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The Visitor [1] design pattern is one of the twenty-three well-known Gang of Four design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
The single-serving visitor pattern should be used when visitors do not need to remain in memory. This is often the case when visiting a hierarchy of objects (such as when the visitor pattern is used together with the composite pattern) to perform a single task on it, for example counting the number of cameras in a 3D scene.
Using a visitor is a complementary approach to doing something with a collection of objects. In particular, a core idea of the visitor pattern is that you can prescribe different operations for different polymorphic (or static) types you visit. Also, with the visitor pattern, different objects have control over traversal order.
Single-serving visitor pattern Optimise the implementation of a visitor that is allocated, used only once, and then deleted Specification pattern Recombinable business logic in a boolean fashion State pattern A clean way for an object to partially change its type at runtime Strategy pattern Algorithms can be selected on the fly, using composition
Many object-oriented design patterns are expressible in functional programming terms: for example, the strategy pattern simply dictates use of a higher-order function, and the visitor pattern roughly corresponds to a catamorphism, or fold.
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