231 232 base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_); 233 if (value != 0 && value != base::internal::kBeingCreatedMarker) { 234 // See the corresponding HAPPENS_BEFORE below. 235 ANNOTATE_HAPPENS_AFTER(&instance_); 236 return reinterpret_cast<Type*>(value); 237 } 238 239 // Object isn't created yet, maybe we will get to create it, let's try... 240 if (base::subtle::Acquire_CompareAndSwap( 241 &instance_, 0, base::internal::kBeingCreatedMarker) == 0) { 242 // instance_ was NULL and is now kBeingCreatedMarker. Only one thread 243 // will ever get here. Threads might be spinning on us, and they will 244 // stop right after we do this store. 245 Type* newval = Traits::New(); 246 247 // This annotation helps race detectors recognize correct lock-less 248 // synchronization between different threads calling get(). 249 // See the corresponding HAPPENS_AFTER below and above. 250 ANNOTATE_HAPPENS_BEFORE(&instance_); 251 base::subtle::Release_Store( 252 &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval)); 253 254 if (newval != NULL && Traits::kRegisterAtExit) 255 base::AtExitManager::RegisterCallback(OnExit, NULL); 256 257 return newval; 258 } 259 260 // We hit a race. Wait for the other thread to complete it. 261 value = base::internal::WaitForInstance(&instance_); 262 263 // See the corresponding HAPPENS_BEFORE above. 264 ANNOTATE_HAPPENS_AFTER(&instance_); 265 return reinterpret_cast<Type*>(value); 266 } 267 268 // Adapter function for use with AtExit(). This should be called single 269 // threaded, so don't use atomic operations. 270 // Calling OnExit while singleton is in use by other threads is a mistake. 271 static void OnExit(void* /*unused*/) { 272 // AtExit should only ever be register after the singleton instance was 273 // created. We should only ever get here with a valid instance_ pointer. 274 Traits::Delete( 275 reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_))); 276 instance_ = 0; 277 } 278 static base::subtle::AtomicWord instance_; 279}; 280 281template <typename Type, typename Traits, typename DifferentiatingType> 282base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>:: 283 instance_ = 0; 284 285#endif // BASE_MEMORY_SINGLETON_H_ 286