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From: Ehud Lamm <mslamm@mscc.huji.ac.il>
Subject: Re: OO puzzle
Date: 1999/12/23
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Date: 1999-12-23T00:00:00+00:00
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In article <83qtap$pri$1@nnrp1.deja.com>,
  Ted Dennison <dennison@telepath.com> wrote:
> In article
> <Pine.A41.3.96-heb-2.07.991222103646.28794B-
100000@pluto.mscc.huji.ac.il
> Well, I'll assume in Ada you would be talking about a routine that
> takes in driver'class and also has a "motor_vehicle" parameter on
which
> it dispatches, and it does *not* dispatch on the driver. One
possibility
> would be to have truck's version of register_driver internally check
> that the driver is in truck_driver'class:
>
> if d not in truck_driver'class then
>    raise Constraint_Error;
> end if;
>

Right. But than we are dealing with a runtime check. See the discussion
per Tucker's post.

> Another would be to disallow calls to the original register altogether
> (via "abstract" perhaps?) and to make a new one that only takes in
> truck_driver'class. That looses the dynamic dispatch from
> Motor_Vehicle'class, though.

But than you lose the ability to define the abstract interface (for
Motor_Vehicle) and know that it is supported by all descendants.

Trade offs all arround...

--
Ehud Lamm mslamm@mscc.huji.ac.il
http://purl.oclc.org/NET/ehudlamm
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