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