-- example: type class (this) --

library ieee;
   use ieee.std_logic_1164.all;
package element_pack_typeclass is

type Element is class
   class attribute Value: integer;
   impure function get_value return integer;
   impure function equal(Other: Element'CLASS) return boolean;
   impure function different(Other: Element'CLASS) return boolean;
end class Element;

type Element is class body
   impure function get_value return integer is
   begin
      return Value;
   end;
   impure function equal(Other: Element'CLASS) return boolean is
   begin
      return (Value = Other.get_value);
   end;
   impure function different(Other: Element'CLASS) return boolean is
   begin
      return not this.equal(Other); -- access to current object
   end;
end class body Element;

type Child_Of_Element is new class Element with
   impure function equal(Other: Element'CLASS) return boolean;
   impure function different2(Other: Element'CLASS) return boolean;
end class Child_Of_Element;

type Child_Of_Element is class body
   impure function equal(Other: Element'CLASS) return boolean is
   -- redefines former method "equal"
   begin
      return (Value = Other.get_value + 5);
   end;

   impure function different2(Other: Element'CLASS) return boolean is
   begin
      return different(Other); -- which "equal" in "different" 
                               -- will be used?
   end;

end class body Child_Of_Element;
end package;