Re: GCC 4.0 Ada.Containers Cursor danger.

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

"Mikhail Terekhov" <terekhov@emc.com> wrote in message
news:gb_Ce.8815$LC6.6336@fe06.lga...
....
> That is wonderful! Almost any example of using cursor doesn't really
> need cursors. Why this burden then?

Not at all true. Any algorithm that needs to modify the container during the
iteration, or needs to iterate through multiple containers in lockstep, will
need something like cursors. Ada.Containers wants Iterate to be safe, so
modifications of the container raise Program_Error (otherwise, it would be
impractical to insure that the iteration would continue safely).
...
> If you need just a set of references to some elements, then you are all
> set. Ada provides access types, why cursor?

Because access types aren't safe in Ada and cannot be made safe. If we
returned access types from functions (and we considered it), a modification
to the container could destroy the access value unpredictably -- and there
is no way for future uses of the access value to detect that. A cursor is a
higher level abstraction than an access type, and it can be checked. (I
personally think it is unfortunate that we didn't *require* it to be checked
in most cases, but at least an implementation *can* check -- which is
impossible with access types.)

                          Randy.