Visitor

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.

Applicability

• Need to perform operations on the objects in a structure, and these objects have differing interfaces
• Need to perform distrinct and unrelated operations on objects, and you want to avoid "polluting" their classes with these operations
• Need to define new operations over a rarely-changing structure

Consequences

• Makes adding new operations easy
• Gathers related operations and separates unrelated ones
• Adding new ConcreteElements is hard
• Works across objects of differing types
• Can accumulate state
• May force a break in encapsulation