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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: A bad counterintuitive behaviour of Ada about OO
Date: Fri, 8 Aug 2014 17:32:23 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:21592
Date: 2014-08-08T17:32:23-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:2extr7bydwfw.tcda12f9xnq9.dlg@40tude.net...
...
> The only case where this may happen is parallel type hierarchies when two
> types are inherited synchronously, e.g.
>
>   type Object is tagged ...
>   type Handle is tagged ...
>   function Access (X : Handle_To_Object) return Object;
>
>   type New_Object is new Object with ...
>
> must produce
>
>   type New_Handle is new Handle with ...
>
> and
>
>   function Access (X : New_Handle) return New_Object;
>
> No OOPL I know can handle this.

I've toyed with proposing this for Ada (seriously). The containers in 
particular are tagged but cannot be usefully extended because of this 
problem. It's unclear, though, that the model would actually work.

In any case, this is the only way to do covariance of unrelated types. 
Otherwise, you're just in the realm of overloading (usually a better 
solution anyway, IMHO).

                                Randy.