can we override alloc and dealloc in objective C?

Question

I know that this is rarely required to override the alloc or dealloc methods,but if required is it possible in iPhone programming?

Answer 1

You can and indeed, you should (if using manual memory management) override dealloc to release any resources you hold (not forgetting to call [super dealloc] when finished). Overriding alloc is possible but, as you say, rarely needed.

Answer 2

In general, overriding alloc is only done when you wish to, eg, allocate an object from a pool of available instances, or perhaps allocate a variable amount of storage for the object based on some external parameter. (In C++ you can access the new parameters and allocate based on them, but Objective-C does not give you access to the initXXX parameters.)

I've never attempted any of this, and I suspect that its a bit of a minefield -- you need to study up on the structures and be pretty careful.

As Adam said, you should ALWAYS (in a reference counted environment) override dealloc if there are any retained objects held by your object.

Update: An interesting thing you can do ... in RedClass or a superclass of it code something like:

+(id)alloc {
    if (self == [RedClass class]) {
        return [BlueClass alloc];
    }
    else {
        return [super alloc];
    }
}

The net result is that whenever you execute [RedClass alloc] a BlueCLass object will be returned. (NB: Presumably BlueClass is a subclass of RedClass, or things will get seriously mucked up shortly after the object is returned.)

Not saying that it's a good idea to do this, but it's possible (and I don't offhand know of any cases where it wouldn't work reliably for vanilla user-defined classes). And it does have a few possible uses.

Additional note: In some cases one might want to use [self isSubclassOf:[RedClass class]] rather than == (though that has some serious pitfalls).