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/logging.h" 26 #include "base/macros.h" 27 #include "base/memory/aligned_memory.h" 28 #include "base/threading/thread_restrictions.h" 29 30 namespace base { 31 namespace internal { 32 33 // Our AtomicWord doubles as a spinlock, where a value of 34 // kBeingCreatedMarker means the spinlock is being held for creation. 35 static const subtle::AtomicWord kBeingCreatedMarker = 1; 36 37 // We pull out some of the functionality into a non-templated function, so that 38 // we can implement the more complicated pieces out of line in the .cc file. 39 BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance); 40 41 class DeleteTraceLogForTesting; 42 43 } // namespace internal 44 45 46 // Default traits for Singleton<Type>. Calls operator new and operator delete on 47 // the object. Registers automatic deletion at process exit. 48 // Overload if you need arguments or another memory allocation function. 49 template<typename Type> 50 struct DefaultSingletonTraits { 51 // Allocates the object. NewDefaultSingletonTraits52 static Type* New() { 53 // The parenthesis is very important here; it forces POD type 54 // initialization. 55 return new Type(); 56 } 57 58 // Destroys the object. DeleteDefaultSingletonTraits59 static void Delete(Type* x) { 60 delete x; 61 } 62 63 // Set to true to automatically register deletion of the object on process 64 // exit. See below for the required call that makes this happen. 65 static const bool kRegisterAtExit = true; 66 67 #if DCHECK_IS_ON() 68 // Set to false to disallow access on a non-joinable thread. This is 69 // different from kRegisterAtExit because StaticMemorySingletonTraits allows 70 // access on non-joinable threads, and gracefully handles this. 71 static const bool kAllowedToAccessOnNonjoinableThread = false; 72 #endif 73 }; 74 75 76 // Alternate traits for use with the Singleton<Type>. Identical to 77 // DefaultSingletonTraits except that the Singleton will not be cleaned up 78 // at exit. 79 template<typename Type> 80 struct LeakySingletonTraits : public DefaultSingletonTraits<Type> { 81 static const bool kRegisterAtExit = false; 82 #if DCHECK_IS_ON() 83 static const bool kAllowedToAccessOnNonjoinableThread = true; 84 #endif 85 }; 86 87 88 // Alternate traits for use with the Singleton<Type>. Allocates memory 89 // for the singleton instance from a static buffer. The singleton will 90 // be cleaned up at exit, but can't be revived after destruction unless 91 // the Resurrect() method is called. 92 // 93 // This is useful for a certain category of things, notably logging and 94 // tracing, where the singleton instance is of a type carefully constructed to 95 // be safe to access post-destruction. 96 // In logging and tracing you'll typically get stray calls at odd times, like 97 // during static destruction, thread teardown and the like, and there's a 98 // termination race on the heap-based singleton - e.g. if one thread calls 99 // get(), but then another thread initiates AtExit processing, the first thread 100 // may call into an object residing in unallocated memory. If the instance is 101 // allocated from the data segment, then this is survivable. 102 // 103 // The destructor is to deallocate system resources, in this case to unregister 104 // a callback the system will invoke when logging levels change. Note that 105 // this is also used in e.g. Chrome Frame, where you have to allow for the 106 // possibility of loading briefly into someone else's process space, and 107 // so leaking is not an option, as that would sabotage the state of your host 108 // process once you've unloaded. 109 template <typename Type> 110 struct StaticMemorySingletonTraits { 111 // WARNING: User has to deal with get() in the singleton class 112 // this is traits for returning NULL. NewStaticMemorySingletonTraits113 static Type* New() { 114 // Only constructs once and returns pointer; otherwise returns NULL. 115 if (subtle::NoBarrier_AtomicExchange(&dead_, 1)) 116 return NULL; 117 118 return new(buffer_.void_data()) Type(); 119 } 120 DeleteStaticMemorySingletonTraits121 static void Delete(Type* p) { 122 if (p != NULL) 123 p->Type::~Type(); 124 } 125 126 static const bool kRegisterAtExit = true; 127 static const bool kAllowedToAccessOnNonjoinableThread = true; 128 129 // Exposed for unittesting. ResurrectStaticMemorySingletonTraits130 static void Resurrect() { subtle::NoBarrier_Store(&dead_, 0); } 131 132 private: 133 static AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_; 134 // Signal the object was already deleted, so it is not revived. 135 static subtle::Atomic32 dead_; 136 }; 137 138 template <typename Type> 139 AlignedMemory<sizeof(Type), ALIGNOF(Type)> 140 StaticMemorySingletonTraits<Type>::buffer_; 141 template <typename Type> 142 subtle::Atomic32 StaticMemorySingletonTraits<Type>::dead_ = 0; 143 144 // The Singleton<Type, Traits, DifferentiatingType> class manages a single 145 // instance of Type which will be created on first use and will be destroyed at 146 // normal process exit). The Trait::Delete function will not be called on 147 // abnormal process exit. 148 // 149 // DifferentiatingType is used as a key to differentiate two different 150 // singletons having the same memory allocation functions but serving a 151 // different purpose. This is mainly used for Locks serving different purposes. 