From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM
	autolearn=unavailable autolearn_force=no version=3.4.4
X-Received: by 10.67.21.205 with SMTP id hm13mr54824380pad.16.1436190575604;
        Mon, 06 Jul 2015 06:49:35 -0700 (PDT)
X-Received: by 10.140.36.170 with SMTP id p39mr526058qgp.28.1436190575559;
 Mon, 06 Jul 2015 06:49:35 -0700 (PDT)
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!news.glorb.com!qs7no894542igc.0!news-out.google.com!4ni66449qgh.1!nntp.google.com!w90no958914qge.0!postnews.google.com!glegroupsg2000goo.googlegroups.com!not-for-mail
Newsgroups: comp.lang.ada
Date: Mon, 6 Jul 2015 06:49:35 -0700 (PDT)
In-Reply-To: <mndq80$h9m$1@dont-email.me>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com;
 posting-host=94.107.233.114;
 posting-account=6m7axgkAAADBKh082FfZLdYsJ24CXYi5
NNTP-Posting-Host: 94.107.233.114
References: <0ade6abf-34c0-46e2-8bd9-d60a00056321@googlegroups.com>
 <mndq80$h9m$1@dont-email.me>
User-Agent: G2/1.0
MIME-Version: 1.0
Message-ID: <28d2406c-d579-4710-9b34-108317fe9abf@googlegroups.com>
Subject: Re: Declaring constants for a abstract tagged type and concrete sub
 classes
From: Serge Robyns <serge.robyns@gmail.com>
Injection-Date: Mon, 06 Jul 2015 13:49:35 +0000
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Xref: news.eternal-september.org comp.lang.ada:26635
Date: 2015-07-06T06:49:35-07:00
List-Id: <comp.lang.ada>

On Monday, 6 July 2015 13:55:02 UTC+2, G.B.  wrote:
> On 06.07.15 12:10, Serge Robyns wrote:
> > package Abstract_Root is
> >     type T_Root is abstract tagged with private;
> >     procedure Set_XYZ (Self: in out T_Root; ABC : in Integer);
> >     function Get_XYZ (Self: in T_Root) return Integer;
> > private
> >     type T_Root is abstract tagged record
> >        XYZ : Integer;
> >     end record;
> > end Abstract_Root;
>=20
> In case you can afford a publicly abstract but
> privately non-abstract type,
>=20
> package Abstract_Root is
>     type T_Root is abstract tagged  private;
>     procedure Set_XYZ (Self: in out T_Root; ABC : in Integer);
>     function Get_XYZ (Self: in T_Root) return Integer;
>     No_Element : constant T_Root;  -- or T_Root'Class;
> private
>     type T_Root is tagged record
>        XYZ : Integer;
>     end record;
>     No_Element : constant T_Root :=3D T_Root'(Xyz =3D> 666);
> end Abstract_Root;
>=20
> Every object needs to be of a concrete type, whether constant
> or not.
>=20
> If what is wanted is some information about any object
> in T_Root'Class (its derivation hierarchy), such as
> being a "special" object acting as "no object", say,
> a function (to override) might do.

Seems not to work as expected, at least not on GNAT 2015.

If I use No_Element : constant T_Root; =3D> I get "type of object cannot be=
 abstract" compile error, which is what I used to have.  I thought your sol=
ution was to address a freezing rule issue.

When I use 'Class I get "type of aggregation cannot by class-wide" compile =
error.

Currently, I'm using an access pointer and use null to validate the result =
of searches.  It works fine, but I'm having for example Ada Containers that=
 merely contains a access pointer to the an allocated object.  And the whol=
e pointer mess I would love to avoid.  This is how I used to work in my C-d=
ays, while I've chosen Ada for my project for a set of reasons such as read=
ability, reliability and it's track record in mission critical systems.