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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: Classwide Parameter?
Date: Tue, 12 Oct 2004 10:10:37 +0200
Organization: AdaCL
Message-ID: <14269526.aqDQ52X6SC@linux1.krischik.com>
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Date: 2004-10-12T10:10:37+02:00
List-Id: <comp.lang.ada>

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

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