Command Pattern vs. Visitor Pattern

Question

Is it generally acceptable to allow a Visitor to modify state of the Receiver, or should that be a Command pattern instead?

Advocation

Answer 1

The purpose of the visitor pattern is to allow new operations to be added to a class heirarchy without modification to that heirarchy. I've never seen anyone suggesting that only read-only operations are acceptable. The only limitation is that the added operations should only use the public interface of the class heirarchy.

Cardie

Answer 2

I don't think you can make a blanket statement whether it's good or bad to modify state of anything. I would think it's ok to modify the states as long as it does not affect the visiting logic itself. For example, you might write a visitor that visits all files under folder structure and renames the file name to upper case.

Accumulative

There are definitely cases where one can make the statement that changing the state is a good or bad thing. The Visitor pattern is expected to be used in such a way as a state change is possible and even likely. Whereas with the observer pattern, the observer should not be changing the state. – Elsieelsinore

what about an user, event for commenting, punisher listener which would ban user after saying some violent words? :-) – Savor

Answer 3

Microsoft's example of a visitor modifying the receiver is the ExpressionVisitor. The purpose of the ExpressionVisitor class is to modify an Expression tree. So I guess Microsoft at least thinks it's acceptable.

Ferrocyanide

Microsoft isn't always the best guide as to what is right and wrong: ayende.com/blog/35841/… – Boony

The standard interpretation when someone says "authority X thinks Y is acceptable" is that you are endorsing Y by appeal to authority X. cf. argumentum ad verecundiam. – Boony

@Jason Sure I'll consider Microsoft an authority. I'm saying here is one instance this authority finding this pattern acceptable. But am I saying they are the "best guide"? No . I leave it up to the asker to decide whether they're right. – Ferrocyanide

Answer 4

Each pattern has it's own pros, cons and use cases.

You can use Command pattern to

1. Decouple the invoker & receiver of command

2. Implement callback mechanism

3. Implement undo and redo functionality

4. Maintain a history of commands

Use Visitor pattern in below scenarios:

1. Similar operations have to be performed on objects of different types grouped in a structure
2. You need to execute many distinct and unrelated operations. It separates Operation from objects Structure
3. New Operations have to be added without change in object structure

Related posts:

Using Command Design pattern

When should I use the Visitor Design Pattern?

Centripetal