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I always thought an object needs the data and the messages to act on it. When would you want a method that is extrinsic to the object? What rule of thumb do you follow to have a visitor? This is supposing that you have full control of the object graph.
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2 Answers. Save this answer. Show activity on this post. The visitor pattern is useful when you want to process a data structure containing different kinds of objects, and you want to perform a specific operation on each of them, depending on its type.
In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying the structures.
The accept() method accepts a visitor. It is provided by the Visitable class. The visit() method is called every time when we visit an element. It is implemented by the Visitor class.
The visitor pattern is particularly useful when applying an operation to all elements of a fairly complicated data structure for which traversal is non-trivial (e.g. traversing over the elements in parallel, or traversing a highly interconnected data structure) or in implementing double-dispatch. If the elements are to be processed sequentially and if double-dispatch is not needed, then implementing a custom Iterable and Iterator is usually the better choice, especially since it fits in better with the other APIs.
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