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TMessagesProj/jni/voip/webrtc/base/lazy_instance.h

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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_