152 // 153 // Example usage: 154 // 155 // In your header: 156 // namespace base { 157 // template <typename T> 158 // struct DefaultSingletonTraits; 159 // } 160 // class FooClass { 161 // public: 162 // static FooClass* GetInstance(); <-- See comment below on this. 163 // void Bar() { ... } 164 // private: 165 // FooClass() { ... } 166 // friend struct base::DefaultSingletonTraits<FooClass>; 167 // 168 // DISALLOW_COPY_AND_ASSIGN(FooClass); 169 // }; 170 // 171 // In your source file: 172 // #include "base/memory/singleton.h" 173 // FooClass* FooClass::GetInstance() { 174 // return base::Singleton<FooClass>::get(); 175 // } 176 // 177 // Or for leaky singletons: 178 // #include "base/memory/singleton.h" 179 // FooClass* FooClass::GetInstance() { 180 // return base::Singleton< 181 // FooClass, base::LeakySingletonTraits<FooClass>>::get(); 182 // } 183 // 184 // And to call methods on FooClass: 185 // FooClass::GetInstance()->Bar(); 186 // 187 // NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance 188 // and it is important that FooClass::GetInstance() is not inlined in the 189 // header. This makes sure that when source files from multiple targets include 190 // this header they don't end up with different copies of the inlined code 191 // creating multiple copies of the singleton. 192 // 193 // Singleton<> has no non-static members and doesn't need to actually be 194 // instantiated. 195 // 196 // This class is itself thread-safe. The underlying Type must of course be 197 // thread-safe if you want to use it concurrently. Two parameters may be tuned 198 // depending on the user's requirements. 199 // 200 // Glossary: 201 // RAE = kRegisterAtExit 202 // 203 // On every platform, if Traits::RAE is true, the singleton will be destroyed at 204 // process exit. More precisely it uses AtExitManager which requires an 205 // object of this type to be instantiated. AtExitManager mimics the semantics 206 // of atexit() such as LIFO order but under Windows is safer to call. For more 207 // information see at_exit.h. 208 // 209 // If Traits::RAE is false, the singleton will not be freed at process exit, 210 // thus the singleton will be leaked if it is ever accessed. Traits::RAE 211 // shouldn't be false unless absolutely necessary. Remember that the heap where 212 // the object is allocated may be destroyed by the CRT anyway. 213 // 214 // Caveats: 215 // (a) Every call to get(), operator->() and operator*() incurs some overhead 216 // (16ns on my P4/2.8GHz) to check whether the object has already been 217 // initialized. You may wish to cache the result of get(); it will not 218 // change. 219 // 220 // (b) Your factory function must never throw an exception. This class is not 221 // exception-safe. 222 // 223 224 template <typename Type, 225 typename Traits = DefaultSingletonTraits<Type>, 226 typename DifferentiatingType = Type> 227 class Singleton { 228 private: 229 // Classes using the Singleton<T> pattern should declare a GetInstance() 230 // method and call Singleton::get() from within that. 231 friend Type* Type::GetInstance(); 232 233 // Allow TraceLog tests to test tracing after OnExit. 234 friend class internal::DeleteTraceLogForTesting; 235 236 // This class is safe to be constructed and copy-constructed since it has no 237 // member. 238 239 // Return a pointer to the one true instance of the class. get()240 static Type* get() { 241 #if DCHECK_IS_ON() 242 // Avoid making TLS lookup on release builds. 243 if (!Traits::kAllowedToAccessOnNonjoinableThread) 244 ThreadRestrictions::AssertSingletonAllowed(); 245 #endif 246 247 // The load has acquire memory ordering as the thread which reads the 248 // instance_ pointer must acquire visibility over the singleton data. 249 subtle::AtomicWord value = subtle::Acquire_Load(&instance_); 250 if (value != 0 && value != internal::kBeingCreatedMarker) { 251 return reinterpret_cast<Type*>(value); 252 } 253 254 // Object isn't created yet, maybe we will get to create it, let's try... 255 if (subtle::Acquire_CompareAndSwap(&instance_, 0, 256 internal::kBeingCreatedMarker) == 0) { 257 // instance_ was NULL and is now kBeingCreatedMarker. Only one thread 258 // will ever get here. Threads might be spinning on us, and they will 259 // stop right after we do this store. 260 Type* newval = Traits::New(); 261 262 // Releases the visibility over instance_ to the readers. 263 subtle::Release_Store(&instance_, 264 reinterpret_cast<subtle::AtomicWord>(newval)); 265 266 if (newval != NULL && Traits::kRegisterAtExit) 267 AtExitManager::RegisterCallback(OnExit, NULL); 268 269 return newval; 270 } 271 272 // We hit a race. Wait for the other thread to complete it. 273 value = internal::WaitForInstance(&instance_); 274 275 return reinterpret_cast<Type*>(value); 276 } 277 278 // Adapter function for use with AtExit(). This should be called single 279 // threaded, so don't use atomic operations. 280 // Calling OnExit while singleton is in use by other threads is a mistake. OnExit(void *)281 static void OnExit(void* /*unused*/) { 282 // AtExit should only ever be register after the singleton instance was 283 // created. We should only ever get here with a valid instance_ pointer. 284 Traits::Delete(reinterpret_cast<Type*>(subtle::NoBarrier_Load(&instance_))); 285 instance_ = 0; 286 } 287 static subtle::AtomicWord instance_; 288 }; 289 290 template <typename Type, typename Traits, typename DifferentiatingType> 291 subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::instance_ = 0; 292 293 } // namespace base 294 295 #endif // BASE_MEMORY_SINGLETON_H_ 296