[Data]

a function returning class a (which is in reality a class B)
then casting that "a" to a " b"

that's dynamic casting example

a* input;

b* result= dynamic_cast<b*>(input);

if b is not in reality a a then null is returned (well I think actually a exception is thrown)

you do this let a function accept both "b" and "c" through the "a" baseclass pointer.