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Date: Mon, 6 Jul 2015 03:10:22 -0700 (PDT)
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Subject: Declaring constants for a abstract tagged type and concrete sub
 classes
From: Serge Robyns <serge.robyns@gmail.com>
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Xref: number.nntp.giganews.com comp.lang.ada:193937
Date: 2015-07-06T03:10:22-07:00
List-Id: <comp.lang.ada>

Hi,

I'm new on Ada OO-programming and I'm facing this dilemma on a class hierar=
chy.

The idea is to have a root type with some common data parts and common inte=
rface and some concrete procedures and functions but to force to have real =
variables through subclasses.

As such I've therefore the following (using Get/Set as examples only):

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;

As I want this class hierarchy to be usable without access type is was plan=
ning to define No_Element constants.  However, the compiler refuses me to c=
reate constant variables, as the type is abstract.  This isn't a problem wh=
en the record has only a few components, but when there are a lot it is pai=
nful, as I was "hoping" to use the No_Element as a part of the No_Element o=
f the subclasses.  This idea came from the Ada Containers.  And to be able =
to use No_Element in if statements like

Something along:
   =20
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 :=3D (Abstract_Root.No_Element, ...);
end Concrete;


and then:

    A_Concrete : T_Concrete;
    ...
    A_Concrete :=3D A_List.Find (Expression);
       -- A_List could be a complex object with multiple components,
       -- including T_Concrete.
    if A_Concrete =3D No_Element then
       -- some error handling
    end if;


Any advise on how to handle this need in Ada?  I'll face this issue too in =
a few potential design patters I want to use (Proxy for example).  Moreover=
, this constant allows me to initialize variables at the moment of my choic=
e;

Regards,
Serge