Is it defined behavior to change an Objective-C object's isa?

Question

In Objective-C, you can change an object's dynamic type at runtime by assigning to it's isa member variable:

id object = ...; object->isa = [SomeClass class];

Is this undefined behavior? I'm currently doing this as a kludge for something else, and it appears to be working, but I feel so dirty doing it this way. The new class I'm setting doesn't add any member variables, it just overrides one method and adds a new one, so the class size is the same. I feel like if I changed the object size, much badness would result.

possible duplicate of Objective-C: How to change the class of an object at runtime? — Gabriele Petronella
– Gabriele Petronella, Commented Aug 28, 2013 at 15:34
You can run into to problems with KVO. They use isa-swizzling too. Mike Ash has a complete solution however: mikeash.com/pyblog/… — nielsbot
– nielsbot, Commented Aug 28, 2013 at 15:42

Darron · Accepted Answer · 2009-02-17 16:59:03Z

I think this is, as you say, dirty.

I suspect it will work if:

The newly assigned class is a subclass of the real class.
The new class doesn't add any member variables.

But I'll admit I don't know the nuts-and-bolts implementation of your Objective-C runtime system. I just know what makes sense to me for an implementation.

benzado · Accepted Answer · 2013-08-28 15:30:53Z

The Objective-C runtime now provides a function to do this: object_setClass. The documentation doesn't say what restrictions there are, but (as others have stated) I imagine you would want to restrict the new class to a class with exactly the instance variable layout as the original class.

olliej · Accepted Answer · 2009-02-18 05:05:30Z

It's really bad form to change these, and it makes all sorts of assumptions about runtime behaviour -- you're much better off using the runtime functions although they don't provide you with a mechanism to directly change the class.

Community · Accepted Answer · 2017-05-23 10:34:46Z

This answer by Joshua Weinberg provides the right way to do this. It worked for me. :)

And Darron was right with his educated guess :) it will only work if it's a superclass of the current object, and if you don't add any members.

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