Re: Improving a tutorial on the Visitor pattern

A website promoting a book on design patterns has the code below. Presumably the virtual destructor has been incorrectly omitted, but that might be forgiven because it's not really relevant to the main point.

However, I am concerned about the code duplication here -- the line v.visit(this) is just repeated verbatim regardless of what member of the Element hierarchy v represents. This doesn't strike me as good design.

Any ideas of how to resolve this? (Feel free to suggest ideas that break the Visitor pattern).

Thank You,

Paul

#include <iostream>
#include <string>
using namespace std;

// 1. Add an accept(Visitor) method to the "element" hierarchy
class Element
{
  public:
    virtual void accept(class Visitor &v) = 0;
};

class This: public Element
{
  public:
     /*virtual*/void accept(Visitor &v);
    string thiss()
    {
        return "This";
    }
};

class That: public Element
{
  public:
     /*virtual*/void accept(Visitor &v);
    string that()
    {
        return "That";
    }
};

class TheOther: public Element
{
  public:
     /*virtual*/void accept(Visitor &v);
    string theOther()
    {
        return "TheOther";
    }
};

// 2. Create a "visitor" base class w/ a visit() method for every "element" type
class Visitor
{
  public:
    virtual void visit(This *e) = 0;
    virtual void visit(That *e) = 0;
    virtual void visit(TheOther *e) = 0;
};

 /*virtual*/void This::accept(Visitor &v)
{
  v.visit(this);
}

 /*virtual*/void That::accept(Visitor &v)
{
  v.visit(this);
}

 /*virtual*/void TheOther::accept(Visitor &v)
{
  v.visit(this);
}

// 3. Create a "visitor" derived class for each "operation" to do on "elements"
class UpVisitor: public Visitor
{
     /*virtual*/void visit(This *e)
    {
        cout << "do Up on " + e->thiss() << '\n';
    }
     /*virtual*/void visit(That *e)
    {
        cout << "do Up on " + e->that() << '\n';
    }
     /*virtual*/void visit(TheOther *e)
    {
        cout << "do Up on " + e->theOther() << '\n';
    }
};

class DownVisitor: public Visitor
{
     /*virtual*/void visit(This *e)
    {
        cout << "do Down on " + e->thiss() << '\n';
    }
     /*virtual*/void visit(That *e)
    {
        cout << "do Down on " + e->that() << '\n';
    }
     /*virtual*/void visit(TheOther *e)
    {
        cout << "do Down on " + e->theOther() << '\n';
    }
};

int main()
{
  Element *list[] =
  {
    new This(), new That(), new TheOther()
  };
  UpVisitor up; // 4. Client creates
  DownVisitor down; // "visitor" objects
  for (int i = 0; i < 3; i++)
  // and passes each
    list[i]->accept(up);
  // to accept() calls
  for (int i = 0; i < 3; i++)
    list[i]->accept(down);
}