lazy_instance.h - mozsearch

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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.

//

// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! DEPRECATED !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

// Please don't introduce new instances of LazyInstance<T>. Use a function-local

// static of type base::NoDestructor<T> instead:

//

// Factory& Factory::GetInstance() {

//   static base::NoDestructor<Factory> instance;

//   return *instance;

// }

// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

//

// 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>::Leaky inst = LAZY_INSTANCE_INITIALIZER;

//   void SomeMethod() {

//     inst.Get().SomeMethod();  // MyClass::SomeMethod()

//

//     MyClass* ptr = inst.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/debug/leak_annotations.h"

#include "base/lazy_instance_helpers.h"

#include "base/logging.h"

#include "base/threading/thread_restrictions.h"

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

// initialization, which does not require a constructor.

#define LAZY_INSTANCE_INITIALIZER {}

namespace base {

template <typename Type>

struct LazyInstanceTraitsBase {

  static Type* New(void* instance) {

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

    // 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 CallDestructor(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 {

// This traits class causes destruction the contained Type at process exit via

// AtExitManager. This is probably generally not what you want. Instead, prefer

// Leaky below.

template <typename Type>

struct DestructorAtExitLazyInstanceTraits {

  static const bool kRegisterOnExit = true;

#if DCHECK_IS_ON()

  static const bool kAllowedToAccessOnNonjoinableThread = false;

#endif

  static Type* New(void* instance) {

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

  static void Delete(Type* instance) {

    LazyInstanceTraitsBase<Type>::CallDestructor(instance);

};

// 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;

#if DCHECK_IS_ON()

  static const bool kAllowedToAccessOnNonjoinableThread = true;

#endif

  static Type* New(void* instance) {

    ANNOTATE_SCOPED_MEMORY_LEAK;

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

  static void Delete(Type* instance) {

};

template <typename Type>

struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};

}  // namespace internal

template <

    typename Type,

    typename Traits =

        internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<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;

  typedef LazyInstance<Type, internal::DestructorAtExitLazyInstanceTraits<Type>>

      DestructorAtExit;

  Type& Get() {

    return *Pointer();

  Type* Pointer() {

#if DCHECK_IS_ON()

    if (!Traits::kAllowedToAccessOnNonjoinableThread)

      ThreadRestrictions::AssertSingletonAllowed();

#endif

    return subtle::GetOrCreateLazyPointer(

        &private_instance_, &Traits::New, private_buf_,

        Traits::kRegisterOnExit ? OnExit : nullptr, this);

  // Returns true if the lazy instance has been created.  Unlike Get() and

  // Pointer(), calling IsCreated() will not instantiate the object of Type.

  bool IsCreated() {

    // Return true (i.e. "created") if |private_instance_| is either being

    // created right now (i.e. |private_instance_| has value of

    // internal::kLazyInstanceStateCreating) or was already created (i.e.

    // |private_instance_| has any other non-zero value).

    return 0 != subtle::NoBarrier_Load(&private_instance_);

  // MSVC gives a warning that the alignment expands the size of the

  // LazyInstance struct to make the size a multiple of the alignment. This

  // is expected in this case.

#if defined(OS_WIN)

#pragma warning(push)

#pragma warning(disable: 4324)

#endif

  // 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.

  alignas(Type) char private_buf_[sizeof(Type)];

#if defined(OS_WIN)

#pragma warning(pop)

#endif

 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_