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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// PLEASE READ: Do you really need a singleton?
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// Singletons make it hard to determine the lifetime of an object, which can
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// lead to buggy code and spurious crashes.
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// Instead of adding another singleton into the mix, try to identify either:
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// a) An existing singleton that can manage your object's lifetime
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// b) Locations where you can deterministically create the object and pass
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// If you absolutely need a singleton, please keep them as trivial as possible
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// and ideally a leaf dependency. Singletons get problematic when they attempt
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// to do too much in their destructor or have circular dependencies.
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#ifndef BASE_MEMORY_SINGLETON_H_
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#define BASE_MEMORY_SINGLETON_H_
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#include "base/at_exit.h"
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#include "base/atomicops.h"
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#include "base/base_export.h"
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#include "base/memory/aligned_memory.h"
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#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
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#include "base/threading/thread_restrictions.h"
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// Our AtomicWord doubles as a spinlock, where a value of
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// kBeingCreatedMarker means the spinlock is being held for creation.
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static const subtle::AtomicWord kBeingCreatedMarker = 1;
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// We pull out some of the functionality into a non-templated function, so that
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// we can implement the more complicated pieces out of line in the .cc file.
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BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance);
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} // namespace internal
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// TODO(joth): Move more of this file into namespace base
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// Default traits for Singleton<Type>. Calls operator new and operator delete on
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// the object. Registers automatic deletion at process exit.
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// Overload if you need arguments or another memory allocation function.
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template<typename Type>
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struct DefaultSingletonTraits {
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// Allocates the object.
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// The parenthesis is very important here; it forces POD type
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// Destroys the object.
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static void Delete(Type* x) {
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// Set to true to automatically register deletion of the object on process
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// exit. See below for the required call that makes this happen.
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static const bool kRegisterAtExit = true;
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// Set to false to disallow access on a non-joinable thread. This is
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// different from kRegisterAtExit because StaticMemorySingletonTraits allows
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// access on non-joinable threads, and gracefully handles this.
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static const bool kAllowedToAccessOnNonjoinableThread = false;
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// Alternate traits for use with the Singleton<Type>. Identical to
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// DefaultSingletonTraits except that the Singleton will not be cleaned up
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template<typename Type>
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struct LeakySingletonTraits : public DefaultSingletonTraits<Type> {
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static const bool kRegisterAtExit = false;
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static const bool kAllowedToAccessOnNonjoinableThread = true;
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// Alternate traits for use with the Singleton<Type>. Allocates memory
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// for the singleton instance from a static buffer. The singleton will
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// be cleaned up at exit, but can't be revived after destruction unless
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// the Resurrect() method is called.
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// This is useful for a certain category of things, notably logging and
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// tracing, where the singleton instance is of a type carefully constructed to
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// be safe to access post-destruction.
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// In logging and tracing you'll typically get stray calls at odd times, like
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// during static destruction, thread teardown and the like, and there's a
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// termination race on the heap-based singleton - e.g. if one thread calls
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// get(), but then another thread initiates AtExit processing, the first thread
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// may call into an object residing in unallocated memory. If the instance is
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// allocated from the data segment, then this is survivable.
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// The destructor is to deallocate system resources, in this case to unregister
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// a callback the system will invoke when logging levels change. Note that
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// this is also used in e.g. Chrome Frame, where you have to allow for the
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// possibility of loading briefly into someone else's process space, and
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// so leaking is not an option, as that would sabotage the state of your host
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// process once you've unloaded.
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template <typename Type>
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struct StaticMemorySingletonTraits {
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// WARNING: User has to deal with get() in the singleton class
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// this is traits for returning NULL.
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// Only constructs once and returns pointer; otherwise returns NULL.
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if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1))
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return new(buffer_.void_data()) Type();
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static void Delete(Type* p) {
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static const bool kRegisterAtExit = true;
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static const bool kAllowedToAccessOnNonjoinableThread = true;
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// Exposed for unittesting.
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static void Resurrect() {
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base::subtle::NoBarrier_Store(&dead_, 0);
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static base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_;
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// Signal the object was already deleted, so it is not revived.
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static base::subtle::Atomic32 dead_;
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template <typename Type> base::AlignedMemory<sizeof(Type), ALIGNOF(Type)>
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StaticMemorySingletonTraits<Type>::buffer_;
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template <typename Type> base::subtle::Atomic32
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StaticMemorySingletonTraits<Type>::dead_ = 0;
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// The Singleton<Type, Traits, DifferentiatingType> class manages a single
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// instance of Type which will be created on first use and will be destroyed at
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// normal process exit). The Trait::Delete function will not be called on
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// abnormal process exit.
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// DifferentiatingType is used as a key to differentiate two different
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// singletons having the same memory allocation functions but serving a
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// different purpose. This is mainly used for Locks serving different purposes.
