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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_