Visitor Pattern - Motivation

Motivation

Consider the design of a 2D CAD system. At its core there are several types to represent basic geometric shapes like circles, lines and arcs. The entities are ordered into layers, and at the top of the type hierarchy is the drawing, which is simply a list of layers, plus some additional properties.

A fundamental operation on this type hierarchy is saving the drawing to the system's native file format. At first glance it may seem acceptable to add local save methods to all types in the hierarchy. But then we also want to be able to save drawings to other file formats, and adding more and more methods for saving into lots of different file formats soon clutters the relatively pure geometric data structure we started out with.

A naive way to solve this would be to maintain separate functions for each file format. Such a save function would take a drawing as input, traverse it and encode into that specific file format. But if you do this for several different formats, you soon begin to see lots of duplication between the functions, e.g. lots of type-of if statements and traversal loops. Another problem with this approach is how easy it is to miss a certain shape in some saver.

Instead, you could apply the Visitor pattern. The Visitor pattern encodes a logical operation on the whole hierarchy into a single class containing one method per type. In our CAD example, each save function would be implemented as a separate Visitor subclass. This would remove all duplication of type checks and traversal steps. It would also make the compiler complain if you leave out a shape.

Another motivation is to reuse iteration code. For example iterating over a directory structure could be implemented with a visitor pattern. This would allow you to create file searches, file backups, directory removal, etc. by implementing a visitor for each function while reusing the iteration code.