////   This file contains the C++ code from Program 5.4 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_04.cpp//class NullObject : public Object{    static NullObject instance;    NullObject ();protected:    int CompareTo (Object const&) const;public:    bool IsNull () const;    HashValue Hash () const;    void Put (ostream& s) const;    static NullObject& Instance ();};