Re: Converting C++ class to Ada

["Norman H. Cohen" <ncohen@watson.ibm.com>]

(Removing comp.lang.c++ from list of newsgroups.)

Jon S Anthony wrote:
 
> In article <mheaney-ya023280001812962110410001@news.ni.net> mheaney@ni.net (Matthew 
> Heaney) writes:
> 
...
> > I thought that access discriminants (and parameters) without an explicit
> > type raise Program_Error when you pass "null" as the value?
> 
> CE is raised if a null value is passed to an access _parameter_.  I
> have never seen anything indicating that CE (or PE) is raised if a
> null value is given for the value of an access _discriminant_.  A good
> thing IMO on both counts.  See in particular 11.5(11)
> 
> Of course, given a recent little discussion on the GNAT chat list I
> could simply have not found it yet... :-) But really, this point seems
> pretty clear!

No, Constraint_Error is raised for both a null access parameter and a
null access discriminant.  This point was just raised on the GNAT chat
list (probably as a result of the discussion here in comp.lang.ada), and
I replied, in part, as follows:

A null access discriminant is a run-time error.

11.5 is just a list of the language-defined checks, together with an
informal description of when the checks occur.  The definitive rules
about when a check is performed will be found in the "Dynamic Semantics"
sections describing the execution, evaluation, or elaboration of the
relevant constructs.

The relevant construct in this case is a discriminant constraint:  It is
during the elaboration of a discriminant constraint that a null access
discriminant would be caught.  RM 3.7.1(12) says:  "For the elaboration
of a discriminant_constraint, the expressions in the
discriminant_associations are evaluated in an arbitrary order and
converted to the type of the associated discriminant (which might raise
Constraint_Error--see 4.6)...."   Hidden in 4.6(49) is the rule, "If the
target type is an anonymous access type, a check is made that the value
of the operand is not null...."  (4.6(57) states that the exception
raised when this check fails is Constraint_Error.  The rule in RM
4.6(49) can be found using the index entry for "Access check".)

-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen