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+// Copyright (c) 2011 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.
+//
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+// PLEASE READ: Do you really need a singleton? If possible, use a
+// function-local static of type base::NoDestructor<T> instead:
+//
+// Factory& Factory::GetInstance() {
+//   static base::NoDestructor<Factory> instance;
+//   return *instance;
+// }
+// !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+//
+// Singletons make it hard to determine the lifetime of an object, which can
+// lead to buggy code and spurious crashes.
+//
+// Instead of adding another singleton into the mix, try to identify either:
+//   a) An existing singleton that can manage your object's lifetime
+//   b) Locations where you can deterministically create the object and pass
+//      into other objects
+//
+// If you absolutely need a singleton, please keep them as trivial as possible
+// and ideally a leaf dependency. Singletons get problematic when they attempt
+// to do too much in their destructor or have circular dependencies.
+
+#ifndef BASE_MEMORY_SINGLETON_H_
+#define BASE_MEMORY_SINGLETON_H_
+
+#include "base/at_exit.h"
+#include "base/atomicops.h"
+#include "base/base_export.h"
+#include "base/lazy_instance_helpers.h"
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/threading/thread_restrictions.h"
+
+namespace base {
+
+// Default traits for Singleton<Type>. Calls operator new and operator delete on
+// the object. Registers automatic deletion at process exit.
+// Overload if you need arguments or another memory allocation function.
+template<typename Type>
+struct DefaultSingletonTraits {
+  // Allocates the object.
+  static Type* New() {
+    // The parenthesis is very important here; it forces POD type
+    // initialization.
+    return new Type();
+  }
+
+  // Destroys the object.
+  static void Delete(Type* x) {
+    delete x;
+  }
+
+  // Set to true to automatically register deletion of the object on process
+  // exit. See below for the required call that makes this happen.
+  static const bool kRegisterAtExit = true;
+
+#if DCHECK_IS_ON()
+  // Set to false to disallow access on a non-joinable thread.  This is
+  // different from kRegisterAtExit because StaticMemorySingletonTraits allows
+  // access on non-joinable threads, and gracefully handles this.
+  static const bool kAllowedToAccessOnNonjoinableThread = false;
+#endif
+};
+
+
+// Alternate traits for use with the Singleton<Type>.  Identical to
+// DefaultSingletonTraits except that the Singleton will not be cleaned up
+// at exit.
+template<typename Type>
+struct LeakySingletonTraits : public DefaultSingletonTraits<Type> {
+  static const bool kRegisterAtExit = false;
+#if DCHECK_IS_ON()
+  static const bool kAllowedToAccessOnNonjoinableThread = true;
+#endif
+};
+
+// Alternate traits for use with the Singleton<Type>.  Allocates memory
+// for the singleton instance from a static buffer.  The singleton will
+// be cleaned up at exit, but can't be revived after destruction unless
+// the ResurrectForTesting() method is called.
+//
+// This is useful for a certain category of things, notably logging and
+// tracing, where the singleton instance is of a type carefully constructed to
+// be safe to access post-destruction.
+// In logging and tracing you'll typically get stray calls at odd times, like
+// during static destruction, thread teardown and the like, and there's a
+// termination race on the heap-based singleton - e.g. if one thread calls
+// get(), but then another thread initiates AtExit processing, the first thread
+// may call into an object residing in unallocated memory. If the instance is
+// allocated from the data segment, then this is survivable.
+//
+// The destructor is to deallocate system resources, in this case to unregister
+// a callback the system will invoke when logging levels change. Note that
+// this is also used in e.g. Chrome Frame, where you have to allow for the
+// possibility of loading briefly into someone else's process space, and
+// so leaking is not an option, as that would sabotage the state of your host
+// process once you've unloaded.
+template <typename Type>
+struct StaticMemorySingletonTraits {
+  // WARNING: User has to support a New() which returns null.
+  static Type* New() {
+    // Only constructs once and returns pointer; otherwise returns null.
+    if (subtle::NoBarrier_AtomicExchange(&dead_, 1))
+      return nullptr;
+
+    return new (buffer_) Type();
+  }
+
+  static void Delete(Type* p) {
+    if (p)
+      p->Type::~Type();
+  }
+
+  static const bool kRegisterAtExit = true;
+
+#if DCHECK_IS_ON()
+  static const bool kAllowedToAccessOnNonjoinableThread = true;
+#endif
+
+  static void ResurrectForTesting() { subtle::NoBarrier_Store(&dead_, 0); }
+
+ private:
+  alignas(Type) static char buffer_[sizeof(Type)];
+  // Signal the object was already deleted, so it is not revived.
+  static subtle::Atomic32 dead_;
+};
+
+template <typename Type>
+alignas(Type) char StaticMemorySingletonTraits<Type>::buffer_[sizeof(Type)];
+template <typename Type>
+subtle::Atomic32 StaticMemorySingletonTraits<Type>::dead_ = 0;
+
+// The Singleton<Type, Traits, DifferentiatingType> class manages a single
+// instance of Type which will be created on first use and will be destroyed at
+// normal process exit). The Trait::Delete function will not be called on
+// abnormal process exit.
