Visitor

Visitor represents an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates. The pattern has behavioral purpose and applies to the objects.

When to use

• an object structure contains many classes of objects with differing interfaces, and you want to perform operations on these objects that depend on their concrete classes
• many distinct and unrelated operations need to be performed on objects in an object structure, and you want to avoid "polluting" their classes with these operations
• the classes defining the object structure rarely change, but you often want to define new operations over the structure