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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: What makes a procedure call 'dispatching' in Ada?
Newsgroups: comp.lang.ada
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Date: Fri, 20 Nov 2009 15:31:26 +0100
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Date: 2009-11-20T15:31:26+01:00
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On Fri, 20 Nov 2009 15:56:50 +0200, Niklas Holsti wrote:

> Dmitry has philosophical objections to redispatching, which is why he 
> suggests that you should declare P with a class-type parameter THIS in 
> the first place, so that A (THIS) will dispatch.

I would not call them philosophical. If you see the code like

if X = 1 then
   ...
   if X = 1 then
      ...
   end if;
   ...
   if X = 1 then
      ...
   end if;
   ...

you start suggesting that there could be something wrong here. Re-dispatch
is not an error it is an indication of a potential design problem. Why
would you need to check twice the same tag? That requires an explanation in
the design document and comments in the code.

A technical issue is that if we had tagged types of value semantics (i.e.
classes rooted in Boolean, Integer etc), then re-dispatch could not work
with them. I.e. from Ada's agnostic point of view on by-value vs.
by-reference, re-dispatch (or speaking more generally, view conversion to
T'Class) is an implementation-specific kludge.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de