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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: How to write TYPECASE in Ada 95?
Date: 1999/02/06
Message-ID: <m3u2wzxn4j.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <79fct8$9k3$1@murdoch.acc.Virginia.EDU>
 <0JJEkbcU#GA.219@pet.hiwaay.net> <m3679fz3wv.fsf@mheaney.ni.net>
NNTP-Posting-Date: Sat, 06 Feb 1999 12:59:05 PDT
Newsgroups: comp.lang.ada
Date: 1999-02-06T00:00:00+00:00
List-Id: <comp.lang.ada>

Matthew Heaney <matthew_heaney@acm.org> writes:

> with Points; use Points;
> procedure Test_Points is
> begin
> 
>    declare
>       P : Tall_Point := (3.0, 4.0, 5.0);
>    begin
>       Points.Show_Type (P);
>       Points.Show_Properties (Point'Class (P));
>    end;
> 
>    declare
>       P : Painted_Point := (3.0, 4.0, Points.Blue);
>    begin
>       Points.Show_Type (P);
>       Points.Show_Properties (Point'Class (P));
>    end;
> 
> end Test_Points;


I should have pointed out that you don't actually need to dispatch
Show_Properties.  At the invocation, you know the specific type of the
point, so why convert it to the class-wide type?

procedure Test_Points is
begin

   declare
      P : Tall_Point := (3.0, 4.0, 5.0);
   begin
      Points.Show_Type (P);
      Points.Show_Properties (P);
   end;

   declare
      P : Painted_Point := (3.0, 4.0, Points.Blue);
   begin
      Points.Show_Type (P);
      Points.Show_Properties (P);
   end;

end Test_Points;


The object model of Ada95 was designed to allow the caller to choose
whether dispatching occurs.  All the calls in the example above are
static, which is all they need to be.