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 // Copyright (c) 2012 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 LAZY_INSTANCE_INITIALIZER initializer.

 //

 // 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 = LAZY_INSTANCE_INITIALIZER;

 //   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 <new>  // For placement new.

 #include "base/atomicops.h"

 #include "base/base_export.h"

 #include "base/basictypes.h"

 #include "base/debug/leak_annotations.h"

 #include "base/logging.h"

 #include "base/memory/aligned_memory.h"

 #include "base/third_party/dynamic_annotations/dynamic_annotations.h"

 #include "base/threading/thread_restrictions.h"

 // LazyInstance uses its own struct initializer-list style static

 // initialization, as base's LINKER_INITIALIZED requires a constructor and on

 // some compilers (notably gcc 4.4) this still ends up needing runtime

 // initialization.

 #define LAZY_INSTANCE_INITIALIZER {0}

 namespace base {

 template <typename Type>

 struct DefaultLazyInstanceTraits {

   static const bool kRegisterOnExit = true;

   static const bool kAllowedToAccessOnNonjoinableThread = false;

   static Type* New(void* instance) {

     DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)

         << ": Bad boy, the buffer passed to placement new is not aligned!\n"

         "This may break some stuff like SSE-based optimizations assuming the "

         "<Type> objects are word aligned.";

     // Use placement new to initialize our instance in our preallocated space.

     // The parenthesis is very important here to force POD type initialization.

     return new (instance) Type();

   }

   static void Delete(Type* instance) {

     // Explicitly call the destructor.

     instance->~Type();

   }

 };

 // We pull out some of the functionality into non-templated functions, so we

 // can implement the more complicated pieces out of line in the .cc file.

 namespace internal {

 // Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:

 // base::LazyInstance<T>::Leaky my_leaky_lazy_instance;

 // instead of:

 // base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >

 // my_leaky_lazy_instance;

 // (especially when T is MyLongTypeNameImplClientHolderFactory).

 // Only use this internal::-qualified verbose form to extend this traits class

 // (depending on its implementation details).

 template <typename Type>

 struct LeakyLazyInstanceTraits {

   static const bool kRegisterOnExit = false;

   static const bool kAllowedToAccessOnNonjoinableThread = true;

   static Type* New(void* instance) {

     ANNOTATE_SCOPED_MEMORY_LEAK;

     return DefaultLazyInstanceTraits<Type>::New(instance);

   }

   static void Delete(Type* instance) {

   }

 };

 // Our AtomicWord doubles as a spinlock, where a value of

 // kBeingCreatedMarker means the spinlock is being held for creation.

 static const subtle::AtomicWord kLazyInstanceStateCreating = 1;

 // Check if instance needs to be created. If so return true otherwise

 // if another thread has beat us, wait for instance to be created and

 // return false.

 BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);

 // After creating an instance, call this to register the dtor to be called

 // at program exit and to update the atomic state to hold the |new_instance|

 BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,

                                       subtle::AtomicWord new_instance,

                                       void* lazy_instance,

                                       void (*dtor)(void*));

 }  // namespace internal

 template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >

 class LazyInstance {

  public:

   // Do not define a destructor, as doing so makes LazyInstance a

   // non-POD-struct. We don't want that because then a static initializer will

   // be created to register the (empty) destructor with atexit() under MSVC, for

   // example. We handle destruction of the contained Type class explicitly via

   // the OnExit member function, where needed.

   // ~LazyInstance() {}

   // Convenience typedef to avoid having to repeat Type for leaky lazy

   // instances.

   typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;

   Type& Get() {

     return *Pointer();

   }

   Type* Pointer() {

 #ifndef NDEBUG

     // Avoid making TLS lookup on release builds.

     if (!Traits::kAllowedToAccessOnNonjoinableThread)

       ThreadRestrictions::AssertSingletonAllowed();

 #endif

     // If any bit in the created mask is true, the instance has already been

     // fully constructed.

     static const subtle::AtomicWord kLazyInstanceCreatedMask =

         ~internal::kLazyInstanceStateCreating;

     // We will hopefully have fast access when the instance is already created.

     // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating

     // at most once, the load is taken out of NeedsInstance() as a fast-path.

     // The load has acquire memory ordering as a thread which sees

     // private_instance_ > creating needs to acquire visibility over

     // the associated data (private_buf_). Pairing Release_Store is in

     // CompleteLazyInstance().

     subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);

     if (!(value & kLazyInstanceCreatedMask) &&

         internal::NeedsLazyInstance(&private_instance_)) {

       // Create the instance in the space provided by |private_buf_|.

       value = reinterpret_cast<subtle::AtomicWord>(

           Traits::New(private_buf_.void_data()));

       internal::CompleteLazyInstance(&private_instance_, value, this,

                                      Traits::kRegisterOnExit ? OnExit : NULL);

     }

     // This annotation helps race detectors recognize correct lock-less

     // synchronization between different threads calling Pointer().

     // We suggest dynamic race detection tool that "Traits::New" above

     // and CompleteLazyInstance(...) happens before "return instance()" below.

     // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).

     ANNOTATE_HAPPENS_AFTER(&private_instance_);

     return instance();

   }

   bool operator==(Type* p) {

     switch (subtle::NoBarrier_Load(&private_instance_)) {

       case 0:

         return p == NULL;

       case internal::kLazyInstanceStateCreating:

         return static_cast<void*>(p) == private_buf_.void_data();

       default:

         return p == instance();

     }

   }

   // Effectively private: member data is only public to allow the linker to

   // statically initialize it and to maintain a POD class. DO NOT USE FROM

   // OUTSIDE THIS CLASS.

   subtle::AtomicWord private_instance_;

   // Preallocated space for the Type instance.

   base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;

  private:

   Type* instance() {

     return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));

   }

   // Adapter function for use with AtExit.  This should be called single

   // threaded, so don't synchronize across threads.

   // Calling OnExit while the instance is in use by other threads is a mistake.

   static void OnExit(void* lazy_instance) {

     LazyInstance<Type, Traits>* me =

         reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);

     Traits::Delete(me->instance());

     subtle::NoBarrier_Store(&me->private_instance_, 0);

   }

 };

 }  // namespace base

 #endif  // BASE_LAZY_INSTANCE_H_