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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Simple example on interfaces
Date: Tue, 26 Jan 2021 09:55:03 +0100
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On 2021-01-26 09:17, Mario Blunk wrote:

> thanks for your help. I reordered things in the example. The outcome can be seen here:
> https://github.com/Blunk-electronic/ada_training/tree/master/src/interfaces
> 
> But as you already said, p2 can not be a real record element as we can see in
> https://github.com/Blunk-electronic/ada_training/blob/master/src/interfaces/lib/pac_1.adb
> There is no source to fetch the "content" of p2 as there is no element p2.
> 
> Perhaps I did not understand your approach completely.

You must provide an implementation for P2:

    type A2 is new A1 and P2_Interface with private;
    overriding function P2 (Object : A2) return Enum;

    type B1 is new Base and P2_Interface with private;
    overriding function P2 (Object : B1) return Enum;

private
    type A2 is new A1 and P2_Interface with record
       P2_Implementation : Enum;
    end record;

    type B1 is new Base and P2_Interface with record
       P2_Implementation : Enum;
    end record;

You cannot re-use this implementation. Here you re-use the interface only.

There are tricks to re-use implementation as well. As I said I would not 
recommend them.

Anyway, for example, this is how generics can be used to hang an 
interface on a type.

generic
    type Victim is abstract new Base with private;
package Generic_Add_P2 is
    type Victim_With_P2 is
       abstract new Victim and P2_Interface with private;
    overriding function P2 (Object : Victim_With_P2) return Enum;
private
    type Victim_With_P2 is new Victim and P2_Interface with record
       P2_Implementation : Enum;
    end record;
end Generic_Add_P2;

package body Generic_Add_P2 is
    function P2 (Object : Victim_With_P2) return Enum is
    begin
       return Object.P2_Implementation;
    end P2;
end Generic_Add_P2;

package Do_A2 is new Generic_Add_P2 (A1);
type A2 is new Do_A2.Victim_With_P2 with record
    -- other members
end record;

package Do_B1 is new Generic_Add_P2 (Base);
type B1 is new Do_B1.Victim_With_P2 with record
    -- other members
end record;

You can extend that to have a setter as well.

It might even be possible to achieve the getter/setter's syntax to

    X := A2.P2;
    A2.P2 := Low;

rather than

    X := A2.P2;
    Set (A2, Low);

But I am too lazy to figure it out, as it would require dozens of helper 
and access types. I never use this horrific new Ada 2012+ stuff. Maybe 
someone else might help...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de