A keyed object class exists, which is derived from another keyed object class – the key classes of these two classes are different. There is an object of the derived class which has a pointer type of the base class. If the key of the object is accessed the key would be interpreted in the “format” of the base class. Since this is generally erroneous, this type of access is forbidden.
The object-key can only be accessed by a pointer of the own class.