I was working on a utility class and was playing with the idea of using the id of an object (e.g. NSObject) as a key in NSDictionary. Setting it directly doesn’t work though: NSObject *obj = [[[NSObject alloc] init] autorelease]; // we’ll use this as the key NSMutableDictionary *dict = [[[NSMutableDictionary alloc] init] autorelease]; [dict [...]