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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/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_