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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: two questions on allocators
Date: Sat, 24 Feb 2018 20:17:30 -0600
Organization: JSA Research & Innovation
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Date: 2018-02-24T20:17:30-06:00
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"Mehdi Saada" <00120260a@gmail.com> wrote in message 
news:dfa8abf3-6016-45dc-88de-91f31fac853a@googlegroups.com...
>> Finally, the phrase "new not null B" is in error, as you cannot say "new 
>> not null" in Ada.
> Here's a simpler exemple ( I think I mixed up A and B, which made my 
> statement meaningless):
> type A is access INTEGER;
> type B is access not null A;
> Object_B : B := new A; --- **
> allowed
> type A is access INTEGER;
> type B is access A;
> Object_B : B := new not null A; -- illegal,
> but wouldn't that be the same than * ?
> Anyway, let's forget it.

Naw, let's answer it.

Look in the AARM (4.8(2.a/3) [directly under the rule in question]): "Such 
an uninitialized allocator would necessarily raise Constraint_Error, as the 
default value is null. Also note that the syntax does not allow a 
null_exclusion in an initialized allocator, so it makes sense to make the 
uninitialized case illegal as well."

That is, an access value is default-initialized to the null value. So this 
allocator is just a silly way to write (raise Constraint_Error).

Another such silly way is:
     Object_B : not null B; -- Always raises Constraint_Error.

If you're going to be interested at this level of detail, I suggest looking 
in the AARM, which has various "annotations" explaining rules, and with 
links to the AIs that generated the rules (if they're new). You can usually 
figure out the rules this way. The AARM can be found in most of the usual 
places, in particular on www.ada-auth.org.

                                                 Randy.