Re: calenday (was Re: IBM Acquires Rational Ada

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

Marin David Condic wrote in message ...
>Tagged records probably present the worst problems if you try to throw
>representation clauses on them. I've only tinkered with one compiler in
this
>regard, so I can't speak for all of them and I realize that there are
>special problems in this respect. But if you're going to allow rep
clauses
>on tagged records, there ought to be some ability to actually make them
>work. Two problems I had were that a) you have no mechanism to find the
>position/size of the tag, so you have no mechanism to offset everything
from
>it and b) the compiler insisted that all extensions of the record start
on a
>longword boundary and there was no apparent way to tell it to start
>elsewhere - even though it would have been an even byte or word
boundary. It
>seems to me that those are problems that *could* be addressed and maybe
the
>rep clause would be more useful.


a) is not a problem in practice. We did this in Claw, and all compilers
put the tag at offset 0. (A few can be told to put it elsewhere, but
that wasn't generally available a few years ago when we did that.) The
size is a problem in that it is implementation-defined.
Ada.Tags.Tag'Size is the correct value, but it isn't static, so you
can't use it in representation clauses. We ended up declaring a constant
TAG_SIZE that we use in the rep. clauses (and compare to
Ada.Tags/Tag'Size as a validity check). However, it is 4 (bytes) in all
of the compilers that we tried, so it really wasn't necessary.

b) is clearly compiler specific. I'd suggest trying to convince your
compiler vendor to improve their compiler. Janus/Ada doesn't require any
alignment for the extension part. And I don't recall having problems
with this in Claw, although perhaps we were lucky.

The biggest non-portability is in the ability (or lack thereof) to put
extension components into "holes" in the parent. This is useful, but can
be really hard to implement (it causes problems with the predefined "="
operation, and I believe that there are other problems that I don't
remember at the moment).

            Randy Brukardt.