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       1             : /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
       2             : /* vim: set ts=8 sts=2 et sw=2 tw=80: */
       3             : // Copyright (c) 2012 The Chromium Authors. All rights reserved.
       4             : // Use of this source code is governed by a BSD-style license that can be
       5             : // found in the LICENSE file.
       6             : 
       7             : #ifndef BASE_ATOMIC_SEQUENCE_NUM_H_
       8             : #define BASE_ATOMIC_SEQUENCE_NUM_H_
       9             : 
      10             : #include "base/atomicops.h"
      11             : #include "base/basictypes.h"
      12             : 
      13             : namespace base {
      14             : 
      15             : class AtomicSequenceNumber;
      16             : 
      17             : // Static (POD) AtomicSequenceNumber that MUST be used in global scope (or
      18             : // non-function scope) ONLY. This implementation does not generate any static
      19             : // initializer.  Note that it does not implement any constructor which means
      20             : // that its fields are not initialized except when it is stored in the global
      21             : // data section (.data in ELF). If you want to allocate an atomic sequence
      22             : // number on the stack (or heap), please use the AtomicSequenceNumber class
      23             : // declared below.
      24             : class StaticAtomicSequenceNumber {
      25             :  public:
      26          15 :   inline int GetNext() {
      27             :     return static_cast<int>(
      28          15 :         base::subtle::NoBarrier_AtomicIncrement(&seq_, 1) - 1);
      29             :   }
      30             : 
      31             :  private:
      32             :   friend class AtomicSequenceNumber;
      33             : 
      34             :   inline void Reset() {
      35             :     base::subtle::Release_Store(&seq_, 0);
      36             :   }
      37             : 
      38             :   base::subtle::Atomic32 seq_;
      39             : };
      40             : 
      41             : // AtomicSequenceNumber that can be stored and used safely (i.e. its fields are
      42             : // always initialized as opposed to StaticAtomicSequenceNumber declared above).
      43             : // Please use StaticAtomicSequenceNumber if you want to declare an atomic
      44             : // sequence number in the global scope.
      45             : class AtomicSequenceNumber {
      46             :  public:
      47             :   AtomicSequenceNumber() {
      48             :     seq_.Reset();
      49             :   }
      50             : 
      51             :   inline int GetNext() {
      52             :     return seq_.GetNext();
      53             :   }
      54             : 
      55             :  private:
      56             :   StaticAtomicSequenceNumber seq_;
      57             :   DISALLOW_COPY_AND_ASSIGN(AtomicSequenceNumber);
      58             : };
      59             : 
      60             : }  // namespace base
      61             : 
      62             : #endif  // BASE_ATOMIC_SEQUENCE_NUM_H_