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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: tagged record child: override constructor?
Newsgroups: comp.lang.ada
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Date: Wed, 14 Sep 2005 09:49:13 +0200
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Date: 2005-09-14T09:49:06+02:00
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On Tue, 13 Sep 2005 21:29:06 +0200, Ludovic Brenta wrote:

> Martin Krischik <krischik@users.sourceforge.net> writes:
>> That's because all class members are virtual in Ada.
> 
> I think this needs to be elaborated on.
> 
> All _primitive operations of a tagged type_ are _potentially_
> "virtual" in Ada.  Each caller of such a primitive operation decides,
> at the call site, whether the dispatching is static or dynamic.

Ada's model is quite simple and logical. If T appears as a formal parameter
or the result of a primitive operation of T, then this parameter is
controlled (= the operation can dispatch on it.)

Dispatching is determined not by any site but exclusively by the actual
parameter type. It must be of T'Class.

For this reason re-dispatching never happens. To re-dispatch, one should
explicitly convert T to T'Class.

> In contrast, in (my recollection of) C++, all virtual methods dispatch
> dynamically from all call sites: the decision is not made at the point
> of call but at the point of definition of the method.

C++ model is more tricky and in some aspects more limited. Only the prefix
(distinguished) parameter can be controlled (the "this-parameter".) The
result cannot be controlled as well. There is no difference between T and
T'Class (the source of countless problems.) So all operations re-dispatch
except for constructors and destructors, which don't. I.e. it is rather C++
where the call site determines whether a virtual function dispatches.

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