Question

Please help me making this clear about instance methods in Objective C:

• Can send messages to self and super inside
  • both dispatch the message to the calling object, but use different implementations
  • if a superclass of yours calls a method on self, it will [execute] your implementation (if one exists)

Lecture 3, Stanford University, Objectve-C course

Answer 1

Okay, now that it’s actually possible to understand the question:

Let’s say you have a class Foo with methods doSomething and doSomethingElse, and you make a subclass of Foo called Bar.

In your implementation of Bar, if you wanted to call doSomething you could either do [self doSomething] or [super doSomething].

• [super doSomething] would use the superclass’s implementation of doSomething—specifically, Foo‘s implementation.
• [self doSomething] would use the class itself’s implementation of doSomething—that is, Bar‘s implementation. Note that if Bar didn’t actually override doSomething, then [self doSomething] would end up calling the superclass’s implementation.

As for what happens if a superclass calls a method on self, let’s say Bar overrides doSomething, but doesn’t override doSomethingElse, and let’s say doSomethingElse looks like this:

- (void)doSomethingElse
{
    [self doSomething];
}

What happens if you call doSomethingElse on Foo *aFoo and Bar *aBar? The result of [aFoo doSomethingElse] is clear: it does [self doSomething] where self is a Foo, so Foo‘s implementation of doSomething will be executed.

But when you do [aBar doSomethingElse] is where things get interesting, and is what Paul was getting at. since Bar doesn’t override doSomethingElse, Foo‘s implementation will be called, which in turn does [self doSomething]. But this time, self is an instance of Bar, and so Bar‘s implementation of doSomething will be called.

Why would [self doSomething] in the implementation of Foo end up executing code from the subclass Bar? This is because of how messages are dispatched in Objective-C. [self doSomething] sends the message doSomething to the object self, and it is up to whatever object self is to decide what code gets executed. Since self, in this situation, would be a Bar, Bar‘s implementation of doSomething is executed.