Re: Discriminated types with default discriminants

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

"Adam Beneschan" <adam@irvine.com> wrote in message
news:1131062882.814405.147150@g44g2000cwa.googlegroups.com...
> Robert A Duff wrote:
>
> > > A few compilers will add a level of indirection, and store the
> > > variable-sized stuff elsewhere, reallocating storage when the amount
> > > available is not big enough.
> >
> > It is quite difficult to get the indirect approach to work right.
> > Think about renaming a subcomponent of the thing,
>
> Ummm ... isn't this exactly what's disallowed by 8.5.1(5)?

Yes, it is. Bob is thinking about cases where a non-discriminant component
is moved by the changing of a discriminant. (I know this because he and I
have had this particular discussion repeatedly.) An implementation like that
is bound to fail, IMHO. On the other hand, an implementation like
Janus/Ada's (in which discriminant dependent components are just descriptors
to the actual data) doesn't have this problem. (There is some complication
determining the correct storage pool to use - think about a discriminant
changing assignment on a subprogram parameter - but its not particularly
hard to get right.)

                             Randy.