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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: using an Ada.Container as a private type
Date: Thu, 03 Apr 2008 12:00:24 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2008-04-03T12:00:24-04:00
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Graham <graham.stark@virtual-worlds.biz> writes:

>    type Payment_Stream is new Payment_Stream_Package.Map;
>
> but I can't do this. Gnat complains that "type derived from tagged
> type must have extension".
> I'm fairly new to Ada but thought I was getting on top of it! What is
> the principal that stops me doing this? What should I do instead?

Not really a "principle" -- more like an arbitrary restriction.
Map is tagged, and tagged types have to obey that rule.

You should say:

    type Payment_Stream is new Payment_Stream_Package.Map
        with null record;

to create an empty extension.

- Bob