Re: C++ to Ada95, help please

[stt@henning.camb.inmet.com (Tucker Taft)]

Mike Meier (mjmeie@ss5.magec.com) wrote:

> Tucker Taft (stt@henning.camb.inmet.com) wrote:
> : I believe GNAT supports a pragma "Unchecked_Union" (approximately) which
> : is applied to a discriminated record to make it look like a C
> : union.

> : <snip>

> : It is hoped that this pragma will become a defacto standard for Ada 95
> : compilers that support interfacing to C.

> : -Tucker Taft  stt@inmet.com
> : Intermetrics, Inc.

> I hate to pick a bone with Tuck just after approval of Ada 95.  But, if
> he hopes this will become a defacto standard, why wasn't it just included
> in the language definition?

There is a fairly big difference between a defacto standard and a dejure 
standard in terms of lead-time, energy required, etc.  If such a pragma had 
been proposed a few years ago, we could have done the work to write it up 
precisely in Reference-Manual-ease, and sent it around for review, and 
revised it, and polished it, and ...  Unfortunately, the consensus developed
around this pragma only in the past 6 months.

In any case, I would expect there to be many de-facto standards
in the area of implementation-defined pragmas, attributes, and
generally useful packages.  The Uniformity Rapporteur Group (URG)
of the ISO Working Group on Ada (WG9) was established specifically
to encourage uniformity in areas that were left open to implementor
and user creativity by the inevitably limited standardization process.

I would expect GNAT to be the vanguard for defacto standards, since
it will be freely available on so many platforms.  Other Ada 95 vendors
will probably try to follow GNAT's lead, unless there is a real
dispute about the appropriateness of a pragma or attribute.
The URG will probably emerge as the forum for resolving such disputes.

> Mike Meier
> Magnavox Electronic Systems Company

-Tucker Taft  stt@inmet.com
Intermetrics, Inc.