Problem Statement

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: 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.

Let's start with the solution:

'======
class T
'======

float vv

method constructor(float a=0) {vv=a}
method destructor    {}
method copy as T     {new T ob : ob<=vv : return ob}
method mA() as float {return vv*2}
method mB() as float {return vv*3}

end class


'======
class S
'======

has T

method mB() as float {return vv*4} 'ovveride

end class

'====
'TEST
'====

new T objA(10.5)

let objB = cast S objA.copy

print objA.mb 'result 31.5
print objB.mb 'result 42

del objA : del objB