Homework #7, Problem 5.

#5.   Aiming a derived class pointer at a base class object may result in 
pointers to nonexistent members in the base-class object, as demonstrated 
in problem #3.  On the other hand, if a base-class pointer is aimed at a 
derived class object (the usual case), then there is no problem in downcasting 
the base-class pointer to a derived-class pointer since the object is a 
derived-class object.