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From: stt@henning.camb.inmet.com (Tucker Taft)
Subject: Re: Ada Objects Help!
Message-ID: <D32y67.78C@inmet.camb.inmet.com>
Sender: news@inmet.camb.inmet.com
Organization: Intermetrics, Inc.
References: <3f9g1u$j4m@nps.navy.mil> <D2tKqM.54v@inmet.camb.inmet.com>
 <1995Jan24.230910.2241@nbivax.nbi.dk> <D30tDn.41x@world.std.com>
Date: Fri, 27 Jan 1995 19:58:54 GMT
Date: 1995-01-27T19:58:54+00:00
List-Id: <comp.lang.ada>

In article <D30tDn.41x@world.std.com>,
Robert A Duff <bobduff@world.std.com> wrote:
> ...
>By the way, dispatching on result is an interesting twist that most OOP
>languages don't have:
>
>    function Union(X, Y: Set) return Set;
>    function Empty_Set return Set;
>
>    X: Set'Class := ...;
>    Y: Set'Class := Union(X, Empty_Set);

>The appropriate version of function Empty_Set to call is determined (at
>run time) based on the tag of Y, since Empty_Set itself doesn't have any
                               ^^ "X"
>controlling parameters.

Although I presume Bob meant "X" in the above paragraph,
you actually *can* use the left-hand side of an assignment statement
(rather than an initialization) to control the dispatching.  
For example, presuming Y was declared as above, we can later 
assign into Y again as follows:

    Y := Union(Empty_Set, Empty_Set); 

In this example, Y is the only thing with a pre-determined tag, and it 
controls all dispatching.

>- Bob

-Tucker Taft  stt@inmet.com
Intermetrics, Inc.