//前置声明 因为在使用sharedptr时可能会产生循环引用的缺陷，所以使用weakedptr进行辅助 template <class T> class WeakedPtr; template<class T> class MySharedPtr { public: //定义为友元类      friend class WeakedPtr<T>;      MySharedPtr(T* ptr =  NULL) :_ptr(ptr), _refCount(new int(1))      {}      ~MySharedPtr()      {           if (--(*_refCount) == 0)           {               delete _ptr;               delete _refCount;           }      }      //s1(s2)      MySharedPtr(const MySharedPtr<T>& sp) :_ptr(sp._ptr), _refCount(sp._refCount)      {           (*_refCount)++;      }      //sp1 = sp2      MySharedPtr<T>& operator=(MySharedPtr<T>& sp)      {               if (_ptr != sp._ptr)               {                    if (--(*_refCount) == 0)                    {                         delete _ptr;                         delete _refCount;                    }                    _ptr = sp._ptr;                    _refCount = sp._refCount;                    (*_refCount)++;               }           return *this;      }      //为了像指针一样才进行*\->的重载      //->的重载      T* operator->()      {           return _ptr;      }      //*的重载      T& operator*()      {           return *_ptr;      }      //查看引用计数的多少      int UseCount()      {           return *_refCount;      } private:      T* _ptr;      int* _refCount;//一块空间有一个指针 }; template <class T> class WeakedPtr { public:      WeakedPtr() :_ptr(NULL)      {}      WeakedPtr(const MySharedPtr<T>& sp)           :_ptr(sp._ptr)      {}      WeakedPtr<T>& operator=(const MySharedPtr<T>&sp)      {           _ptr = sp._ptr;           return *this;      }      T& operator* ()      {           return *_ptr;      }      T* operator->()      {           return _ptr;      } private:      T* _ptr; }; struct  ListNode {      int _data;      WeakedPtr<ListNode> _next;      WeakedPtr<ListNode> _prev;      ~ListNode()      {           cout << "~ListNode" << endl;      } }; void testMySharedPtr() {      //SharePtr<int> sp1(new int(1));      //SharePtr<int> sp2(sp1);      MySharedPtr<ListNode> cur(new ListNode);      MySharedPtr<ListNode> next(new ListNode);      cout << cur.UseCount() << endl;      cout << next.UseCount() << endl;      cur->_next = next;      next->_prev = cur;      cout << cur.UseCount() << endl;      cout << next.UseCount() << endl; }