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// template <typename T> struct DefaultSingletonTraits;
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// static FooClass* GetInstance(); <-- See comment below on this.
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// void Bar() { ... }
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// FooClass() { ... }
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// friend struct DefaultSingletonTraits<FooClass>;
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// DISALLOW_COPY_AND_ASSIGN(FooClass);
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// In your source file:
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// #include "base/memory/singleton.h"
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// FooClass* FooClass::GetInstance() {
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// return Singleton<FooClass>::get();
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// And to call methods on FooClass:
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// FooClass::GetInstance()->Bar();
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// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance
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// and it is important that FooClass::GetInstance() is not inlined in the
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// header. This makes sure that when source files from multiple targets include
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// this header they don't end up with different copies of the inlined code
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// creating multiple copies of the singleton.
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// Singleton<> has no non-static members and doesn't need to actually be
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// This class is itself thread-safe. The underlying Type must of course be
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// thread-safe if you want to use it concurrently. Two parameters may be tuned
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// depending on the user's requirements.
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// RAE = kRegisterAtExit
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// On every platform, if Traits::RAE is true, the singleton will be destroyed at
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// process exit. More precisely it uses base::AtExitManager which requires an
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// object of this type to be instantiated. AtExitManager mimics the semantics
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// of atexit() such as LIFO order but under Windows is safer to call. For more
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// information see at_exit.h.
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// If Traits::RAE is false, the singleton will not be freed at process exit,
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// thus the singleton will be leaked if it is ever accessed. Traits::RAE
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// shouldn't be false unless absolutely necessary. Remember that the heap where
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// the object is allocated may be destroyed by the CRT anyway.
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// (a) Every call to get(), operator->() and operator*() incurs some overhead
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// (16ns on my P4/2.8GHz) to check whether the object has already been
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// initialized. You may wish to cache the result of get(); it will not
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// (b) Your factory function must never throw an exception. This class is not
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template <typename Type,
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typename Traits = DefaultSingletonTraits<Type>,
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typename DifferentiatingType = Type>
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// Classes using the Singleton<T> pattern should declare a GetInstance()
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// method and call Singleton::get() from within that.
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friend Type* Type::GetInstance();
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// Allow TraceLog tests to test tracing after OnExit.
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friend class DeleteTraceLogForTesting;
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// This class is safe to be constructed and copy-constructed since it has no
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// Return a pointer to the one true instance of the class.
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// Avoid making TLS lookup on release builds.
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if (!Traits::kAllowedToAccessOnNonjoinableThread)
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base::ThreadRestrictions::AssertSingletonAllowed();
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// The load has acquire memory ordering as the thread which reads the
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// instance_ pointer must acquire visibility over the singleton data.
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base::subtle::AtomicWord value = base::subtle::Acquire_Load(&instance_);
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if (value != 0 && value != base::internal::kBeingCreatedMarker) {
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// See the corresponding HAPPENS_BEFORE below.
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ANNOTATE_HAPPENS_AFTER(&instance_);
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return reinterpret_cast<Type*>(value);
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// Object isn't created yet, maybe we will get to create it, let's try...
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if (base::subtle::Acquire_CompareAndSwap(
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&instance_, 0, base::internal::kBeingCreatedMarker) == 0) {
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// instance_ was NULL and is now kBeingCreatedMarker. Only one thread
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// will ever get here. Threads might be spinning on us, and they will
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// stop right after we do this store.
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Type* newval = Traits::New();
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// This annotation helps race detectors recognize correct lock-less
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// synchronization between different threads calling get().
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// See the corresponding HAPPENS_AFTER below and above.
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ANNOTATE_HAPPENS_BEFORE(&instance_);
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// Releases the visibility over instance_ to the readers.
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base::subtle::Release_Store(
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&instance_, reinterpret_cast<base::subtle::AtomicWord>(newval));
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if (newval != NULL && Traits::kRegisterAtExit)
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base::AtExitManager::RegisterCallback(OnExit, NULL);
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// We hit a race. Wait for the other thread to complete it.
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value = base::internal::WaitForInstance(&instance_);
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// See the corresponding HAPPENS_BEFORE above.
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ANNOTATE_HAPPENS_AFTER(&instance_);
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return reinterpret_cast<Type*>(value);
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// Adapter function for use with AtExit(). This should be called single
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// threaded, so don't use atomic operations.
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// Calling OnExit while singleton is in use by other threads is a mistake.
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static void OnExit(void* /*unused*/) {
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// AtExit should only ever be register after the singleton instance was
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// created. We should only ever get here with a valid instance_ pointer.
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reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_)));
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static base::subtle::AtomicWord instance_;
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template <typename Type, typename Traits, typename DifferentiatingType>
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base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
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#endif // BASE_MEMORY_SINGLETON_H_