Re: Run-time accessibility checks

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 7 Dec 2008 06:56:49 -0800 (PST), Ludovic Brenta wrote:

> On Dec 7, 9:44 am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> 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.
> 
> My definition of correctness is a bit more demanding. I want to be
> *certain* that the pointer *cannot* become dangling, *ever*,
> especially in the face of software maintenance where the program
> changes. That's what accessibility checks give me, and I am grateful
> for their existence and I will design my programs so as not to fail
> accessibility checks.

No, it is *less* demanding than mine. Mine is that the program is either
correct or not, statically. This leaves no room for run-time checks on
correctness. Consequently, if accessibility check fails at run-time then a
correct program shall handle the corresponding exception. It does not add
safety.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de