base/lazy_instance.h - chromium/src - Git at Google

 // 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/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;
 #ifndef NDEBUG
   static const bool kAllowedToAccessOnNonjoinableThread = false;
 #endif

   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;
 #ifndef NDEBUG
   static const bool kAllowedToAccessOnNonjoinableThread = true;
 #endif

   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);
     }
     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_
	// 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/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;
	#ifndef NDEBUG
	static const bool kAllowedToAccessOnNonjoinableThread = false;
	#endif

	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;
	#ifndef NDEBUG
	static const bool kAllowedToAccessOnNonjoinableThread = true;
	#endif

	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);
	}
	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_