Question

In a book, I saw that if a subclass is overriding a superclass’s method, we may have

self = [super init];

First, is this supposed to be done in the subclass’s init method?

Second, I wonder why the call is not just

[super init];

? I mean, at the time of calling init, the memory is allocated by alloc already (I think by [Foobar alloc] where Foobar is the subclass’s name. So can’t we just call [super init] to initialize the member variables? Why do we have to get the return value of init and assign to self? I mean, before calling [super init], self should be pointing to a valid memory allocation chuck… so why assigning something to self again?

(if assigning, won’t [super init] just return self‘s existing value?)

Answer 1

So why assign the value returned from [super init] to self? Looking at
a typical initializer method:

 - (id)initWithString:(NSString *)aString {
     self = [super init];
     if (self)
     {
         instanceString = [aString retain];
     }
     return self; }

Why do we assign [super init] to self here?

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 and the
instanceString is set on in the original object (the line
instanceString = [aString retain]; could have been the first line of
the method and the result would be the same).

In the third case, the initialization has failed. self is set to nil,
no further action is taken and nil is returned.

The rationale for assigning to self is associated with the second
case: if the returned object is different, we want the:

instanceString = [aString retain]; which gets converted to

self->instanceString = [aString retain]; to act on the correct value,

so we have to change the value of self to point to this new object.

hoping this helps…

From Cocoa with Love