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