Question

I’m reading through Mark Dalrymple’s Learn Objective-C on the Mac (only at the chapter on Protocols, so still relatively newbish) and trying to figure something out:

Why would you ever reference a class by its own name? If I had a class called Foo, why would I ever want to write, say,

[[Foo alloc] init]

and not

[[[self class] alloc] init]

If I had a subclass Bar, wouldn’t the first option invalidate me from writing

[[Bar alloc] init]

whereas the second option would allow it? When would the first option be better?

Answer 1

Generally, within a class method, you do use [[self alloc] init]. For example, the canonical way to write a convenience method for a class is:

+ (id)fooWithBar:(Bar *)aBar
{
    return [[[self alloc] initWithBar:aBar] autorelease];
}

(Note that in a class method, self refers to the class object.)

However, you would use [[Foo alloc] init] (that is, an explicit class name) if you actually want an instance of the Foo class (and not a subclass).