The Visitor pattern

The Visitor pattern stands out from the other classic object-oriented patterns due to its complexity. On the one hand, the basic structure of the Visitor pattern is quite complex and involves many coordinated classes that must work together to form the pattern. On the other hand, even the description of the Visitor pattern is complex—there are several very different ways to describe the same pattern. Many patterns can be applied to multiple kinds of problems, but the Visitor pattern goes beyond that—there are several ways to describe what it does that use completely different languages, talk about seemingly unrelated problems, and overall have nothing in common. However, they all describe the same pattern. Let's start by examining the many faces of the Visitor pattern, and then move on to the implementation.

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