////   This file contains the C++ code from Program 5.15 of//   "Data Structures and Algorithms//    with Object-Oriented Design Patterns in C++"//   by Bruno R. Preiss.////   Copyright (c) 1998 by Bruno R. Preiss, P.Eng.  All rights reserved.////   http://www.pads.uwaterloo.ca/Bruno.Preiss/books/opus4/programs/pgm05_15.cpp//NullIterator::NullIterator ()    {}void NullIterator::Reset ()    {}bool NullIterator::IsDone () const    { return true; }Object& NullIterator::operator * () const    { return NullObject::Instance (); }void NullIterator::operator ++ ()    {}