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

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

ipc/chromium/src/base/lazy_instance.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) 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.
 // The LazyInstance<Type, Traits> class manages a single instance of Type,
 // which will be lazily created on the first time it's accessed.  This class is
 // useful for places you would normally use a function-level static, but you
 // need to have guaranteed thread-safety.  The Type constructor will only ever
 // be called once, even if two threads are racing to create the object.  Get()
 // and Pointer() will always return the same, completely initialized instance.
 // When the instance is constructed it is registered with AtExitManager.  The
 // destructor will be called on program exit.
 //
 // LazyInstance is completely thread safe, assuming that you create it safely.
 // The class was designed to be POD initialized, so it shouldn't require a
 // static constructor.  It really only makes sense to declare a LazyInstance as
 // a global variable using the base::LinkerInitialized constructor.
 //
 // LazyInstance is similar to Singleton, except it does not have the singleton
 // property.  You can have multiple LazyInstance's of the same type, and each
 // will manage a unique instance.  It also preallocates the space for Type, as
 // to avoid allocating the Type instance on the heap.  This may help with the
 // performance of creating the instance, and reducing heap fragmentation.  This
 // requires that Type be a complete type so we can determine the size.
 //
 // Example usage:
 //   static LazyInstance<MyClass> my_instance(base::LINKER_INITIALIZED);
 //   void SomeMethod() {
 //     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()
 //
 //     MyClass* ptr = my_instance.Pointer();
 //     ptr->DoDoDo();  // MyClass::DoDoDo
 //   }
 #ifndef BASE_LAZY_INSTANCE_H_
 #define BASE_LAZY_INSTANCE_H_
 #include "base/atomicops.h"
 #include "base/basictypes.h"
 namespace base {
 template <typename Type>
 struct DefaultLazyInstanceTraits {
   static void New(void* instance) {
     // Use placement new to initialize our instance in our preallocated space.
     // The parenthesis is very important here to force POD type initialization.
     new (instance) Type();
   }
   static void Delete(void* instance) {
     // Explicitly call the destructor.
     reinterpret_cast<Type*>(instance)->~Type();
   }
 };
 // We pull out some of the functionality into a non-templated base, so that we
 // can implement the more complicated pieces out of line in the .cc file.
 class LazyInstanceHelper {
  protected:
   enum {
     STATE_EMPTY    = 0,
     STATE_CREATING = 1,
     STATE_CREATED  = 2
   };
   explicit LazyInstanceHelper(LinkerInitialized x) { /* state_ is 0 */ }
   // Declaring a destructor (even if it's empty) will cause MSVC to register a
   // static initializer to register the empty destructor with atexit().
   // Make sure that instance is created, creating or waiting for it to be
   // created if neccessary.  Constructs with |ctor| in the space provided by
   // |instance| and registers dtor for destruction at program exit.
   void EnsureInstance(void* instance, void (*ctor)(void*), void (*dtor)(void*));
   base::subtle::Atomic32 state_;
  private:
   DISALLOW_COPY_AND_ASSIGN(LazyInstanceHelper);
 };
 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
 class LazyInstance : public LazyInstanceHelper {
  public:
   explicit LazyInstance(LinkerInitialized x) : LazyInstanceHelper(x) { }
   // Declaring a destructor (even if it's empty) will cause MSVC to register a
   // static initializer to register the empty destructor with atexit().
   Type& Get() {
     return *Pointer();
   }
   Type* Pointer() {
     Type* instance = reinterpret_cast<Type*>(&buf_);
     // We will hopefully have fast access when the instance is already created.
     if (base::subtle::NoBarrier_Load(&state_) != STATE_CREATED)
       EnsureInstance(instance, Traits::New, Traits::Delete);
     return instance;
   }
  private:
   int8_t buf_[sizeof(Type)];  // Preallocate the space for the Type instance.
   DISALLOW_COPY_AND_ASSIGN(LazyInstance);
 };
 }  // namespace base
 #endif  // BASE_LAZY_INSTANCE_H_

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

ipc/chromium/src/base/lazy_instance.h