Visitor Pattern Jeff Schott CS590L Spring 2004

What is the Purpose of the Visitor Pattern ? n Represent an operation to be performed on the elements of an object structure n Define a new operation without changing the classes of the elements on which it operates

When do you use the Visitor Pattern? n An object structure contains many classes of objects n Many distinct and unrelated operations on these objects - want to avoid “polluting” their classes n Classes defining the object structure rarely change, but operations change frequently

Visitor Pattern Structure

Visitor Pattern Participants n Visitor n Concrete Visitor n Element n Concrete Element n Object Structure

Visitor Pattern Collaborations

Benefits of the Visitor Pattern n Makes adding new operations easy n Gathers related operations and separates unrelated operations n Visitors can visit objects that don’t have a common parent class n Visitors can accumulate state as they visit each element

Liabilities of the Visitor Pattern n Adding new Concrete Element classes is hard n Allowing a Visitor to access the internal state of a Concrete Element breaks encapsulation

Visitor Pattern Example: Compiler Syntax Tree n Operations on Syntax Tree –Type Checking –Code Generation –Pretty Printing n Different Classes for Tree Nodes –Assignment Statements –Variables –Arithmetic Expressions

Visitor Pattern Example: Node Class Hierarchy n Distributing operations across classes is confusing n Mixing operations is confusing n Adding new operations requires recompiling all classes

Visitor Pattern Example: Visitor Class Hierarchy

Visitor Pattern Example: New Node Class Hierarchy

Visitor Pattern Implementation Issues n Double Dispatch n Who is Responsible for Traversing the Object Structure? –Object Structure –Visitor –Iterator Object

Visitor Pattern Sample Code: Element

Visitor Pattern Sample Code: Visitor

Visitor Pattern Sample Code: Concrete Elements

Visitor Pattern Sample Code: Concrete Visitor

Visitor Pattern Sample Code: Concrete Visitor (cont’d)

Visitor Pattern Sample Code: Client

Visitor Pattern Known Uses n Smalltalk-80 Compiler n IRIS Inventor Toolkit n X Consortium’s Fresco Application Toolkit n Bistro Programming Language Compiler

Visitor Pattern: Related Patterns n Composite n Interpreter n Iterator

Visitor Pattern Questions?