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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Breaking a circularity
Date: Mon, 28 Mar 2011 22:01:24 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2011-03-28T22:01:24-05:00
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"Gene" <gene.ressler@gmail.com> wrote in message 
news:f22049a1-0983-4c22-ba94-b5da81aaba88@s9g2000yqm.googlegroups.com...
On Mar 28, 7:37 am, Simon Wright <si...@pushface.org> wrote:
>Yes and no.  The only reason for making Event_Queue generic is
>so that a State can be passed through the dispatching Handle call.
>Since this is an "in out" parameter that's invariably a record type,
>it's being passed by pointer behind the scenes.  Yet there doesn't
>seem
>to be a way to take advantage of *this* pointer for breaking the
>circularity.  (In (eeewwww) C  we could use a void* or other pointer
>trick here.)

The way we handled this problem in Claw was to make State an abstract tagged 
type (rather than using a generic). Then, the parameter to the Handle call 
can be class-wide, which allows it to access any extension of the type.

I suspect that this also breaks the circularity (since the State type 
doesn't need to be defined until much later), but I haven't tried to figure 
it out.

                    Randy.