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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: why this construct causes aliasing problem in 2012 with functions
 allowing In Out?
Date: Thu, 21 Jun 2012 15:06:15 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-06-21T15:06:15-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:ikgnsser1io8$.1ofpbj9hcpcio.dlg@40tude.net...
...
> Well, there exist possibilities to address the issue.

The issue *is* addressed. Either the slide or the summary of it is wrong.

> Let the language provided means to contract the evaluation order of some
> operations.
>
> Arguments having contracted side effects (like change(...) with its in out
> parameters) would be illegal for any operation having no contracted
> evaluation order, e.g. "-". This would make
>
>   two := change(one,two) + change(one,two)
>
> illegal.

No need to "make it" illegal; it IS illegal.

 But when the order is specified, e.g. left to right for string
> "&", then side effects in arguments would be permitted:
>
>   Name : String := Read (Socket) & Read (Socket);

I don't think the order is specified for "&"; that's the first I've heard of 
it. In any case, this is the actual rule used in Ada 2012 (it only applies 
when the order is unspecified, see the quote I previously posted).

The result is pretty complex, but normally, the compiler will tell you when 
you did something bad; otherwise, it isn't worth much energy to understand.

But this comes back to Adam's point: it wasn't worth trying to make a 
special case for operators that are communative when used alone, so there is 
no exception for "+" or any other case when the result does not actually 
depend on the order (but could have with a different body).

                                    Randy.