Re: Construction initialization problem

[Adam Beneschan <adam@irvine.com>]

On Dec 4, 2:17 pm, "Randy Brukardt" <ra...@rrsoftware.com> wrote:
> "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de> wrote in messagenews:oscmgxrpod50$.g7h7snlssha0$.dlg@40tude.net...
>
> > Consider this:
>
> >   type A is tagged limited null record;
> >   type B (X : not null access A'Class) is tagged limited null record;
>
> > Now we want to specialize B so that its instances would already contain A:
>
> You never explained why you would want the discriminant in the first place.
> I would think that it would be better to do what you've said about C
> (include the instance directly in it) from the beginning, and the problem
> doesn't come up.
>
> Access discriminants have a fairly high overhead in Ada (because of the
> possibility of coextensions), so unless you *need* coextensions, they ought
> to be avoided.
>
> One could use a named access type instead to declare the discriminant if you
> really need a discriminant. But the only time you must use a discriminant is
> if you have a discriminant-dependent component, and there is no interesting
> way to make a component dependent on an access discriminant. So I'd prefer
> to just avoid the discriminant.

I thought access discriminants---at least those with anonymous access
type---were put into Ada 95 (before there were coextensions) to allow
objects to point to other objects in a nested scope, in a restricted
way (i.e. only in limited types) that prevented those references to a
nested objects from staying around after the nested object
disappeared.  I wasn't aware that there were enough new features in
Ada 2005 to provide this ability without using access discriminants.
So what am I missing?

                                 -- Adam