Re: Latest on Windoze Navy software

From: Tom Linden (tom_at_kednos.com)
Date: 10/20/04 

Date: Tue, 19 Oct 2004 17:41:51 -0700

On 19 Oct 2004 12:20:04 -0500, <briggs@encompasserve.org> wrote:

> In article <opsf4i3fjezgicya@hyrrokkin>, "Tom Linden" <tom@kednos.com>
> writes:
>> On 19 Oct 2004 10:01:59 -0500, Larry Kilgallen <Kilgallen@SpamCop.net>
>> wrote:
>>> As has been explained to you before, descriptors are an implementation
>>> issue, not a language issue.
>>
>> Well, %DESCR is an extension, which would make a source program
>> employing
>> it non-portable. So, from that point of view it is a language issue,
>> even
>> though it appears to be a preprocessor function.
>
> If you use a non-portable extension then the behavior of a program
> that uses that extension is an implementation issue, not a language
> issue.

I had a different understanding of the term. I was thinking of
specific implementations which did not impact the language reference,
e.g. how you allocate storage. I view this as an extension to the
language reference. If wrong, my misunderstanding.

>
> If you need to use a non-portable extension then the behavior that
> you are trying to influence must be implementation-specific rather
> than language-defined.
>
> The language definition is silent on both the question of parameter
> passing mechanism (reference versus descriptor) and on the question
> of interpretation of the %DESCR extension. Accordingly, neither is
> a language issue. Both are implementation issues.
>
> One should distinguish between the semantics of parameter passing
> and the mechanism of parameter passing.
>
> Call by value, by reference or by copy-in, copy-back -- those are
> semantics. You can write a program to tell which semantics
> are in use. And the language definition may prescribe particular
> semantics. Or it may be left as an implementation decision.
>
> Call by value, by reference or by descriptor -- those are mechanisms.
> You cannot, in general, write a program to tell which mechanism is in
> use. Parameter passing mechanism is almost always an implementation
> decision.
>
> One popular choice used by language definers is to prescribe
> copy-in, copy-back semantics by default but to allow by-reference
> semantics for structured objects.
>
> On VAX:
>
> You can implement by-value semantics using value, reference or
> descriptor.
>
> You can implement by-reference semantics using reference or descriptor
>
> You can implement copy-in, copy-back semantics using reference or
> descriptor
>
>
>
> John Briggs 

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