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From: "G.B." <bauhaus@futureapps.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Declaring constants for a abstract tagged type and concrete sub
 classes
Date: Mon, 6 Jul 2015 13:55:01 +0200
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Date: 2015-07-06T13:55:01+02:00
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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;

In case you can afford a publicly abstract but
privately non-abstract type,

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 := T_Root'(Xyz => 666);
end Abstract_Root;

Every object needs to be of a concrete type, whether constant
or not.

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.