The Tor Browser: ipc/chromium/src/base/singleton.h@6474c204b198 (annotated)

ipc/chromium/src/base/singleton.h@6474c204b198 (annotated)

ipc/chromium/src/base/singleton.h

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 // 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_,
 ,
                                              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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ipc/chromium/src/base/singleton.h@6474c204b198 (annotated)

ipc/chromium/src/base/singleton.h