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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Operation can be dispatching in only one type
Date: Wed, 2 Dec 2009 23:29:49 -0600
Organization: Jacob Sparre Andersen
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Date: 2009-12-02T23:29:49-06:00
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1jcbtmi5rztyp$.norvlhez9i9$.dlg@40tude.net...
...
> No the problem is whether to treat a call to the body as legal. The 
> problem
> is that you could not rely on its contract (out Car), in order to be able
> to decide whether this call would "initialize" the object.
>
> BTW, you consider ":=" as an initialization, but why? Where is a guaranty
> that it initializes the object by a "valid" value? I feel that very 
> concept
> is somewhat inconsistent with ADT.

To expand on this point, there are a number of places in Ada where objects 
can become "deinitialized". (Compilation strategies can prevent some of 
these, but at a cost of more checks with the associated loss of performance. 
For instance, Janus/Ada prevents Unchecked_Conversion from deinitializing 
scalar objects -- but to do so requires that range/validity checks be done 
on the results of some conversions -- hardly "unchecked").

Other places (such as the result of Stream'Read and the results of 
Sequential_IO.Read for composite types, and especially abort) can't be 
eliminated in any reasonable way.

If you really care about this topic, I suggest you read and reread 13.9.1 
until you understand it. (Then join the ARG and explain it to the rest of 
us. ;-)! This is not at all a simple topic, and the sort of simple rule used 
by Java does little other than give a false sense of security (and a lot of 
annoying errors in code that has no problems).

                               Randy.