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Source: http://www.vincehuston.org/dp/visitor.html

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 elemtns 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 heterogenous 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.

Example

The Visitor pattern represents an operation to be performed on the elements of an object structure without changing the classes on which it operates. This pattern can be observed in the operation of a taxi company. When a person calls a taxi company (accepting a visitor), the company dispatches a cab to the customer. Upon entering the taxi thecustomer, or Visitor, is no longer in control of his or her own transportation, the taxi (driver) is.