Re: Access parameters and accessibility

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

"Michael B." <michaelb@example.com> wrote in message 
news:m6n3kr$t51$1@speranza.aioe.org...
> I'm reading the book "Programming in Ada 2005" of John Barnes and I have 
> difficulties to understand the chapter about access parameters.
> In chapter 10.7 on pages 197/198 he writes:
>
> ---
> An access parameter can be passed on to another access parameter; 
> typically the accessibility indication is passed on unchanged but in the 
> unusual circumstance where the called subprogram is internal to the 
> calling subprogram, the accessibility level is replaced by that of the 
> (statically known) formal calling parameter if less than the original 
> actual parameter.
> ---
>
> Could someone give an example/explanation where this rule is used?
> I tried to read the RM. Chapter 3.10.2 seems to be the right one but I 
> don't understand it either...

We in the ARG call discussing accessibility a "trip to the Heart of 
Darkness", and indeed, 3.10.2 is now informally called the "Heart of 
Darkness". I even put that into an AARM note (3.10.2(3.b/3)). Only the 
desperate or foolhardy venture there.

I had heaard about the issue that John mentions in this paragraph, but I 
never understood it (I don't think Janus/Ada does it). I've had it explained 
to me a couple of times, and I sort of understand it, but you'd have to pay 
me to spend an hour finding an example and writing up an appropriate 
explanation. It's that draining - I wouldn't do it for free. Sorry.

Besides, I agree with the others that it has nothing to do with OOP. Claw 
only uses anonymous access parameters to get the effect of in out parameters 
in functions (which isn't a problem with Ada 2012 anyway), and as Dmitry 
noted, it doesn't work very well. Anonymous access parameters: just say no!!

Finally, the model that John is talking about isn't sufficient to implement 
dynamic accessibility checks in Ada 2005 or Ada 2012. He's really talking 
about an implementation short-cut that should be irrelevant to understanding 
the feature anyway. (That is, it doesn't even belong in his book.) And that 
short-cut doesn't work. Indeed, no Ada compiler has ever actually 
implemented correct accessibility checking for Ada 2005 or later. We believe 
it is possible (Steve Baird and I have worked out correct implementation 
models, but no one has tried either yet). We're supposed to be writing an 
Ada-Europe paper -- but of course that requires a trip to the Heart of 
Darkness, and neither of us has been in much a hurry to take that trip!

The only thing to worry about vis-a-vis dynamic accessibility checking is 
that the object has to have a lifetime the same or longer than the access 
type. How that's implemented should not matter (unless of course you are 
trying to fix a compiler bug -- something most people here aren't going to 
do).

                                             Randy.