Re: Counter-proposal for variable arrays

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

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:1065709998.992831@master.nyc.kbcfp.com...
> Dmitry A. Kazakov wrote:
> > It is already so. Consider:
> > type Tricky (I, J : Positive) is record
> >    X : String (1..I);
> >    Y : String (1..J);
> > end record;
> > The offset to Y is variable and depends on the value of I.
>
> I Didn't Know You Could Do That In Ada(tm). :-)
> It really works that way? And you can have procedures
> which just take unconstrained Tricky parameters, and
> they'll know how to deal with all instances? Cool!

You can write this, but it often isn't implemented with variable offsets.
What Janus/Ada does is to use discontinous storage for
depends-on-discriminant components. The component itself is a descriptor,
and has fixed size. The array data is allocated separately. That way, we can
actually change the data memory when the discriminants are changed by
assignment, and we don't use more memory than necessary.

Most other compilers don't do that, but I know that some at least use extra
pointers in cases like the above to avoid having non-static offsets.

On the other hand, if you do generic code sharing (as Janus/Ada does), you
end up with dynamic offsets, dispatching slots, tags, array strides, object
sizes, and pretty much everything else. (But usually not all at the same
time!)

                Randy.



>
> > For good or bad, neither Ada is C++ nor I have Stroustrup's influence
> > on the language design. There should be some consensus reached. And of
> > course, any change undertaken have to be carefully reviewed.
>
> Don't underestimate the power of Just Doing It. You know how they
> say that it's easier to get forgiveness than permission. If the
> feature were actually added to a popular compiler and worked well,
> that would be a big step in gaining acceptance.
>