Fair enough.

Here's how you'd make that class on the fly:

   Class mySubclass = objc_allocateClassPair([NSObject class], "MySubclass", 0);

Here's how you'd add a description method to a newly created class:

   static NSString *Description(id self, SEL _cmd){
        return [NSString stringWithFormat: @"<%@ %p: foo=%@>",[self class], self, [self foo]];
   }

    // add Description to mySubclass
    // grab NSObject's description signature so we can borrow it
    Method description = class_getInstanceMethod([NSObject class],
                                                 @selector(description));
    const char *types = method_getTypeEncoding(description);
    
    // now add
    class_addMethod(mySubclass, @selector(description), (IMP)Description, types);

And your iVars:

   class_addIvar(mySubclass, "foo", sizeof(id), rint(log2(sizeof(id))), @encode(id));