Visitor Pattern

From eqqon

(Difference between revisions)

Revision as of 09:10, 21 November 2007

Motivation for applying the Visitor Pattern

The complete example program

using System;
using System.Collections.Generic;
using System.Text;

namespace CodeSnippet
{

public class A
{
public virtual void Print(VisitorPattern.Printer printer) { printer.Print(this); } // calls Print(A element)
}

public class B : A
{
public override void Print(VisitorPattern.Printer printer) { printer.Print(this); } // calls Print(B element)
}

public class C : B
{
public override void Print(VisitorPattern.Printer printer) { printer.Print(this); } // calls Print(C element)
}

namespace NaiveApproach
{
public class Printer
{
public void Print(A element) { Console.WriteLine("A"); }

public void Print(B element) { Console.WriteLine("B"); }

public void Print(C element) { Console.WriteLine("C"); }

public void Print(A[] array)
{
foreach (A element in array)
{
// here we reimplement polymorphic method dispatching!
if (element is C)
Print(element as C);
else if (element is B)
Print(element as B);
else if (element is A)
Print(element);
}
}
}
}

namespace VisitorPattern
{
public class Printer
{
public void Print(A element) { Console.WriteLine("A"); }

public void Print(B element) { Console.WriteLine("B"); }

public void Print(C element) { Console.WriteLine("C"); }

public void Print(A[] array)
{
foreach (A element in array)
{
// here we make use of the language's polymorphic method dispatching
element.Print(this);
}
}
}
}

class Program
{
static void Main(string[] args)
{
A[] array = { new A(), new B(), new C() };
Console.WriteLine("Naive Approach:");
new NaiveApproach.Printer().Print(array);
Console.WriteLine("Visitor Pattern:");
new VisitorPattern.Printer().Print(array);
Console.ReadLine();
}
}
}

@@ Line 3: / Line 3: @@
 == Motivation for applying the Visitor Pattern ==
-== The complete example Program ==
+== The complete example program ==
 <span><span class="S5">using</span><span class="S0"> </span>System<span class="S10">;</span><br />