Classwide Parameter?

[matthias_k <nospam@digitalraid.com>]

Hiya,

I have experimented a little, and I can't really figure out what I need 
classwide parameters for. Consider this code:

package Shape_Pkg is
    type Shape is abstract tagged null record;
    procedure Draw (obj: Shape);
end;

package Circle_Pkg is
    type Circle is new Shape with null record;
    procedure Draw (obj: Circle);
end;

...

procedure Test is
    c: Circle;
begin
    Draw( c ); -- calls Circle_Pkg.Draw
end Test;

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Now, if I change Shape_Pkg.Draw's formal parameter to Shape'Class, 
what's the difference? In both cases it's legal to call Draw on a Circle.

- Matthias