Assigning to self in Objective-C

Question

I'm from the C++ world so the notion of assigning this makes me shudder:

this = new Object; // Gah!

But in Objective-C there is a similar keyword, self, for which this is perfectly acceptable:

self = [super init]; // wait, what?

A lot of sample Objective-C code uses the above line in init routines. My questions:

1) Why does assignment to self make sense (answers like "because the language allows it" don't count)

2) What happens if I don't assign self in my init routine? Am I putting my instance in some kind of jeopardy?

3) When the following if statement fails, what does it mean and what should I do to recover from it:

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

    if (self)
    {
        self.my_foo = 42;
    }

    return self;
}

Answer 1

This is a topic that is frequently challenged by newcomers:

• Wil Shipley: self = [stupid init];
• Matt Gallagher: What does it mean when you assign [super init] to self?
• Apple documentation: Implementing Initializers
• Cocoa-Dev: self = [super init] debate

Basically, it stems from the idea that a superclass may have over-ridden the designated initializer to return a different object than the one returned from +alloc. If you didn't assign the return value of super's initializer into self, then you could potentially be dealing with a partially initialized object (because the object that super initialized isn't the same object that you're initializing).

On the whole, it's pretty rare for super to return something different, but it does happen in a couple of cases.