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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Garbage Collection ???
Date: Thu, 19 Apr 2012 15:26:03 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-04-19T15:26:03-05:00
List-Id: <comp.lang.ada>

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message 
news:wcc4nsf4wm4.fsf@shell01.TheWorld.com...
> Julian Leyh <julian@vgai.de> writes:
>
>> Am Samstag, 14. April 2012 20:21:17 UTC+2 schrieb Robert A Duff:
>>> In Ada 2012, you can say:
>>>
>>>     type T_Ptr_Opt is access all T;
>>>     subtype T_Ptr is T_Ptr_Opt with
>>>         Dynamic_Predicate => T_Ptr /= null;
>>
>> In Ada 2005, you can already say:
>>
>>     type T_Ptr_Opt is access all T;
>>     subtype T_Ptr is not null T_Ptr_Opt;
>
> Right, but that's less useful, because it forces you
> to initialize your variables (e.g. record components)
> before you're ready to.

I don't think that's really true, since the rule that you're referring to 
applies if *any* components are initialized. So while it is true (but a bad 
idea for access types, IMHO) for stand-alone objects, depending on it is 
iffy at best in records.

In all honesty, I think the rule that makes these work different is just a 
bug in Ada 2012. The rule *I* was thinking about only applied to things that 
aren't initialized at all (i.e. scalars), not to things that are initialized 
automatically (access types and Default_Value aspects). In the latter case, 
you can use the object immediately and *never* initialize it -- having the 
predicate not enforced means that it cannot ever be trusted on such objects. 
That seems like a bad way to go (and different from constraints).

Thus, I think the current rule (3.2.4(31/3)) is OK for composite types, but 
it is wrong for elementary types when those are automatically initialized.

(And a null-excluding component - by any means - is almost never a good 
idea. You always need a "nothing/unknown" indicator. Null-exclusions are 
almost exclusively useful for parameters, where you don't need "nothing" 
simply because you don't make the call in that case.)

                                Randy.