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From: Ted Dennison <dennison@telepath.com>
Subject: Re: Freezing question
Date: 1999/07/20
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Date: 1999-07-20T00:00:00+00:00
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In article <37937E54.8D3AA9F0@mitre.org>,
  "Robert I. Eachus" <eachus@mitre.org> wrote:
>     The usual is either to declare a class wide operation, and
dispatch
> in the body or to define the operation over a wider scope than
necessary
> and raise an exception when necessary.
>     type Widget is abstract tagged private;
>     procedure Add (Child  : in out Widget;
>                    Parent : in out Widget'Class);
>     type Container is abstract new Widget with private;
>
>     ...
>     procedure Add (Child  : in out Widget
>                    Parent : in out Widget'Class) is
>     begin
>       if not Parent in Container'Class then raise ... end if;

That's pretty much what I ended up doing. The only difference is that I
had a "Container" operation to call, so I upcasted Parent to
"Container'Class". I should get a constraint_error if it isn't in
Container'Class.

>     Of course, IMHO, you want Widget and Container to be related so
that
> Parent is actually a wider class than Widget, but that is another
> issue...

I don't think so. There are operations that Containers need to perform
that don't make sense for all Widgets (eg: Adding a child widget to
manage). There are operations that all Widgets including containers need
to be able to perform (redrawing themselves). Containers are just a kind
of widget that has the extra capability of containing and managing other
widgets. But a Container should be able to contain a widget that is not
itself a container. %-)

--
T.E.D.


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