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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Converting C++ class to Ada
Date: 1996/12/20
Message-ID: <E2q4Ds.942@world.std.com>#1/1
X-Deja-AN: 205144143
references: <32ADF183.7195@lmtas.lmco.com> <JSA.96Dec18184555@alexandria>
 <E2o1vz.76y@world.std.com> <32BAB8C8.7046@gsfc.nasa.gov>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-12-20T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <32BAB8C8.7046@gsfc.nasa.gov>,
Stephen Leake  <Stephen.Leake@gsfc.nasa.gov> wrote:
>I can't find a rule in the RM that says this.

Yeah, it's not easy to find.  :-(

Basically, lots of the rules about checks on parameter passing,
assignment, etc., are defined in terms of type conversion.  E.g. an
actual parameter is always implicitly converted to the type of the
formal (6.4.1(11)).  Then 4.6 specifies all the run-time checks for
conversions, including this null check.

Also, the *literal* null is illegal for these types.

It would have been helpful if we had said "An access param/discrim can
never be null." somewhere as a NOTE, instead of letting you figure it
out.  A statement like that is a lot more useful (though less specific)
than telling which combination of compile-time and run-time rules ensure
that it's true.  That is, to the programmer, the rule is "don't try to
make one of these things null", and it's not really important to know
the exact points of checking.  The compiler writer of course has to know
the details.

- Bob