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refCount.h
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1 //
2 // Copyright 2016 Pixar
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5 // with the following modification; you may not use this file except in
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24 #ifndef PXR_BASE_TF_REF_COUNT_H
25 #define PXR_BASE_TF_REF_COUNT_H
26 
27 /// \file tf/refCount.h
28 /// \ingroup group_tf_Memory
29 
30 #include "pxr/pxr.h"
31 
32 #include "pxr/base/arch/inttypes.h"
33 #include "pxr/base/tf/api.h"
34 #include <atomic>
35 
37 
38 template <typename T> class TfRefPtr;
39 
40 /// \class TfRefCount
41 /// \ingroup group_tf_Memory
42 ///
43 /// Reference counter class
44 ///
45 /// This class is intended to be embedded in other classes, for use as
46 /// a reference counter. Unless you need to provide extraordinary
47 /// customization, you should forgo direct use of this class and instead
48 /// make use of the base class \c TfRefBase.
49 ///
50 /// Initialization of a reference counter is somewhat counterintuitive.
51 /// Consider an object T with a reference counter R. When T is
52 /// initialized, R should be initialized to one, even if T is
53 /// copy-constructed. This implies that \e all constructors of \c
54 /// TfRefCount set the counter to one, even the copy constructor.
55 ///
56 /// Conversely, if T is assigned to, the reference counter R in T should not
57 /// change. This implies that the assignment operator for \c TfRefCount
58 /// does not change the counter's value.
59 ///
60 /// Finally, for thread-safety, the counter should be atomic.
61 ///
62 /// This class was written primarily for use in classes whose access is
63 /// encapsulated by means of the \c TfRefPtr interface; such classes require
64 /// reference counting semantics as described above. Note that
65 /// the behavior of a \c TfRefCount in a class T is invariant with respect
66 /// to T's copy constructors and assignment operators.
67 ///
68 /// Again, please do not directly embed a \c TfRefCount in a structure
69 /// unless the functionality of \c TfRefBase is insufficient for your needs.
70 ///
71 class TfRefCount {
72 public:
73  /// Initialize counter to one.
74  TfRefCount() : _counter(1) {
75  }
76 
77  /// Initialize counter to one.
78  ///
79  /// Even if you copy from a reference counter, you want the
80  /// newly constructed counter to start at one.
81  TfRefCount(const TfRefCount&) : _counter(1) {
82  }
83 
84  /// Returns counter's value.
85  int Get() const {
86  return _counter;
87  }
88 
89  /// Assignment to a reference counter has no effect.
90  const TfRefCount& operator=(const TfRefCount&) const {
91  return *this;
92  }
93 private:
94  /// Decrements counter by \c 1, returning true if the result is 0.
95  bool _DecrementAndTestIfZero() const {
96  return (--_counter == 0);
97  }
98 
99  /// Adds \c amount to the count, returning the prior value.
100  int _FetchAndAdd(int amount) const {
101  return _counter.fetch_add(amount);
102  }
103 
104 private:
105  mutable std::atomic<int> _counter;
106  template <typename T> friend class TfRefPtr;
108  friend struct Tf_RefPtr_Counter;
109 };
110 
112 
113 #endif // PXR_BASE_TF_REF_COUNT_H
TfRefCount()
Initialize counter to one.
Definition: refCount.h:74
const TfRefCount & operator=(const TfRefCount &) const
Assignment to a reference counter has no effect.
Definition: refCount.h:90
TfRefCount(const TfRefCount &)
Definition: refCount.h:81
PXR_NAMESPACE_CLOSE_SCOPE PXR_NAMESPACE_OPEN_SCOPE
Definition: path.h:1346
#define PXR_NAMESPACE_CLOSE_SCOPE
Definition: pxr.h:91
int Get() const
Returns counter's value.
Definition: refCount.h:85