Slide R-18


// ERROR; can't instantiaten abstract class; class person has
// pure virtual method walk()
person paula;  

// OK; freshman is subclass of student
student *stu = new freshman();

// OK; statically dispatches to method student::enroll(); if
// enroll() were declared virtual in student or a superclass
// of student, method freshman::enroll() would have been
// called
stu->enroll();

// OK; *stu's dynamic type is freshman, which is a
// subclass of student; sara gets "student" parts (i.e. not
// freshman-specific) of *stu; loss of information
student sara = *stu;

// OK; statically dispatches to method person::run() because
// sara's type is student, and student inherits but does not
// override this method
sara.run();

// OK; student is a subclass of person; pp now points to
// dyn. allocated freshman instance
person *pp = stu;

// OK; run() declared virtual in person, so dynamically
// dispatches to freshman::run()
pp->run();

// OK; walk() declared (pure) virtual in person, so
// would dynamically dispatch to freshman::walk(), but
// freshman doesn't define walk(), so runs inherited
// student::walk() 
pp->walk();

// ERROR; pp is a person ptr., and person is not a subclass
// of freshman (and is obviously not the same class,
// freshman) 
freshman *fred = pp;

// INVALID; see above
fred->enroll();