1. Visitor Pattern Jeff Schott CS590L Spring 2004

2. 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

3. 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

4. Visitor Pattern Structure

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

6. Visitor Pattern Collaborations

7. 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

8. 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

9. 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

10. 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

11. Visitor Pattern Example: Visitor Class Hierarchy

12. Visitor Pattern Example: New Node Class Hierarchy

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

14. Visitor Pattern Sample Code: Element

15. Visitor Pattern Sample Code: Visitor

16. Visitor Pattern Sample Code: Concrete Elements

17. Visitor Pattern Sample Code: Concrete Visitor

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

19. Visitor Pattern Sample Code: Client

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

21. Visitor Pattern: Related Patterns n Composite n Interpreter n Iterator

22. Visitor Pattern Questions?