The Tor Browser: comparison ipc/chromium/src/base/singleton.h

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+:b170897a865b
+// 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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comparison: ipc/chromium/src/base/singleton.h

ipc/chromium/src/base/singleton.h