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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: What makes a procedure call 'dispatching' in Ada?
Newsgroups: comp.lang.ada
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References: <he64pl$d8i$1@nntp.ilk.net>
Date: Fri, 20 Nov 2009 14:27:14 +0100
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Date: 2009-11-20T14:27:14+01:00
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On Fri, 20 Nov 2009 14:15:33 +0100, Markus Schoepflin wrote:

> I'm trying to fell may way around object oriented Ada programming, and I 
> think I must be missing something absolutely basic.

> Please consider the following package:
> 
> ---%<---
> package FOOS is
> 
>     type FOO is abstract tagged null record;
> 
>     procedure P (THIS : in FOO);
>     procedure A (THIS : in FOO) is abstract;
> 
> end FOOS;
> 
> package body FOOS is
> 
>     procedure P (THIS : in FOO)
>     is
>     begin
>        A (THIS);
>     end;
> 
> end FOOS;
> --->%---
> 
> When trying to compile this, I get:
> 
> foos.adb:6:07: call to abstract function must be dispatching
> gnatmake: "foos.adb" compilation error
> 
> What is the compiler trying to tell me here? And how do I go about calling 
> abstract procedures?

It tells you that the type of THIS is FOO, so you cannot call to A, because
A is not defined on FOO.

If P is to be defined in terms of any type from the class FOO, then P has
to be declared differently (class-wide):

package FOOS is
    type FOO is abstract tagged null record;
    procedure P (THIS : in FOO'Class);
        -- I am SAME for the whole class rooted in FOO
    procedure A (THIS : in FOO) is abstract;
        -- I have an implementation in each instance of the class except
        -- for the abstract ones like FOO.
end FOOS;

package body FOOS is
   procedure P (THIS : in FOO'Class) is
   begin
      A (THIS);
         -- I do not know what kind of FOO is THIS. So I
         -- dispatch to A according to the tag of THIS.
   end P;
end FOOS;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de