Re: GCC 4.0 Ada.Containers Cursor danger.

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

"Georg Bauhaus" <bauhaus@arcor.de> wrote in message
news:42C98672.3020705@arcor.de...
> Dmitriy Anisimkov wrote:
> > IMHO the cursors in the Ada.Containers implemented in GCC 4.0
> > dangerouse like pointers in C/C++.
>
> I think saying that Cursors are as dangerous as Ada pointers
> is more to the point. IIRC there are significant
> differences between Ada.Containers and STL, more checks i.e.
>
> Not sure though why Replace_Element doesn't raise an exception,
> possibly because messing around with two pointers to the same object
> and then deleting the object can lead to erroneous execution:

Yes. It's very expensive to make such checks that would be bulletproof (a
cursor would need to be a controlled type with an indirect reference to the
container, and the container would need to maintain a list of all existence
cursors). Thus the checks aren't mandated.

However, these checks are relatively easy to make in cases like the one
given. And nothing in the standard prevents an implementation from making a
check in this case. I believe that Ada implementations ought to make such
checks in the easy cases (certainly the Janus/Ada implementation will). One
of the real advantages to cursors in this case is that checks *can* be made;
if we had used access types directly, no such checks would be possible.

One can imagine a debugging version of the containers that made all possible
checks, no matter what the cost. (I think my design is good enough for
general use, because we can check almost all errors for a small additional
cost in the size of cursors.)

                  Randy Brukardt

                          Randy Brukardt