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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.

 #ifndef BASE_ATOMIC_SEQUENCE_NUM_H_

 #define BASE_ATOMIC_SEQUENCE_NUM_H_

 #include "base/atomicops.h"

 #include "base/basictypes.h"

 namespace base {

 class AtomicSequenceNumber;

 // Static (POD) AtomicSequenceNumber that MUST be used in global scope (or

 // non-function scope) ONLY. This implementation does not generate any static

 // initializer.  Note that it does not implement any constructor which means

 // that its fields are not initialized except when it is stored in the global

 // data section (.data in ELF). If you want to allocate an atomic sequence

 // number on the stack (or heap), please use the AtomicSequenceNumber class

 // declared below.

 class StaticAtomicSequenceNumber {

  public:

   inline int GetNext() {

     return static_cast<int>(

         base::subtle::NoBarrier_AtomicIncrement(&seq_, 1) - 1);

   }

  private:

   friend class AtomicSequenceNumber;

   inline void Reset() {

     base::subtle::Release_Store(&seq_, 0);

   }

   base::subtle::Atomic32 seq_;

 };

 // AtomicSequenceNumber that can be stored and used safely (i.e. its fields are

 // always initialized as opposed to StaticAtomicSequenceNumber declared above).

 // Please use StaticAtomicSequenceNumber if you want to declare an atomic

 // sequence number in the global scope.

 class AtomicSequenceNumber {

  public:

   AtomicSequenceNumber() {

     seq_.Reset();

   }

   inline int GetNext() {

     return seq_.GetNext();

   }

  private:

   StaticAtomicSequenceNumber seq_;

   DISALLOW_COPY_AND_ASSIGN(AtomicSequenceNumber);

 };

 }  // namespace base

 #endif  // BASE_ATOMIC_SEQUENCE_NUM_H_