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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Declaring constants for a abstract tagged type and concrete sub
 classes
Date: Mon, 06 Jul 2015 16:42:17 +0100
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Date: 2015-07-06T16:42:17+01:00
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Serge Robyns <serge.robyns@gmail.com> writes:

> 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;

> package Concrete is
>    type T_Concrete is new T_Root with private;
>    No_Element : constant T_Concrete;
>    ....
> private
>    type T_Concrete is new T_Root with record
>       ...
>    end record;
>    No_Element : constant T_Concrete := (Abstract_Root.No_Element, ...);
> end Concrete;

You could write

   with Abstract_Root;
   package Concrete is
      type T_Concrete is new Abstract_Root.T_Root with private;
      No_Element : constant T_Concrete;
   private
      type T_Concrete is new Abstract_Root.T_Root with record
         ABC : Integer;
      end record;
      No_Element : constant T_Concrete
        := (Abstract_Root.T_Root with ABC => -42);
   end Concrete;

(or whatever will suit you)

but I think you'll have to give T_Root.XYZ an equivalent default value,
or equality won't work.