An object is polymorphic when its specific type may vary.
The types a specific value may take, is called class.

It is trivial to copy an object if its type is known:

int x;
int y = x;
Here x is not polymorphic, so y is declared of same type (int) as x.
But if the specific type of x were unknown, then y could not be declared of any specific type.

The task: let a polymorphic object contain an instance of some specific type S derived from a type T.
The type T is known.
The type S is possibly unknown until run time.
The objective is to create an exact copy of such polymorphic object (not to create a reference, nor a pointer to).
Let further the type T have a method overridden by S.
This method is to be called on the copy to demonstrate that the specific type of the copy is indeed S.

/*REXX program to  copy  (polymorphically)  one variable's value into another variable. */
b= 'old value.'
a= 123.45
b= a                                             /*copy a variable's value into another.*/
if a==b  then say "copy did work."
         else say "copy didn't work."            /*didn't work, maybe ran out of storage*/
                                                 /*stick a fork in it,  we're all done. */