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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Callback in Ada (User Data argument)
Date: Mon, 29 Nov 2010 19:35:52 -0600
Organization: Jacob Sparre Andersen
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Date: 2010-11-29T19:35:52-06:00
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"Warren" <ve3wwg@gmail.com> wrote in message 
news:Xns9E3F69D4E88CDWarrensBlatherings@188.40.43.230...
...
> The problem with the "user data" argument, is that you don't
> know what the user's type is going to be (in C, this is given
> as a void *). The Ada dirty approach might be to use
> 'System_Address, but that seems wrong. Still another approach
> is to use a generic.
>
> Perhaps the best avenue might be to declare a base tagged
> type, that the user can extend. Then the user data argument
> could be the base classwide type.

That is the best avenue, unless the "user data" type can be (or needs to be) 
part of the "root" tagged object. For instance, in Claw, we used the "user 
data" type in order to provide a link between the Windows handle and the 
Claw object. That's OK in that if you need private "user data", you can just 
add it to your own extension of the Claw data type.

                             Randy.