Re: Default comparison of tagged variant records

["Randy Brukardt" <randy@rrsoftware.com>]

<msimonides@power.com.pl> wrote in message
news:1165822851.141177.13460@79g2000cws.googlegroups.com...
>
> Randy Brukardt napisal(a):
> > "Adam Beneschan" <adam@irvine.com> wrote in message
> > news:1165602630.553040.134960@n67g2000cwd.googlegroups.com...
>
> > > This looks like a compiler bug to me.  It should return TRUE in those
> > > all cases.
> >
> > I agree with Adam that it *looks* like a compiler bug. OTOH, I tried it
on
> > Janus/Ada and it also gave FALSE. Something else is going on...
...
> > ...and so does the call to "=". Your program should print TRUE and this
is
> > clearly a Gnat bug. ;-)
>
> Do you mean comparing the Unbounded_String components of record? This
> works in Gnat too and I've used it to define equality for the
> troublesome type as a workaround.

No, I mean the original full record compare should work properly.

> > (If anyone cares, the problem with Janus/Ada was that it was not picking
up
> > the primitive "=" for the untagged private type Unbounded_String, as it
was
> > only looking for primitive *dispatching* operations. Thus it was using
the
> > predefined one, and that always returned False unless the objects were
the
> > same object. Gnat's problem may very well be different.)
>
> In GNAT "=" also returns True when comparing the same object with
> itself.
>
> It looks like there might be some explanation that has its roots
> somewhere in ARM, that's why I asked.

Not really. Ada 95 was rather silent on this issue: in particular, there was
no requirement that the predefined "=" work right for language-defined
types. So some implementations caused trouble by not considering this issue;
re-emergence could be a problem. The ARG thought that this was bad for
usability, so that problem was addressed in the Corrigendum (2001) by adding
4.5.2(32.1/1):

For all nonlimited types declared in language-defined packages, the "=" and
"/=" operators of the type shall behave as if they were the predefined
equality operators for the purposes of the equality of composite types and
generic formal types.

There is a further note to implementers in the AARM:

If any language-defined types are implemented with a user-defined "="
operator, then either the full type must be tagged, or the compiler must use
"magic" to implement equality for this type. A normal user-defined "="
operator for an untagged type does not meet this requirement.

Janus/Ada did indeed implement Unbounded_String as a tagged type, but a
compiler bug prevented the compiler from noticing this when creating "=" for
a type with Unbounded_String components.

GNAT seems to have a similar bug; but make no mistake, it is a bug, because
the behavior reported for GNAT violates 4.5.2(32.1/1). [As well as good
taste! ;-)]

I suspect that the main reason that implementations have bugs in this area
is that there is no ACATS test for 4.5.2(32.1/1). It was considered a low
priority to test that last time such things were considered; perhaps that
judgement was wrong.

                                        Randy.




                                       Randy.