Editing Visitor pattern (section)

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{{short description|Software design pattern}}

A '''visitor pattern''' is a [[software design pattern]] that separates the [[algorithm]] from the [[Object (computer science)|object]] structure. Because of this separation, new operations can be added to existing object structures without modifying the structures. It is one way to follow the [[open/closed principle]] in [[object-oriented programming]] and [[software engineering]]. 

In essence, the visitor allows adding new [[virtual function]]s to a family of [[Class (computer programming)|classes]], without modifying the classes. Instead, a visitor class is created that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal through [[double dispatch]].

Programming languages with [[Tagged union|sum types]] and [[pattern matching]] obviate many of the benefits of the visitor pattern, as the visitor class is able to both easily branch on the type of the object and generate a compiler error if a new object type is defined which the visitor does not yet handle.