Visitor pattern

A visitor pattern is a software design pattern that separates the algorithm from the 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 functions to a family of 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 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.

Overview

The Visitor

design pattern is one of the twenty-three Gang of Four design patterns.

Problems the pattern can solve

It should be possible to define a new operation for (some) classes of an object structure without changing the classes.

When new operations are needed frequently and the object structure consists of many unrelated classes,

it's inflexible to add new subclasses each time a new operation is required

because "distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."

Solution described by the pattern

Define a separate (visitor) object that implements an operation to be performed on elements of an object structure.