subclassing convenience construtctor

Question

If I have a super class with a convenience constructor as follows (using ARC):

+(id)classA {
    ClassA *foo = [[ClassA alloc] init];

    return foo;
}

If I then subclass ClassA, with a class named ClassB, and I want to override the convenience constructor, is the following correct:

+(id)classB {
    ClassB *foo = [ClassA classA];

    return foo;
}

(Assume that I cannot call alloc and init on ClassB).

Thanks!

Answer 1

No, that is not correct, since that allocates, inits and returns a ClassA, not a ClassB. The only way to do this is not to use ClassA explicitly:

+ (id) classA
{
    return [[self alloc] init];
}

Of course, you could also use old-fashioned new for this:

ClassB *myB = [ClassB new];

FWIW, assuming I would want to do more than just allocate and init, and my class is named Gadget, then I would do something like:

+ (id) gadgetWithNumber: (int) num
{
    return [[self alloc] initWithNumber: num];
    // or without ARC:
    // return [[[self alloc] initWithNumber: num] autorelease];
}

Of course that assumes that there is an initWithNumber: method in my class.

Answer 2

A class method is passed self just like an instance method, only in the case of a class method the value of self is an instance of Class. So change your classA method to:

+ (id) classA
{
   return [[self alloc] init];
}

(Which is just the definition of new you inherit from NSObject. The convention is newX returns a retained reference, while classnameX returns an autoreleased reference. Using ARC the difference is really moot, but I'd go with newX myself.)

Now if ClassB is a sub-class of ClassA then [ClassB classA] will invoke classA with self being ClassB and you will alloc and init a ClassB instance.