Objective-C：强制某个类成为其他类的子类

Question

B is subclass of A. C is a subclass of A. When i create ci do if((self = [NSKeyedUnarchiver unarchiveObjectWithFile:…])) { } inside the .m in C.

So despite i set the the superclass in Ch to be B (which is what i want), when i create the C object from unarchiveObjectWithFile: since this object is subclass of A, i can't force it to be subclass of B. (Hard to explain sorry).

Is there any workaround ???

In other words: when i unarchiveObjectWithFile:… an object that pertains to a class A, and i have another class B that is subclass of A, can the unarchiveObjectWithFile: be a subclass of B? (wich is allowed since B is subclass of a)

Answer 1

If I understand correctly: No.

If you archive an instance of B then when you unarchive it you get an instance of B , likewise for instances of A and C .

However, in non-technical language, a subclass is everything its superclass is plus some extra bits. You can't[*] take an instance of a superclass, created directly or from unarchiving, and make it into an instance of one of its subclasses - the "extra bits" are not there.

HTH

[*] Before someone comments: Yes, this is not absolutely true in all situations, but you are entering obscure, highly specialised, and dangerous waters. Don't go there.

Addendum

Seems you might be in the category where changing the class of an object is safe: you should be able to change the class of an instance to a subclass provided the subclass adds no instance variables or properties. To do this you can use the Objective-C runtime functions :

id obj = ... some instance of class A
object_setClass(obj, [B class]); // where B is a subclass of A which adds no properties or variables

You can also use the unarchiving classes to set the class.

Do not do this casually. Consider alternatives, but sometimes it is an appropriate solution.

Answer 2

The unarchiver wants to create the same type of class that was perviously archived, and that's generally what you want too.

You could look at using setClass:forClassName: to change the class that will be unarchived, but you stand a good chance of ruining everything sooner or later...

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Based on the particle comment:

Another option is to take the superclass instance and 'wrap' it in your subclass instance. So, you can archive and unarchive your subclass instance and it will contain the superclass instance. Any method that your subclass doesn't directly respond to you can forward on to the superclass instance.

In this way you have 2 objects but you aren't tampering with anything or relying on implementation details of classes that you don't own.