Objective-C lets you add ivars without breaking subclasses (by computing and caching ivar offsets at runtime rather than compile time), but you still can't safe

Objective-C lets you add ivars without breaking subclasses (by computing and caching ivar offsets at runtime rather than compile time), but you still can't safely ever add methods to an existing class without running the risk of a subclass unintentionally overriding that method.