Question

The only difference I observed on my own is that respondsToSelector‘s receiver can either be a class or an instance, while instancesRespondToSelector can only have a class receiver. What else makes them different, though? Are there any performance issues with one or the other?

Answer 1

Under the hood, -[NSObject respondsToSelector:] is implemented like this:

- (BOOL)respondsToSelector:(SEL)aSelector {
    return class_respondsToSelector([self class], aSelector);
}

and +[Class instancesRespondToSelector:] is implemented like this:

+ (BOOL)instancesRespondToSelector:(SEL)aSelector {
    return class_respondsToSelector(self, aSelector);
}

(I used Hopper on CoreFoundation to figure this out.)

So, there’s basically no difference. However, you can override respondsToSelector: in your own class to return YES or NO on a per-instance basis (NSProxy does this). You can’t do that with instancesRespondToSelector:.