////   This file contains the C++ code from Program 5.20 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_20.cpp//Object& Association::Key () const    { return *key; }Object& Association::Value () const{    if (value == 0)	return NullObject::Instance ();    else	return *value;}int Association::CompareTo (Object const& object) const{    Association const& association =	dynamic_cast<Association const&> (object);    return Key ().Compare (association.Key ());}void Association::Put (ostream& s) const{    s << "Association {" << *key;    if (value != 0)	s << ", " << *value;    s << "}";}