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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
// Copyright (c) 2006-2008 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.
#ifndef BASE_SINGLETON_H_
#define BASE_SINGLETON_H_
#include "base/at_exit.h"
#include "base/atomicops.h"
#include "base/platform_thread.h"
// 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;
};
// 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;
};
// 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 usages: (none are preferred, they all result in the same code)
// 1. FooClass* ptr = Singleton<FooClass>::get();
// ptr->Bar();
// 2. Singleton<FooClass>()->Bar();
// 3. Singleton<FooClass>::get()->Bar();
//
// Singleton<> has no non-static members and doesn't need to actually be
// instantiated. It does no harm to instantiate it and use it as a class member
// or at global level since it is acting as a POD type.
//
// 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 instanciated. 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.
//
// If you want to ensure that your class can only exist as a singleton, make
// its constructors private, and make DefaultSingletonTraits<> a friend:
//
// #include "base/singleton.h"
// class FooClass {
// public:
// void Bar() { ... }
// private:
// FooClass() { ... }
// friend struct DefaultSingletonTraits<FooClass>;
//
// DISALLOW_EVIL_CONSTRUCTORS(FooClass);
// };
//
// 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 {
public:
// 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() {
// Our AtomicWord doubles as a spinlock, where a value of
// kBeingCreatedMarker means the spinlock is being held for creation.
static const base::subtle::AtomicWord kBeingCreatedMarker = 1;
base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_);
if (value != 0 && value != kBeingCreatedMarker)
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,
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();
base::subtle::Release_Store(
&instance_, reinterpret_cast<base::subtle::AtomicWord>(newval));
if (Traits::kRegisterAtExit)
base::AtExitManager::RegisterCallback(OnExit, NULL);
return newval;
}
// We hit a race. Another thread beat us and either:
// - Has the object in BeingCreated state
// - Already has the object created...
// We know value != NULL. It could be kBeingCreatedMarker, or a valid ptr.
// Unless your constructor can be very time consuming, it is very unlikely
// to hit this race. When it does, we just spin and yield the thread until
// the object has been created.
while (true) {
value = base::subtle::NoBarrier_Load(&instance_);
if (value != kBeingCreatedMarker)
break;
PlatformThread::YieldCurrentThread();
}
return reinterpret_cast<Type*>(value);
}
// Shortcuts.
Type& operator*() {
return *get();
}
Type* operator->() {
return get();
}
private:
// Adapter function for use with AtExit(). This should be called single
// threaded, but we might as well take the precautions anyway.
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.
Traits::Delete(reinterpret_cast<Type*>(
base::subtle::NoBarrier_AtomicExchange(&instance_, 0)));
}
static base::subtle::AtomicWord instance_;
};
template <typename Type, typename Traits, typename DifferentiatingType>
base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
instance_ = 0;
#endif // BASE_SINGLETON_H_
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