The code in the OP resembles a well-known variation of the Visitor design pattern known as an Internal Visitor (see e.g. Extensibility for the Masses. Practical Extensibility with Object Algebras by Bruno C. d. S. Oliveira and William R. Cook). That variation, however, uses generics and return values (instead of void) to solve some of … Read more
In short they come from to different conceptualizations that, in some languages where double dispatch is not natively supported, lead to the visitor pattern as a way to concatenate two (or more) single dispatch in order to have a multi-dispatch surrogate. In long The idea of multiple dispatch is – essentially – allow a call … Read more
The strategy pattern is like a 1:many relationship. When there is one type of object and I want to apply multiple operations to it, I use the strategy pattern. For example, if I have a Video class that encapsulates a video clip, I might want to compress it in different ways. So I create a … Read more
You might want to have a look at the Strategy pattern. This still gives you a separation of concerns while still being able to add new functionality without having to change each class in your hierarchy. class AbstractShape { IXmlWriter _xmlWriter = null; IShapeDrawer _shapeDrawer = null; public AbstractShape(IXmlWriter xmlWriter, IShapeDrawer drawer) { _xmlWriter = … Read more
So you’ve probably read a bajillion different explanations of the visitor pattern, and you’re probably still saying “but when would you use it!” Traditionally, visitors are used to implement type-testing without sacrificing type-safety, so long as your types are well-defined up front and known in advance. Let’s say we have a few classes as follows: … Read more
The visitor pattern’s visit/accept constructs are a necessary evil due to C-like languages’ (C#, Java, etc.) semantics. The goal of the visitor pattern is to use double-dispatch to route your call as you’d expect from reading the code. Normally when the visitor pattern is used, an object hierarchy is involved where all the nodes are … Read more
I’m not very familiar with the Visitor pattern. Let’s see if I got it right. Suppose you have a hierarchy of animals class Animal { }; class Dog: public Animal { }; class Cat: public Animal { }; (Suppose it is a complex hierarchy with a well-established interface.) Now we want to add a new … Read more