Re: Classwide Parameter?

[Simon Wright <simon@pushface.org>]

matthias_k <nospam@digitalraid.com> writes:

> Thanks for that answer. However, I'm still having problems (I have
> rewritten the code to be in one single package now):
> 
> <snip>
> package Graphics is
> 
>     type Shape is abstract tagged null record;
>     procedure Draw (Obj: Shape'Class);

Replace this with 
   procedure Draw (Obj : Shape) is abstract;
as in

package Graphics is

    type Shape is abstract tagged null record;
    procedure Draw (Obj: Shape) is abstract;

    type Circle is new Shape with record
       Radius: Float;
       Center: Float;
    end record;
    procedure Draw (Obj: Circle);

    type Square is new Shape with record
       Size: Float;
    end record;
    procedure Draw (Obj: Square);

end;
with Ada.Text_IO; use Ada.Text_IO;
package body Graphics is

   procedure Draw (Obj: Circle) is
   begin
      Put_Line ("circle");
   end Draw;

   procedure Draw (Obj: Square) is
   begin
      Put_Line ("square");
   end Draw;

end Graphics;
with Graphics;
use Graphics;
procedure Demo is
    type Reference is access all Shape'Class;
    Object: Reference;
begin
    Object := new Circle;
    Draw( Object.all );

    Object := new Square;
    Draw( Object.all );
end;

(use gnatchop)

which said

   smaug.pushface.org[2]$ gnatmake demo
   gcc -c demo.adb
   gcc -c graphics.adb
   gnatbind -x demo.ali
   gnatlink demo.ali
   smaug.pushface.org[3]$ ./demo
   circle
   square
   smaug.pushface.org[4]$ 



If you say
    type Reference is access all Shape'Class;
then your procedure
    procedure Draw (Obj: Shape'Class);
is bound to get called, since A_Reference.all is a Shape'Class.

-- 
Simon Wright                               100% Ada, no bugs.