CSC/ECE 517 Fall 2012/ch2b 2w51 aa

Visitor Pattern, Directory of sites

The Purpose of this article is to act as a "directory of sites", or a simple quick jump page for the reader. The effort is to try and collect good sources (blog posts, forum chats, general articles etc.) which deal with the Visitor pattern and to make this available to the readers with summary. The readers can then explore the links provided, to understand certain topics that they are interested in. Readers are also encouraged to edit the document in case they are able to contribute good material. Since this is just a quick jump page, there will be an apparent lack of examples. This is in line with the requirements given in the writeup, since we are re-directing you to pages where you will get plenty of information.

Introduction

In Object Oriented Programming, the Visitor Pattern is a way of decoupling algorithms to be performed by an object. The way to do this is to create a visitor class which would help to add new virtual function to the classes without modifying their structures. This is accomplished by creating appropriate specializations of the virtual function in visitor class which will take instance reference as input and implements the algorithm using double dispatch.

For more information on the Visitor pattern, you can visit the following links

• http://kasunpanorama.blogspot.com/2010/04/understanding-visitor-pattern.html

This page can provide more introductory information on visitor pattern

• http://en.wikipedia.org/wiki/Visitor_pattern

Wikipedia provides a good condensed view on the topic of the Visitor pattern.

Motivation

Now that some information is present about what the Visitor pattern is and what it should do, let us explore the motivation behind using the Visitor pattern. Some links are provided for the reader to explore the topic and a brief summary is presented to guide the reader

• http://en.wikipedia.org/wiki/Visitor_pattern

Motivation for visitor patterns is nicely depicted in Wikipedia for Visitor pattern.

• http://www.oodesign.com/visitor-pattern.html

One of the top visited sites for design pattern basics , oodesign.com has good introduction, examples and implementation details. Sample code is also available for the reader. The article is good not only from the point of view of explaining the motivation behind Visitor patterns, but also because it follows a good flow in setting up the topic, explanation using examples and concluding with a brief summary

• http://sourcemaking.com/design_patterns/visitor

This page has good detailed description of visitor pattern and the worth reading part is the discussion section of this page which also talks about how double dispatch is related to visitor.

• http://www.dofactory.com/Patterns/PatternVisitor.aspx

Simple introductory page which describes and talks about the pattern in short. It is a very 'bare-bones' reference article, but is a good introductory piece

Visitor Pattern in Java

(1) http://java.dzone.com/articles/design-patterns-visitor (2) http://www.javaworld.com/javatips/jw-javatip98.html (3) http://snehaprashant.blogspot.com/2009/01/visitor-pattern-in-java.html (4) http://en.wikipedia.org/wiki/Visitor_pattern#Java_example

Visitor Pattern in C++

Following are the links which provides examples based on C++:

• http://sourcemaking.com/design_patterns/visitor/cpp/1

A comparison is provided between the 'before' and 'after' code based on the Visitor pattern implementation. This link and the next couple of links are part of the same discussion provided here : http://sourcemaking.com/design_patterns/visitor

• http://sourcemaking.com/design_patterns/visitor/cpp/2
• http://sourcemaking.com/design_patterns/visitor/cpp/3

• http://www.swe.uni-linz.ac.at/research/deco/designPatterns/visitor/visitor.defaultroles.c++.html

This page explains visitor pattern in detail from the basics till implementation using C++. It has implementation details and also discusses issues that could possibly arise when using the Visitor pattern.

Visitor Pattern in Python

Following are two links which provide good examples of implementation of visitor patterns in python:

• https://chris-lamb.co.uk/2006/12/08/visitor-pattern-in-python/

The author sets up the explanation by using the example of an Abstract syntax tree traversal. A brief comparison is also provided about the differences in a Java implementation and a Python implementation for the same.

• http://curtis.schlak.com/2012/01/04/python-visitor-pattern-helper.html

The author presents a similar example as the link before. A point to note about this link, is that there is sufficient amount of code, but not enough explanation. However, the code is still useful.

Visitor Pattern in Ruby

(1) https://gist.github.com/989193 (2) http://www.scribd.com/doc/2217773/Design-Patterns-in-Ruby

Advantages

• This links helps to understand the advantage of a design pattern like visitor, which are the features it combines and how it makes it easy for programmer to implement those ideas. http://compilers.cs.ucla.edu/jtb/jtb-2003/whyvisitors/whyvisitors.html

• The applicability of visitor pattern can be found here: http://www.swe.uni-linz.ac.at/research/deco/designPatterns/visitor/visitor.abstract.html#Applicability

Disadvantages

• http://www.developerfusion.com/article/84939/iterators-composites-and-visitors/ This page has a subsection in the visitor pattern section which discusses issues related to visitor pattern.

• http://www.oodesign.com/visitor-pattern.html This page describes issues with visitor pattern in section Specific problems and implementation

Video Lecture: (1) https://www.youtube.com/watch?v=mTJodk3vfdk (2) https://www.youtube.com/watch?v=UYUopyMcWjw&feature=related