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