Re: Classwide Parameter?

From: Martin Krischik (krischik_at_users.sourceforge.net)
Date: 10/12/04 

Date: Tue, 12 Oct 2004 10:10:37 +0200

matthias_k wrote:

> 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);

You want an abstract method not a class wide procedure:

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;
> </snip>
>
> Now, what I want is to have different implementations of the Draw method
> for each Subtype. However, if I run this program:
>
> <snip>
> 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 );

Only use pointer in Ada if you realy need to:

Test_1:
declare
     Object: Shape'Class := Circle'(Radius => 10.0, Center => 10.0);
begin
   Draw( Object);
end

Test_2:
declare
     Object: Shape'Class := Square'(Size => 10.0);
begin
   Draw( Object);
end

> end;

> </snip>
>
> The Shape's Draw method is always called here but it shouldn't. No late
> binding happens. I have tried to make it abstract, but it didn't even
> compile then. What's wrong?
>
> - Matthias 

-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com

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