struct listNode;  // defined later
typedef listNode* ptrType;

class listClass : public baseListClass
{
public:
// constructors and destructor:
   listClass();
   listClass(const listClass& L);
   virtual ~listClass();

// list operations:
   virtual bool ListIsEmpty() const; 

   virtual int ListLength() const;

   virtual void ListInsert(int NewPosition, 
                           listItemType NewItem, 
                           bool& Success);

   virtual void ListDelete(int Position, 
                           bool& Success);

   virtual void ListRetrieve(int Position, 
                             listItemType& DataItem, 
                             bool& Success) const;

   virtual void DisplayList() const;

protected:
   void SetSize(int NewSize);       // sets Size
   ptrType ListHead() const;        // returns Head
   void SetHead(ptrType NewHead);   // sets Head

   // the next two functions return the list item and 
   // Next pointer of a node in the linked list
   listItemType ListItem(ptrType P) const;
   ptrType ListNext(ptrType P) const;

private:
   int     Size;  // number of items in the list
   ptrType Head;  // pointer to the linked list

   ptrType PtrTo(int Position) const;
};  // end class