Re: GCC 4.0 Ada.Containers Cursor danger.

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

"Maxim Reznik" <reznikmm@front.ru> wrote in message
news:dagnvs$2s9f$1@pandora.alkar.net...
...
> I look in all my Ada programs. There are a few loops per container,
> no more that 2 or 3 at once. Because cursors's main purpose is
> iteration over containers I think there will be a few cursors per
> container usually.

Most programs will have no cursors at all, or only one in stored in a local
variable. It would be almost impossible for there to be any trouble with
such a cursor.

Other programs will store cursors in other data structures. Such a program
could store a large number of cursors.

> If make container remember all its cursors we can implement
> all operations which make cursors invalid so that they
> reset invalid cursors to No_Element.

Yes, that is the "checked" implementation of cursors. The standard certainly
allows such an implementation, and you're welcome to implement such an
implementation. But it is likely to be too slow for any application that
uses a lot of cursors (especially if they create and destroy a lot of
cursors).

I would have preferred that we require a check for dangling cursors so long
as the container itself still exists. (Once the container is destroyed,
checks are pretty much impossible.) But I lost that battle, because the
implementation is not quite bullet-proof.

Anyway, this is a quality of implementation issue. If you care about checks,
you should insist that your Ada vendor provide them. (RRS will certainly
make checks in its implementation of containers. They should catch all
obvious mistakes [like your example], and most others as well.) If you care
about certain types of performance, ditto. Nothing in the standard prevents
either.

                             Randy.