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authorMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
committerMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
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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_