Re: Run-time accessibility checks (was: Construction initialization problem)

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Dmitry A. Kazakov wrote:
> It would be interesting to make a poll, how many programmers
>
> 1. start straight with Unchecked_Access
>
> 2. write Access first and then switch to Unchecked_Access after the first
> compiler message without analyzing the message
>
> 3. try to understand the message and change the design

Put me in category 3. However, changing the design isn't always
desirable because alternative designs may have unacceptable drawbacks.

> My guess is 65-30-5. Yours?

I don't know but I don't remember seeing any Unchecked_Access that
wasn't thoroughly explained in the comments near it; this would
indicate the person who wrote it was at least in category 2.

> > I've been trying to work on this problem, but the obvious solutions would
> > require full dynamic accessibility checks, including passing the
> > accessibility of all by-reference parameters -- and that is way too
> > expensive to consider. Plus dynamic checks provide a new failure mechanism
> > for code; it's not clear that is an advantage.
>
> Ooch, this is the major contributor to the group 1. If I had any danger
> that X'Access might fail at run-time, I would immediately switch to
> X'Unchecked_Access.

Actually, the presence of run-time accessibility checks are the reason
that puts me in category 3. If the run-time overhead or, worse, the
possibility of failure at run-time are unacceptable, then I prefer not
to use access types at all.

> It is absolutely unacceptable to me that a correct
> program might fail at run-time because of accessibility checks.

I differ here; to me, a program that fails an accessibility check at
run time is incorrect.

> The only
> case I could buy it, if exceptions where contracted, so that I would get an
> compile-time error at some other place. Like "Constraint_Error may be
> propagated, but the contract states otherwise."

In my understanding, there is an implicit contract stating that all
subprograms may raise at least Program_Error, Storage_Error or
Constraint_Error. I accept that as a fact and keep it in mind when
designing.

--
Ludovic Brenta.