Yep, categories are exactly that. Since it’s a C derivative, and ultimately stores instance variables in struct-alikes, there’s no way to add instance variables and maintain binary compatibility, at least in today’s Obj-C runtime. So, while regrettable, it sure is a lot more flexibility than C++ programmers have…

Right. In practice, the Ruby approach, where you can extend a class at any time later, is very similar to the usage of Objective-C categories in practice. In Objective-C, if you need per-instance variables in a category, you can maintain a hash table with pointers to objects. This is tricky to implement, however, because object addresses get reused, and unless you can be notified of the destruction of an object, you may not be able to verify that you have the same object as in a previous invocation of the category methods.

A workaround for this inherent limitation of Objective-C is to use poseAs:. poseAs: might be only available to users of OPENSTEP -derived frameworks or GNUstep, or it may be part of the core Obj-C specification (if such a thing exists). I’m not quite sure :)

Moving on, however, poseAs: allows you to specify a class as a subtitute for an existing one that receives all invocations on that class. You could therefore get the behavior you’re asking as follows:

@interface myPosedClass : someOtherClass {
  id holy,crap,look,at,these,additional;
  id instance,variables;
}
-doSomethingSpecial;
@end

[myPosedClass poseAs:[someOtherClass class]];
someInstance = [[[someOtherClass alloc] init]
   doHorribleThings];

And in that case, the object that is instantiated and receiving the message is actually an instance of myPosedClass

Of course, poseAs: currently has its own limitations, namely that you can not un-pose, or re-pose after it has been done. However, if you’re aiming for true dynamism, you could always pose a class that overrides performSelector: and transparently proxies all messages to other classes; this behavior is easily configurable at runtime.

But adding instance variables will break any subclasses of that class.