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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// The LazyInstance<Type, Traits> class manages a single instance of Type,
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// which will be lazily created on the first time it's accessed. This class is
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// useful for places you would normally use a function-level static, but you
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// need to have guaranteed thread-safety. The Type constructor will only ever
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// be called once, even if two threads are racing to create the object. Get()
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// and Pointer() will always return the same, completely initialized instance.
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// When the instance is constructed it is registered with AtExitManager. The
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// destructor will be called on program exit.
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// LazyInstance is completely thread safe, assuming that you create it safely.
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// The class was designed to be POD initialized, so it shouldn't require a
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// static constructor. It really only makes sense to declare a LazyInstance as
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// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.
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// LazyInstance is similar to Singleton, except it does not have the singleton
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// property. You can have multiple LazyInstance's of the same type, and each
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// will manage a unique instance. It also preallocates the space for Type, as
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// to avoid allocating the Type instance on the heap. This may help with the
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// performance of creating the instance, and reducing heap fragmentation. This
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// requires that Type be a complete type so we can determine the size.
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// static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;
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// void SomeMethod() {
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// my_instance.Get().SomeMethod(); // MyClass::SomeMethod()
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// MyClass* ptr = my_instance.Pointer();
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// ptr->DoDoDo(); // MyClass::DoDoDo
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#ifndef BASE_LAZY_INSTANCE_H_
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#define BASE_LAZY_INSTANCE_H_
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#include <new> // For placement new.
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#include "base/atomicops.h"
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#include "base/base_export.h"
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#include "base/basictypes.h"
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#include "base/debug/leak_annotations.h"
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#include "base/logging.h"
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#include "base/memory/aligned_memory.h"
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#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
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#include "base/threading/thread_restrictions.h"
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// LazyInstance uses its own struct initializer-list style static
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// initialization, as base's LINKER_INITIALIZED requires a constructor and on
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// some compilers (notably gcc 4.4) this still ends up needing runtime
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#define LAZY_INSTANCE_INITIALIZER {0}
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template <typename Type>
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struct DefaultLazyInstanceTraits {
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static const bool kRegisterOnExit = true;
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static const bool kAllowedToAccessOnNonjoinableThread = false;
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static Type* New(void* instance) {
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DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)
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<< ": Bad boy, the buffer passed to placement new is not aligned!\n"
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"This may break some stuff like SSE-based optimizations assuming the "
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"<Type> objects are word aligned.";
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// Use placement new to initialize our instance in our preallocated space.
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// The parenthesis is very important here to force POD type initialization.
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return new (instance) Type();
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static void Delete(Type* instance) {
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// Explicitly call the destructor.
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// We pull out some of the functionality into non-templated functions, so we
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// can implement the more complicated pieces out of line in the .cc file.
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// Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:
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// base::LazyInstance<T>::Leaky my_leaky_lazy_instance;
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// base::LazyInstance<T, base::internal::LeakyLazyInstanceTraits<T> >
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// my_leaky_lazy_instance;
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// (especially when T is MyLongTypeNameImplClientHolderFactory).
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// Only use this internal::-qualified verbose form to extend this traits class
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// (depending on its implementation details).
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template <typename Type>
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struct LeakyLazyInstanceTraits {
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static const bool kRegisterOnExit = false;
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static const bool kAllowedToAccessOnNonjoinableThread = true;
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static Type* New(void* instance) {
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ANNOTATE_SCOPED_MEMORY_LEAK;
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return DefaultLazyInstanceTraits<Type>::New(instance);
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static void Delete(Type* instance) {
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// Our AtomicWord doubles as a spinlock, where a value of
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// kBeingCreatedMarker means the spinlock is being held for creation.
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static const subtle::AtomicWord kLazyInstanceStateCreating = 1;
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// Check if instance needs to be created. If so return true otherwise
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// if another thread has beat us, wait for instance to be created and
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BASE_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);
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// After creating an instance, call this to register the dtor to be called
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// at program exit and to update the atomic state to hold the |new_instance|
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BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,
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subtle::AtomicWord new_instance,
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void (*dtor)(void*));
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} // namespace internal
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template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >
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// Do not define a destructor, as doing so makes LazyInstance a
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// non-POD-struct. We don't want that because then a static initializer will
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// be created to register the (empty) destructor with atexit() under MSVC, for
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// example. We handle destruction of the contained Type class explicitly via
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// the OnExit member function, where needed.
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// ~LazyInstance() {}
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// Convenience typedef to avoid having to repeat Type for leaky lazy
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typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;
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// Avoid making TLS lookup on release builds.
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if (!Traits::kAllowedToAccessOnNonjoinableThread)
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ThreadRestrictions::AssertSingletonAllowed();
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// If any bit in the created mask is true, the instance has already been
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// fully constructed.
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static const subtle::AtomicWord kLazyInstanceCreatedMask =
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~internal::kLazyInstanceStateCreating;
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// We will hopefully have fast access when the instance is already created.
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// Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating
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// at most once, the load is taken out of NeedsInstance() as a fast-path.
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// The load has acquire memory ordering as a thread which sees
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// private_instance_ > creating needs to acquire visibility over
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// the associated data (private_buf_). Pairing Release_Store is in
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// CompleteLazyInstance().
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subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);
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if (!(value & kLazyInstanceCreatedMask) &&
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internal::NeedsLazyInstance(&private_instance_)) {
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// Create the instance in the space provided by |private_buf_|.
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value = reinterpret_cast<subtle::AtomicWord>(
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Traits::New(private_buf_.void_data()));
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internal::CompleteLazyInstance(&private_instance_, value, this,
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Traits::kRegisterOnExit ? OnExit : NULL);
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// This annotation helps race detectors recognize correct lock-less
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// synchronization between different threads calling Pointer().
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// We suggest dynamic race detection tool that "Traits::New" above
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// and CompleteLazyInstance(...) happens before "return instance()" below.
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// See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).
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ANNOTATE_HAPPENS_AFTER(&private_instance_);
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bool operator==(Type* p) {
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switch (subtle::NoBarrier_Load(&private_instance_)) {
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case internal::kLazyInstanceStateCreating:
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return static_cast<void*>(p) == private_buf_.void_data();
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return p == instance();
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// Effectively private: member data is only public to allow the linker to
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// statically initialize it and to maintain a POD class. DO NOT USE FROM
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// OUTSIDE THIS CLASS.
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subtle::AtomicWord private_instance_;
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// Preallocated space for the Type instance.
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base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;
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return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));
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// Adapter function for use with AtExit. This should be called single
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// threaded, so don't synchronize across threads.
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// Calling OnExit while the instance is in use by other threads is a mistake.
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static void OnExit(void* lazy_instance) {
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LazyInstance<Type, Traits>* me =
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reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);
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Traits::Delete(me->instance());
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subtle::NoBarrier_Store(&me->private_instance_, 0);
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#endif // BASE_LAZY_INSTANCE_H_