Re: Run-time accessibility checks

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

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:ggyyafzb06rk.1fkbqhnepowug$.dlg@40tude.net...
> On Sat, 6 Dec 2008 09:10:49 -0800 (PST), Ludovic Brenta wrote:
>
>> Dmitry A. Kazakov wrote:
>
>>> 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.
>
> Ah, I used to think this way too. But then I realized that this form of
> constraint is not a semantic one. Comparing to the constraint range 1..10,
> accessibility constraint is an implementation detail. It has no semantic
> meaning. When you assign a value outside the range that is semantically
> incorrect. When you assign a pointer that potentially might become
> dangling, that has no semantic meaning. The pointer is not yet dangling.
> The program correctness depends on whether it will become dangling.

You are wrong here; that occurs when an *static* accessibility check is 
triggered that has to be checked at runtime because the source has dynamic 
accessibility. That conversion between worlds is the problem. IMHO, the 
static accessibility model is a pile of steaming dung - it never, ever works 
(I have yet to successfully write 'Access in anything outside of ACATS 
tests). OTOH, a fully dynamic model would only fail when the pointer really 
is dangling, because the accessibility would stay with the access type and 
there would never be any conversion to the static world. So that would only 
represent a real bug. But of course it is a lot more expensive at runtime.

I'm going to write up a proposal for a fully dynamic model. I don't really 
expect it to get anywhere, but at least we'll have it around the next time 
someone starts realizing that static accessibility is complete junk.

                               Randy.

P.S. Although it is not completely clear from the Standard, Ada has two 
virtually separate sets of definitions for static accessibility and 
"dynamic" accessibility (the latter is called just plain "accessibility" to 
add to the confusion). It's the static model that is such a mess; the 
dynamic model is just fine.


                                        Randy.