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 a

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));