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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: Tue, 7 Jul 2015 19:21:44 +0200
Organization: A noiseless patient Spider
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Xref: news.eternal-september.org comp.lang.ada:26684
Date: 2015-07-07T19:21:44+02:00
List-Id: <comp.lang.ada>

On 07.07.15 18:27, Serge Robyns wrote:

> Now I'm facing my other issue, how to "merge" the root and child part in a new No_Element ...  This is about valid Ada syntax.

> No_Element : constant T_Concrete := ( T_Root.No_Element, 789);

If the derived type stays within the package hierarchy,
i.e. in a child package (that's Abstract_Root.No_Element
in the extension aggregate):

...
private
   No_Element : constant Non_Abstract :=
      Non_Abstract'(No_Element with
                    Abc => -42);

That is, one can start an extension aggregate with
an expression supplying the T_Root stuff from the parent
type. Or,

...
private
   No_Element : constant Non_Abstract :=
      Non_Abstract'(T_Root with
                    Abc => -42);

where T_Root stands for the type's defaults only.
Outside the hierarchy, that could be

...
private
   No_Element : constant Non_Abstract :=
     Non_Abstract'(Abstract_Root.T_Root with
                   Abc => -42);

This presumes, then, that no_element-ness is fully determined
by the component Abc here, since Abstract_Root.No_Element's
private view is unavailable outside the package hierarchy
and so can't be depended upon.