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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Dispatching problem.
Newsgroups: comp.lang.ada
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Date: Tue, 15 Jan 2008 20:49:33 +0100
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On Tue, 15 Jan 2008 10:55:46 -0800 (PST), petter_fryklund@hotmail.com
wrote:

> I get compilation error when I  try to dispatch a call using class
> wide access variables stating that I cannot call abstract program.
> 
> Perhaps oversimplified example is:
> 
> package PBN is
>    type Item_T is abstract tagged null record;
>    type Item_Ptr is access all Item_T'Class;
>    function  Create (From : in String) return access Item_T'Class is
> abstract;
>    function  Img (From : access Item_T'Class) return String is
> abstract;

This is a class-wide operation. As such it is not primitive (and does not
dispatch). For non-primitive operations declaring abstract means
disallowing in Ada.

[ My guess, you wanted:

    function  Img (From : access Item_T) return String is abstract;
(better:  function  Img (From : Item_T) return String is abstract;)

BTW, same applies to Create. It is a factory, so it should be

   function  Create (From : in String)
      return access Item_T is abstract;
(better:  function  Create (From : in String) return Item_T;) ]

> end PBN;
> 
> package PBN.Site is
>    type Site_T is new Item_T with
>       record
>          Name :  ASU.Unbounded_String;
>       end record;
>    type Site_Ptr is access all Site_T;
> 
>    function  Create (From : in String) return Site_Ptr;
>    function  Img (From : access Site_T) return String;

This is an "overloading" for the disallowed operation above.

> end PBN.Site;
> 
> with Ada.Text_IO;
> with PBN;
> with PBN.Site;
> procedure Print is
>     Site :  PBN.Item_Ptr;
> begin
>     Site := PBN.Site.Create ("[Site qwerty]");
>     Ada.Text_IO.Put_Line (Site.Img);             <-----------------
> cannot call abstract program

This calls to the disallowed Img from PBN, the compiler tells you so.

> end Print;
> 
> Any suggestions (apart from taking extensive course in OOD ;-) ?

Use new keywords of Ada 2005 indicating a desire to override, since you are
using that (damned (:-)) prefix notation anyway...

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