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From: AdaMagica <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Wikibook on Ada Programming - Object Orientation
Date: Thu, 8 Jul 2010 02:34:22 -0700 (PDT)
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Date: 2010-07-08T02:34:22-07:00
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I have a problem with an example in
http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation#Multiple_Inheritance

It proposes to use a mixin together with the Rosen trick, however for
unlimited types. This cannot work!

When such an object is assigned, the disciminant will inevitably point
to the wrong object.

For a detailed discussion of the problems involved, see my paper in
Ada Letters, Volume XIX, Number 4, December 1999, which you can also
find here:

http://www.christ-usch-grein.homepage.t-online.de/Ada/Finalization.html

Ada 2005 changed some of the rules; now mix-ins can have access
discriminants to nonlimited types, but the problem persists.

So how should the wikibook be corrected? Are there safe solutions to
the problem of mixins?