1 template 
      
         2 class List  3 {  4     private:  5         struct Node//所有都是公有的  6         {  7             Object data;  8             Node *prev;  9             Node *next; 10  11             Node(const Object & d = Object(),Node *p = NUll，Node *n = Null): 12             data(d) , prev(p) , next(n) 13             { 14             } 15         }; 16     public: 17         class const_iterator 18         { 19                 public: 20                     const_iterator() : current(NULL){} 21                     const Object & operator* () const 22                     { 23                         return retrieve(); 24                     } 25                     const_iterator & operator++ ( ) const 26                     { 27                         current = current->next; 28                         return *this; 29                     } 30                     const_iterator & operator++( int ) 31                     { 32                         const_iterator old = *this; 33                         ++(*this); 34                         return old; 35                     } 36                     bool operator == (const const_iterator * rhs) const 37                     { 38                         return current == rhs.current; 39                     } 40                     bool operator != (const const_iterator & rhs) const 41                     { 42                         return !(*this == rhs); 43                     } 44  45             protected: 46                 Node *current; 47  48                 Object & retrieve() cosnt 49                 { 50                     return current->data; 51                 } 52                 const_iterator(Node *p) : current(p) 53                 { 54                 } 55                 friend class List