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From: Ted Dennison <dennison@telepath.com>
Subject: Re: OO puzzle
Date: 1999/12/22
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In article
<Pine.A41.3.96-heb-2.07.991222103646.28794B-100000@pluto.mscc.huji.ac.il
>,
  Ehud Lamm <mslamm@mscc.huji.ac.il> wrote:

> This example is again from the Eiffel book.
>
> 1. Truck_Driver IS-A Driver (one hierrachy)
>
> 2. Motor_Vehicle (and all its kids)  support the operation
> register_Driver(d:driver) the paramter is of type driver above.
>
>    Car IS-A Motor_Vechicle
>    Truck IS-A Motor_Vehicle
>      Truck wants to override register_driver(d:truck_driver) becuase
> only truck drivers should be allowed here.
>
> Can yuo do this in Ada? Try, and show your error messages! How can you
> do something to achive the same result, in Ada?

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;

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.

--
T.E.D.


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