+//
+// DifferentiatingType is used as a key to differentiate two different
+// singletons having the same memory allocation functions but serving a
+// different purpose. This is mainly used for Locks serving different purposes.
+//
+// Example usage:
+//
+// In your header:
+//   namespace base {
+//   template <typename T>
+//   struct DefaultSingletonTraits;
+//   }
+//   class FooClass {
+//    public:
+//     static FooClass* GetInstance();  <-- See comment below on this.
+//     void Bar() { ... }
+//    private:
+//     FooClass() { ... }
+//     friend struct base::DefaultSingletonTraits<FooClass>;
+//
+//     DISALLOW_COPY_AND_ASSIGN(FooClass);
+//   };
+//
+// In your source file:
+//  #include "base/memory/singleton.h"
+//  FooClass* FooClass::GetInstance() {
+//    return base::Singleton<FooClass>::get();
+//  }
+//
+// Or for leaky singletons:
+//  #include "base/memory/singleton.h"
+//  FooClass* FooClass::GetInstance() {
+//    return base::Singleton<
+//        FooClass, base::LeakySingletonTraits<FooClass>>::get();
+//  }
+//
+// And to call methods on FooClass:
+//   FooClass::GetInstance()->Bar();
+//
+// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance
+// and it is important that FooClass::GetInstance() is not inlined in the
+// header. This makes sure that when source files from multiple targets include
+// this header they don't end up with different copies of the inlined code
+// creating multiple copies of the singleton.
+//
+// Singleton<> has no non-static members and doesn't need to actually be
+// instantiated.
+//
+// This class is itself thread-safe. The underlying Type must of course be
+// thread-safe if you want to use it concurrently. Two parameters may be tuned
+// depending on the user's requirements.
+//
+// Glossary:
+//   RAE = kRegisterAtExit
+//
+// On every platform, if Traits::RAE is true, the singleton will be destroyed at
+// process exit. More precisely it uses AtExitManager which requires an
+// object of this type to be instantiated. AtExitManager mimics the semantics
+// of atexit() such as LIFO order but under Windows is safer to call. For more
+// information see at_exit.h.
+//
+// If Traits::RAE is false, the singleton will not be freed at process exit,
+// thus the singleton will be leaked if it is ever accessed. Traits::RAE
+// shouldn't be false unless absolutely necessary. Remember that the heap where
+// the object is allocated may be destroyed by the CRT anyway.
+//
+// Caveats:
+// (a) Every call to get(), operator->() and operator*() incurs some overhead
+//     (16ns on my P4/2.8GHz) to check whether the object has already been
+//     initialized.  You may wish to cache the result of get(); it will not
+//     change.
+//
+// (b) Your factory function must never throw an exception. This class is not
+//     exception-safe.
+//
+
+template <typename Type,
+          typename Traits = DefaultSingletonTraits<Type>,
+          typename DifferentiatingType = Type>
+class Singleton {
+ private:
+  // A class T using the Singleton<T> pattern should declare a GetInstance()
+  // method and call Singleton::get() from within that. T may also declare a
+  // GetInstanceIfExists() method to invoke Singleton::GetIfExists().
+  friend Type;
+
+  // This class is safe to be constructed and copy-constructed since it has no
+  // member.
+
+  // Returns a pointer to the one true instance of the class.
+  static Type* get() {
+#if DCHECK_IS_ON()
+    if (!Traits::kAllowedToAccessOnNonjoinableThread)
+      ThreadRestrictions::AssertSingletonAllowed();
+#endif
+
+    return subtle::GetOrCreateLazyPointer(
+        &instance_, &CreatorFunc, nullptr,
+        Traits::kRegisterAtExit ? OnExit : nullptr, nullptr);
+  }
+
+  // Returns the same result as get() if the instance exists but doesn't
+  // construct it (and returns null) if it doesn't.
+  static Type* GetIfExists() {
+#if DCHECK_IS_ON()
+    if (!Traits::kAllowedToAccessOnNonjoinableThread)
+      ThreadRestrictions::AssertSingletonAllowed();
+#endif
+
+    if (!subtle::NoBarrier_Load(&instance_))
+      return nullptr;
+
+    // Need to invoke get() nonetheless as some Traits return null after
+    // destruction (even though |instance_| still holds garbage).
+    return get();
+  }
+
+  // Internal method used as an adaptor for GetOrCreateLazyPointer(). Do not use
+  // outside of that use case.
+  static Type* CreatorFunc(void* /* creator_arg*/) { return Traits::New(); }
+
+  // Adapter function for use with AtExit().  This should be called single
+  // threaded, so don't use atomic operations.
+  // Calling OnExit while singleton is in use by other threads is a mistake.
+  static void OnExit(void* /*unused*/) {
+    // AtExit should only ever be register after the singleton instance was
+    // created.  We should only ever get here with a valid instance_ pointer.
+    Traits::Delete(reinterpret_cast<Type*>(subtle::NoBarrier_Load(&instance_)));
+    instance_ = 0;
+  }
+  static subtle::AtomicWord instance_;
+};
+
+template <typename Type, typename Traits, typename DifferentiatingType>
+subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::instance_ = 0;
+
+}  // namespace base
+
+#endif  // BASE_MEMORY_SINGLETON_H_