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From: petter_fryklund@hotmail.com
Newsgroups: comp.lang.ada
Subject: Re: Dispatching problem.
Date: Tue, 15 Jan 2008 12:01:41 -0800 (PST)
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Date: 2008-01-15T12:01:41-08:00
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On 15 Jan, 20:49, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Tue, 15 Jan 2008 10:55:46 -0800 (PST), petter_frykl...@hotmail.com
> wrote:
>
> > I get compilation error when I =A0try to dispatch a call using class
> > wide access variables stating that I cannot call abstract program.
>
> > Perhaps oversimplified example is:
>
> > package PBN is
> > =A0 =A0type Item_T is abstract tagged null record;
> > =A0 =A0type Item_Ptr is access all Item_T'Class;
> > =A0 =A0function =A0Create (From : in String) return access Item_T'Class =
is
> > abstract;
> > =A0 =A0function =A0Img (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:
>
> =A0 =A0 function =A0Img (From : access Item_T) return String is abstract;
> (better: =A0function =A0Img (From : Item_T) return String is abstract;)
>
> BTW, same applies to Create. It is a factory, so it should be
>
> =A0 =A0function =A0Create (From : in String)
> =A0 =A0 =A0 return access Item_T is abstract;
> (better: =A0function =A0Create (From : in String) return Item_T;) ]
>
> > end PBN;
>
> > package PBN.Site is
> > =A0 =A0type Site_T is new Item_T with
> > =A0 =A0 =A0 record
> > =A0 =A0 =A0 =A0 =A0Name : =A0ASU.Unbounded_String;
> > =A0 =A0 =A0 end record;
> > =A0 =A0type Site_Ptr is access all Site_T;
>
> > =A0 =A0function =A0Create (From : in String) return Site_Ptr;
> > =A0 =A0function =A0Img (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
> > =A0 =A0 Site : =A0PBN.Item_Ptr;
> > begin
> > =A0 =A0 Site :=3D PBN.Site.Create ("[Site qwerty]");
> > =A0 =A0 Ada.Text_IO.Put_Line (Site.Img); =A0 =A0 =A0 =A0 =A0 =A0 <------=
-----------
> > 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 ar=
e
> using that (damned (:-)) prefix notation anyway...
>
> --
> Regards,
> Dmitry A. Kazakovhttp://www.dmitry-kazakov.de

Thanks!