////   This file contains the C++ code from Program 16.10 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/pgm16_10.cpp//class CountingVisitor : public Visitor{    unsigned int count;public:    CountingVisitor () : count (0)	{}    void Visit (Object&)	{ ++count; }    unsigned int Count () const	{ return count; }};bool Graph::IsConnected () const{    CountingVisitor visitor;    DepthFirstTraversal (PreOrder (visitor), SelectVertex (0));    return visitor.Count () == numberOfVertices;}