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From: Bob Duff <bobduff@theworld.com>
Newsgroups: comp.lang.ada
Subject: Re: Declaring constants for a abstract tagged type and concrete sub
 classes
Date: Mon, 06 Jul 2015 13:02:06 -0400
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Date: 2015-07-06T13:02:06-04:00
List-Id: <comp.lang.ada>

Serge Robyns <serge.robyns@gmail.com> writes:

> type T_Root ...
> No_Root_Element : T_Root := (a, b, c);
>
> type T_Sub is new T_Root ....
> No_Sub_Element : T_Sub := ( ??? No_Root_Element ???, x, y z);

One possibility is to declare T_Root with default values for the
components, such that those defaults are what you want for a "No_...".
Then you can do:

    No_Sub_Element : T_Sub := (T_Root with X => X, Y => Y, Z => Z);

Or if T_Sub also has meaningful defaults, then:

    No_Sub_Element : T_Sub := (T_Root with others => <>);

I don't much like using default values, but in this kind of case
it might be the best choice.

- Bob