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Subject: Re: Classwide Parameter?
References: <ckeaod$9hh$03$1@news.t-online.com>
From: Matthew Heaney <matthewjheaney@earthlink.net>
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Date: Mon, 11 Oct 2004 16:53:32 GMT
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Date: 2004-10-11T16:53:32+00:00
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matthias_k <nospam@digitalraid.com> writes:

> I have experimented a little, and I can't really figure out what I need
> classwide parameters for.

Class-wide objects dispatch their primitive operations.  The call is
dynamically bound, not statically bound.


> Consider this code:
> 
> package Shape_Pkg is
>     type Shape is abstract tagged null record;
>     procedure Draw (obj: Shape);  -- try adding "is abstract" here
> 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;
> 
> ------------
> 
> 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.

The call to draw in your example above is statically bound.  A better
way to see what's going on is to do this:

procedure Test_Draw (S : Shape'Class) is -- class-wide type
begin
   Draw (S);  -- call is dynamically bound
end;

We can now rewrite your example:

procedure Test is
   C : Circle;
begin
   Test_Draw (C);  --will call Circle_Pkg.Draw
end;


A better example is adding different types that derive from Shape ("are
in the Shape class," in Ada-speak):

procedure Test is
   C : Circle;
   S : Square;
   T : Triangle;
begin
   Test_Draw (C);  --will call Circle_Pkg.Draw
   Test_Draw (S);  --calls Square_Pkg.Draw
   Test_Draw (T);  --calls Triangle_Pkg.Draw
end;