C++ - Linked List code - LNode was not declared in

问题:

I am writing a class for a linked list in C++ and I am having an issue when writing the operator overload for <<. My class' header is:

class LList
{
   private:
      struct LNode
      {
         LNode ();
         int data;
         LNode * next;
      };
   public:
      LList ();
      LList (const LList & other);
      ~LList ();
      LList & operator = (const LList & other);
      bool operator == (const LList & other);
      int Size () const;
      friend ostream & operator << (ostream & outs, const LList & L);
      bool InsertFirst (const int & value);
      bool InsertLast (const int & value);
      bool DeleteFirst ();
      bool DeleteLast ();
   private:
      LNode * first;
      LNode * last;
      int size;
};

and the operator is:

ostream & operator << (ostream & outs, const LList & L){ 
    LNode *np;
    np=L.first;
    while(np!=NULL){
        outs<<np->data;
        np=np->next;
    }
    return outs;
}

When I compile the code I get an error:

LList.cpp: In function ‘std::ostream& operator<<(std::ostream&, const LList&)’:
LList.cpp:36:2: error: ‘LNode’ was not declared in this scope
  LNode *np;
  ^
LList.cpp:36:9: error: ‘np’ was not declared in this scope
  LNode *np;

I thought I could instantiate a struct inside a friend function but it looks like that doesn't work. Any of you know what is happening?

回答1:

Since your operator<< is a global function, you need to access your inner class with:

LList::LNode *np;

Since this function is a friend of LList, then it can access the private LNode class.

回答2:

Since the operator<< overload is a non-member function, you'll need to use LList::LNode.

ostream & operator << (ostream & outs, const LList & L){ 

    LList::LNode *np;
 // ^^^^^

    np=L.first;
    while(np!=NULL){
        outs<<np->data;
        np=np->next;
    }
    return outs;
}

回答3:

That's because LNode isn't in global scope, it's a nested class of LList. You have to write:

ostream & operator << (ostream & outs, const LList & L){ 
    LList::LNode *np = L.first;
    ↑↑↑↑↑↑↑
    /* rest as before */
}