Code Fragment: NodeList1

template <typename Object>
class NodeList {
protected: 
  // ... (insert Node class definition here)
public:
  // ... (insert Position class definition here)
protected:                          // utility to convert Position to node pointer
  NodePtr nodePtr(const Position& p) const throw(InvalidPositionException) {
    if (p.node == NULL)
      throw InvalidPositionException("Attempt to use null position");
    else return p.node;
  }
protected: 			// data members
  int       sz;        		// number of items
  NodePtr   header;		// head of list sentinel
  NodePtr   trailer;		// tail of list sentinel
public:
  NodeList() {			// default constructor
    sz = 0;
    header   = new Node;	// create sentinels
    trailer  = new Node;
    header->next   = trailer;	// head points to trailer
    trailer->prev  = header;	// trailer points to head
  }
  int size() const 		// list size
    { return sz; }
  bool isEmpty() const 		// is the list empty?
    { return (sz == 0); }
  bool isFirst(const Position& p) const	// is this the first?
      throw(InvalidPositionException) {
    NodePtr v = nodePtr(p);
    return v->prev == header;
  }
  bool isLast(const Position& p) const 	// is this the last?
      throw(InvalidPositionException) {
    NodePtr v = nodePtr(p);
    return v->next == trailer;
  }
  // ...