Inheritance: Restricting rather than extending? [duplicate]

Answer 1

No. You can't prevent the users of your subclass from calling the methods of a superclass. You could override them and throw an exception inside, but that would just produce a broken subclass.

Remember that inheritance works as an "is a" extension, that is, instances of your subclasses should behave normally in any context that doesn't know about this particular subclass but knows about its superclass. It's only in places that have explicit knowledge about your subclass that you can benefit from adding extra initialization and other methods.

For example, UIKit has no knowledge of your subclass. So if you want to make your UIView subclass available from a NIB, you need to use the initialization methods that will be called by the NIB loading system, namely initWithCoder:. You can simply call your own initialization methods inside initWithCoder:. But if there are any additional parameters you would like to pass to the init method, you'll have to provide a way to configure them after initialization.

Answer 2

Actually, you CAN restrict with a subclass. You can override whichever methods you want to block in your subclass's .h file. You can make initWithFrame unavailable by placing the following in your .h file.

- (id) initWithFrame:(CGRect) frame __attribute__((unavailable("message")));

This will make the initWithFrame: method unavailable to anyone using your subclass.

To keep other code form calling this method, you can further restrict by putting this in your .m file.

- (id) initWithFrame:(CGRect) frame
{
    return nil;     
}

Answer 3

Actually, you can get compile-time warnings about calling a method on a subclass. Use the __attribute((deprecated)) attribute. If you want people to use -initWithPizza: instead of -initWithFrame:, do this:

@interface MyView : UIView
- (id)initWithPizza:(MyPizza *)pizza;
@end

@interface MyView (Deprecations)
- (id)initWithFrame:(CGRect)frame __attribute((deprecated("Use initWithPizza: instead")));
@end

Putting the -initWithFrame: declaration in a separate category is necessary to avoid Xcode complaining that you declared the method in the header but didn't implement it. Since you're just inheriting it from the superclass, that's fine; you don't have to implement it at all. But if you want to implement it to throw an exception, or call through to -initWithPizza: with a default argument, that's fine.

Of course, this won't stop UIKit from calling -initWithFrame: if it was already going to do so. But if you can guarantee that won't happen, then you're fine.