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编译的时候说我“Stack::Link *p;“这句错了，请问哪有问题呀？  

template  
class Stack{
  struct Link{
    T* data;
    Link* next;
    Link(T* dat, Link* nxt)
      : data(dat), next(nxt) {}
  }* head;
public:
  Stack() : head(0) {}
  ~Stack();
  void push(T* dat) {
    head = new Link(dat, head);
  }
  T* peek() const {  
    return head ? head->data : 0;
  }
  T* pop();
  // Nested iterator class:
  class iterator; // Declaration required
  friend class iterator; // Make it a friend
  class iterator { // Now define it
 Stack::Link *p;
      public:
    iterator(const Stack& tl) : p(tl.head) {}
    // Copy-constructor:
    iterator(const iterator& tl) : p(tl.p) {}
    // The end sentinel iterator:
    iterator() : p(0) {}
    // operator++ returns boolean indicating end:
    bool operator++() {
      if(p->next)
        p = p->next;
      else p = 0; // Indicates end of list
      return bool(p);
    }
    bool operator++(int) { return operator++(); }
    T* current() const {
      if(!p) return 0;
      return p->data;
    }
    // Pointer dereference operator:
    T* operator->() const {  
      require(p != 0,  
        "PStack::iterator::operator->returns 0");
      return current();  
    }
    T* operator*() const { return current(); }
    // bool conversion for conditional test:
    operator bool() const { return bool(p); }
    // Comparison to test for end:
    bool operator==(const iterator&) const {
      return p == 0;
    }
    bool operator!=(const iterator&) const {
      return p != 0;
    }
  };
  iterator begin() const {  
    return iterator(*this);  
  }
  iterator end() const { return iterator(); }
};

template Stack::~Stack() {
  while(head)
    delete pop();
}

template T* Stack::pop() {
  if(head == 0) return 0;
  T* result = head->data;
  Link* oldHead = head;
  head = head->next;
  delete oldHead;
  return result;
}

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