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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
// }
#include <new> // For placement new.
#include "base/atomicops.h"
#include "base/base_export.h"
#include "base/basictypes.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.
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.
// 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) {
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 {
// 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)
// If any bit in the created mask is true, the instance has already been
// fully constructed.
static const subtle::AtomicWord kLazyInstanceCreatedMask =
// 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>(
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(...).
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();
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
subtle::AtomicWord private_instance_;
// Preallocated space for the Type instance.
base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
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);
subtle::NoBarrier_Store(&me->private_instance_, 0);
} // namespace base