Code Fragment: Position

  class Position {					// position in NodeList
  private:
    NodePtr node;					// pointer to the node
  public:
    Position(NodePtr n = NULL)				// constructor
      { node = n; }
    Object& element() const 				// return element
         throw(InvalidPositionException) {
      if (node == NULL) throw InvalidPositionException("Null position");
      return node->element;
    }
    bool isNull() const 				// a null position?
      { return node == NULL; }
    friend class NodeList<Object>;			// allow access
  };