Question

I am dealing with a large codebase that has a lot of classes and a lot of abstract methods on these classes. I am interested in peoples opinions about what I should do in the following situation.

If I have a class Parent-A with an abstract method. There will only be 2 children. If Child-B implements AbstractMethodA but Child-B does not as it doesnt apply.

Should I

1. Remove the abstract keyword from parent and use virtual or dynamic?
2. Provide a empty implementation of the method.
3. Provide an implementation that raises an error if called.
4. Ignore the warning.

Edit: Thanks for all the answers. It confirmed my suspicion that this shouldn’t happen. After further investigation it turns out the methods weren’t used at all so I have removed them entirely.

Answer 1

If AbstractMethodA does not apply to Child-B, then Child-B should not be inheriting from Parent-A.

Or to take the contrapositive, if Child-B inherits from Parent-A, and AbstractMethodA does not apply to the child, then it should not be in the parent either.

By putting a method in Parent-A, you are saying that the method applies to Parent-A and all its children. That’s what inheritance means, and if you use it to mean something different, you will end up in a serious dispute with your compiler.

[Edit – that said, Mladen Prajdic’s answer is fine if the method does apply, but should do nothing for one or more of the classes involved. A method which does nothing is IMO not the same thing as a method which is not applicable, but maybe we don’t mean the same thing by ‘doesn’t apply’]

Another technique is to implement the method in Child-B anyway, but have it do something drastic like always returning failure, or throw an exception, or something. It works, but should be regarded as a bit of a bodge rather than a clean design, since it means that callers need to know that the thing they have that they’re treating as Parent-A is really a child-B and hence they shouldn’t call AbstractMethodA. Basically you’ve discarded polymorphism, which is the main benefit of OO inheritance. Personally I prefer doing it this way over having an exception-throwing implementation in the base class, because then a child class can’t ‘accidentally’ behave badly by ‘forgetting’ to implement the method at all. It has to implement it, and if it implements it to not work then it does so explicitly. A bad situation should be noisy.