Polymorphic copy
You are encouraged to solve this task according to the task description, using any language you may know.
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: <c> int x; int y = x; </c> 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.
Ada
<ada> with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Polymorphic_Copy is
package Base is
type T is tagged null record;
function Name (X : T) return String;
end Base;
use Base;
package body Base is
function Name (X : T) return String is
begin
return "T";
end Name;
end Base;
-- The procedure knows nothing about S
procedure Copier (X : T'Class) is
Duplicate : T'Class := X; -- A copy of X
begin
Put_Line ("Copied " & Duplicate.Name); -- Check the copy
end Copier;
package Derived is
type S is new T with null record;
overriding function Name (X : S) return String;
end Derived;
use Derived;
package body Derived is
function Name (X : S) return String is
begin
return "S";
end Name;
end Derived;
Object_1 : T; Object_2 : S;
begin
Copier (Object_1); Copier (Object_2);
end Test_Polymorphic_Copy; </ada> The procedure Copier does not know the specific type of its argument. Nevertheless it creates an object Duplicate of exactly same type. Sample output:
Copied T Copied S