Visitor Design Pattern

Intent

• Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
• The classic technique for recovering lost type information.
• Do the right thing based on the type of two objects.
• Double dispatch

Problem

Many distinct and unrelated operations need to be performed on node objects in a heterogeneous aggregate structure. You want to avoid "polluting" the node classes with these operations. And, you don't want to have to query the type of each node and cast the pointer to the correct type before performing the desired operation.

 

Understanding the need of Visitor Pattern