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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
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Subject: Re: accessing record element (question to the pros)
Date: Fri, 24 Feb 2006 15:46:05 -0600
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Date: 2006-02-24T15:46:05-06:00
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"Simon Wright" <simon@pushface.org> wrote in message
news:m2oe0w7f3c.fsf@grendel.local...
> "Jeffrey R. Carter" <spam@spam.com> writes:
>
> > However, in general, you can't do what you seem to be trying to
> > do. Variant records do not allow unchecked type conversion; the
> > language rules prohibit it. That's what Ada.Unchecked_Conversion is
> > for.
>
> GNAT (certainly in recent versions) has pragma Unchecked_Union
>
(http://gcc.gnu.org/onlinedocs/gcc-4.0.2/gnat_rm/Pragma-Unchecked_005fUnion.
html#Pragma-Unchecked_005fUnion).
>
> NB, compiler-specific.

It is present in a number of Ada 95 compilers (at least the Ada
Magic-derived ones as well as GNAT), and is part of Ada 2005 (see
http://www.adaic.com/standards/05rm/html/RM-B-3-3.html). So it's not really
compiler-specific.

OTOH, Unchecked_Union is *only* intended for interfacing to C. In
particular, using it to get the effect of Unchecked_Conversion is strongly
discouraged. Indeed, any such use is defined to be erroneous (see the Note
at the end of B.3.3 - there's no explicit rule because it follows from the
rules for check suppression). It might work, but a compiler could raise an
exception or reformat your hard disk as well...

If you want this to work reliably, use Unchecked_Conversion. I don't endorse
tricks with 'Address, either, because they can confuse optimizers and are
just too tricky to understand for the maintenance programmers to follow. Ada
has a special dispensation that Unchecked_Conversion doesn't have to make a
copy (unlike other functions), so there's really no good reason for using
tricky overlays. If Unchecked_Conversion is too slow, lean on your compiler
vendor!!

                         Randy.