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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Access parameters and accessibility
Date: Wed, 17 Dec 2014 16:58:10 -0600
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2014-12-17T16:58:10-06:00
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"Natasha Kerensikova" <lithiumcat@instinctive.eu> wrote in message 
news:slrnm92hsh.nrc.lithiumcat@nat.rebma.instinctive.eu...
> On 2014-12-17, Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
>> On Wed, 17 Dec 2014 07:02:56 +0000 (UTC), Natasha Kerensikova wrote:
>>
>>> Now you got me really worried, because I thouhgt access discriminants
>>> were safe. It was under that assumption that I actually started to
>>> really like them.
>>>
>>> More specifically, the pattern I caught myself using a lot looks like
>>> the following:
>>>
>>>    type Preprocessed_Data (Input_Data : access Input_Data_Type) is 
>>> record
>>
>> not null access?
>
> I tought access discriminants were not allowed to be null.
> Has that changed?
> Or do you advise "not null access" to make it explicit?

Explicit is always better, regardless of the rules.

Ada 95 did not define null for anonymous access types, but all more recent 
versions of Ada do. The only implicit restriction against null that I 
remember in modern Ada is for dispatching anonymous access parameters. See 
3.10(13.1/2). Ahh, yes, AARM 3.10(26.c/2) says:

Inconsistences with Ada 95
Access discriminants and noncontrolling access parameters no longer exclude 
null. A program which passed null to such an access discriminant or access 
parameter and expected it to raise Constraint_Error may fail when compiled 
with Ada 2005. One hopes that there no such programs outside of the ACATS. 
(Of course, a program which actually wants to pass null will work, which is 
far more likely.)

Moral: If you're programming for Ada 2005 or later, always use a null 
exclusion when that makes sense. (It's even a good idea on controlling 
parameters, we would have preferred to require it but that would have been 
severely incompatible with Ada 95.)

                                Randy.