Re: A simple ADA puzzle (I haven't the answer)

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

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:pltjvkdjp69c.11hhxh2y8idq9.dlg@40tude.net...
> On Wed, 23 Jun 2004 16:37:01 -0500, Randy Brukardt wrote:
>
> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> > news:2i5dd0lu0i3a8bokfh7916r4j67g42ri4o@4ax.com...
> > ...
> >>>As I have said, it's extremely simple. Automatic variables are
invisible.
> >>
> >> Huh, how is it better than Ada's limitation on tagged types?
> >
> > The only limitation on tagged types in Ada 2005 is that you can't extend
a
> > formal type in a generic body. See AI-344, approved in Palma de
Mallorca.
> > (There are accessibility checks if you try pass an object of a nested
type
> > outwards.)
>
> But I will be still able to allocate it, return a class-wide pointer and
> then to dispatch to an out-of-scope subroutine. It seems that some
> additional overhead will be needed to catch that.

No. The conversion to the class-wide pointer (presumably global) would fail
its accessibility check. So the program would be illegal. No overhead
needed.

Tucker's idea was simply to move the accessibility checks from the
declaration of the type (the Ada 95 rule) to the uses of the objects (which
turn out to be only a handful of places).

It is true that dispatching to an overridden routine for a type in a nested
scope will be more expensive (there will be a wrapper to set the static link
or display properly). But that's a small price to pay for something that you
can't do at all now. (And it won't make any existing code slower,
obviously.)

> BTW, there could be alternatives to accessibility checks, when overhead is
> accepted. For example, one could forcibly upcast objects if their type
goes
> out of scope.

Silent truncation (upcasting) of a type was considered a very bad thing
during the design of Ada 95. Anything that would require that (such has
having both statically and dynamically tagged operands in a single call -
see 3.9.2(8)) is illegal.

You could argue whether that is a good idea or not, but I don't think that
we're going to change that part of the philosophy.

                          Randy.