Re: Access parameters and accessibility

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

On Mon, 15 Dec 2014 17:52:46 +0100, Michael B. wrote:

> 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?

The rule is: don't ever use this.

Accessibility information is a booby trap placed to blow up your program
later, when you expect it least.

If you know what you are doing, you should when you pass a pointer around,
kill the accessibility information where possible, e.g. by converting to a
named access type, by using X.all'Unchecked_Access etc.

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