c++中的引用计数

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#ifndef   SHARED_PTR_H_     #define   SHARED_PTR_H_         #ifndef   NULL     #define   NULL   0     #endif         namespace   util     {             template   <class   T>             class   shared_ptr;                 template   <class   T>             class   shared_data             {                 private:                     friend   class   shared_ptr<T>;                         explicit   shared_data(T   *pT):   _M_ptr(pT)                     {                             _M_nCount   =   1;                     }                         ~shared_data()                     {                             delete   _M_ptr;                     }                         void   operator++()                     {                             ++_M_nCount;                     }                         void   operator--()                     {                             --_M_nCount;                             if   (_M_nCount   ==   0)                             {                                     delete   this;                             }                     }                         T   &operator   *()const                     {                             return     *_M_ptr;                     }                         T   *operator->()const                     {                             return   _M_ptr;                     }                         bool   operator   ==   (T   *pT)const                     {                             return   _M_ptr   ==   pT;                     }                         T   *get()                     {                             return   _M_ptr;                     }                         int   use_count()                     {                             return   _M_nCount;                     }                     private:                     T   *_M_ptr;                     unsigned   int   _M_nCount;             };                 template   <class   T>             class   shared_ptr             {                     typedef   shared_data<T>   element;                 public:                     explicit   shared_ptr(T   *pT):   _M_pD(NULL)                     {                             _M_pD   =   new   element(pT);                     }                     explicit   shared_ptr():   _M_pD(NULL)     {                     }                     //   copy   constructor                     shared_ptr(const   shared_ptr   <   T   >     &rT)                     {                             _M_pD   =   rT.get_element();                             if   (_M_pD   !=   NULL)                             {                                     ++(*_M_pD);                             }                     }                         ~shared_ptr()                     {                             if   (_M_pD   !=   NULL)                             {                                     --(*_M_pD);                             }                     }                         //   assignment   operator                     shared_ptr   <   T   >     &operator   =   (const   shared_ptr   <   T   >     &rT)                     {                             if   (_M_pD   !=   NULL)                             {                                     --(*_M_pD);                             }                             _M_pD   =   rT.get_element();                                 if   (_M_pD   !=   NULL)                             {                                     ++(*_M_pD);                             }                             return     *this;                     }                             //   assignment   operator                     shared_ptr   <   T   >     &operator   =   (shared_ptr   <   T   >     &rT)                     {                             if   (_M_pD   !=   NULL)                             {                                     --(*_M_pD);                             }                             _M_pD   =   rT.get_element();                                 if   (_M_pD   !=   NULL)                             {                                     ++(*_M_pD);                             }                             return     *this;                     }                         T   &operator   *()const                     {                             return   _M_pD->operator   *();                     }                         T   *operator->()const                     {                             return   _M_pD->operator->();                     }         bool   operator   ==   (shared_ptr   <   T   >     &rT)const                     {                             return   rT.get_element()   ==   _M_pD;                     }                         bool   operator   ==   (T   *pT)const                     {                             if   (_M_pD   ==   NULL)                             {                                     return   pT   ==   NULL;                             }                             return     *_M_pD   ==   pT;                     }                         T   *get()                     {                             if   (_M_pD   ==   NULL)                             {                                     return   NULL;                             }                             else                             {                                     return   _M_pD->get();                             }                     }                         void   release()                     {                             if   (_M_pD   !=   NULL)                             {                                     --(*_M_pD);                                     _M_pD   =   NULL;                             }                     }                         void   reset(T   *pT)                     {                             if   (_M_pD   !=   NULL)                             {                                     --(*_M_pD);                                     _M_pD   =   NULL;                             }                             _M_pD   =   new   element(pT);                     }                     private:                     element   *get_element()const                     {                             return   _M_pD;                     }                         element   *_M_pD;             };     }         #endif
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