How can I avoid empty (noop) method of a visitor pattern implementation?

Question

I have a large collection of different objects that extend the same base class A. To apply different algorithms to the collection, I created an IVisitor interface that has an Accept method for each child of A.

Some of the visitors that implement IVisitor actually use all children of A but many of them don't, so I end up having visitors with a bunch of empty (noop) methods that I would like to avoid. Moreover, everytime I add a new child class of A I need to add a new Accept method to IVisitor and all the classes implement it, which is a common problem of the visitor pattern.

I've tried to implement the Selective Visitor pattern but I don't like to cast objects around as in my case it could be a problem.

I've also evaluated this question but its answers are not really target the problem. I would like to use an interface instead of an abstract class, because in my case multiple inheritance is not allowed.

Is there a way to avoid this kind of hassle and get a cleaner code?

EDIT: Here you find an example on Draw.io

Can you provide a sample code in order to clarify your question? — Polymorphic, Dec 09 '17 at 10:16
Provide a default implementation of all the Visit() methods in the interface, doing nothing (or better, throwing an exception). Which language are you using? — JB Nizet, Dec 10 '17 at 09:45
@JBNizet That's not feasible because the languages I use do not support multiple inheritance so the interface must be an interface, not a class — SeeEn, Dec 10 '17 at 19:31
C# for that matter. In my post I specified that I cannot use multiple inheritance and that should be enough ;-) — SeeEn, Dec 10 '17 at 20:22
This seems like an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). — jaco0646, Dec 11 '17 at 01:00

score 2 · Answer 1 · answered Dec 11 '17 at 05:43

You can define a new pattern ;) for your case: "Selective Generic Visitor". You can have only one Visit method in your IVisit interface that accepts a generic parameter with constraint inherited from BaseClass.

interface IVisitor<T> where T:BaseClass
{
    void Visit(T element);
}

Then Concrete visitors can specify the generic parameter and implement the Visit method. If you can take advantage an IoC container such as Autofac, It serves you as a factory for generic types and easily you can get the proper Visitor instance by requesting IVisitor and pass it to Accept method calls.

How can I avoid empty (noop) method of a visitor pattern implementation?

1 Answers1