please look at these (contradictory?) statements:

If anyone tries to make an object of an abstract class, the compiler prevents them.
Note that pure virtual functions prevent an abstract class from being passed into a function by value.../Thus, it is also a way to
prevent object slicing (which will be described shortly). By making a
class abstract, you can ensure that a pointer or reference is always
used during upcasting to that class./

a class to be passed ???

Could someone tell me how to pass a class? (especially abstract one)