Visitor Pattern

Motivation

How can I add functionalities to legacy systems without violating the open/closed principle?

Visitor Pattern

What is it?

Behavioural design pattern that allow algorithms and objects that the algorithms operate on to be separated
 

Benefits

• OCP: introduce behaviour to classes without changing their implementation.
• SRP: functionality and data in an object can be separated

When to use?

New operations need to be added to all elements of an existing object structure

UML Diagram

• Element classes must have an accept function that invokes the visit function of the Visitor
• New behaviour is added in the visit function of the Visitor

Double Dispatch

Exercise