Question

I have a question about writing your own init methods in objective-c. I’ve read a few different books and have seen a couple of ways to do it but the consensus is the right way to do it is like this:

- (id)init
{
    self = [super init]; 
    if(self!=nil)
    {

    }
    return self;
}

I’m a little confused about the line “self = [super init]”. My understanding is that, there’s no guarantee that [super init] will return the class that you expect it to. I think this is called “class clusters”. But in the normal case, where it does return the class you expect it to, if I set self to point to a class that is returned to me, aren’t I just saying that self is referring to an object of a different class rather than the class that I’m in the init method of?

To summarize, why set self to be the superclass vs the actual class I’m in?

From a blog I read:

The textbook reason is because [super
init] is permitted to do one of three
things:

1) Return its own receiver (the self
pointer doesn’t change) with inherited
instance values initialized. 2) Return a
different object with inherited
instance values initialized. 3) Return
nil, indicating failure. In the first
case, the assignment has no effect on
self…

“The assignment has no effect on self” is what confuses me. Why does it have no effect? If I set something = to something else, shouldn’t that have an effect?

Answer 1

To summarize, why set self to be the superclass vs the actual class I’m in?

This is the Apple suggested way to do things, specifically due to the case of class clusters, as you say.

In general, you should not worry about the fact that self might be of a different class in the “normal” case.

self simply identifies the object you are, not the class (the class is actually a different object in the runtime). If you think of OO inheritance properties, it is at the same time an object of its class and of its superclass (if it is clear what I am trying to say). There is no contradiction in the “normal” case, since the value of self does not change.

Also, you can think of self as a special pointer to your object. In the cluster case, self can change, that is the reason why it can happen that its class change.

Hope this helps clarifying things. You will also find an interesting reading in this article by Wil Shipley.