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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: GNAT GPL 2013 bug?
Date: Mon, 12 Aug 2013 19:03:14 +0200
Organization: cbb software GmbH
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Xref: news.eternal-september.org comp.lang.ada:16822
Date: 2013-08-12T19:03:14+02:00
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On Mon, 12 Aug 2013 09:52:29 -0700 (PDT), Adam Beneschan wrote:

> On Friday, August 9, 2013 11:51:34 PM UTC-7, Dmitry A. Kazakov wrote:
>> On Fri, 09 Aug 2013 16:04:02 -0400, Robert A Duff wrote:
> 
>> I can "dispatch" on null anyway:
>> 
>>    type T is tagged null record;
>>    procedure P (X : T) is null;
>>    procedure R (X : access T'Class);
>> 
>>    procedure R (X : access T'Class) is
>>    begin
>>      X.P;
>>    end R;
> 
> But I could write
> 
>    procedure R (X : access T'Class) is
>    begin
>       if X = null then
>          -- take some special default action
>       else
>          X.P;
>       end if;
>    end R;
> 
> The point is that when a caller calls R, the above procedure is simply
> called.  So it will work to call it with a null parameter; the code in R
> may later run into a Constraint_Error or not, depending on how it's
> written, but there's no problem calling R.  But when P is called, the
> dispatching occurs *at* *that* *point*, before any code in any of the P
> procedures is executed;

It is irrelevant for the client (= user = program semantics) where that
check is placed, an implementation detail, nothing more.

> if the parameter is null, the dispatch cannot
> work.
> 
> I'm really not clear at all on what point you're trying to make.

That implied null-exclusion for controlling parameters does not change/save
anything. Just added language weirdness that

   "access T" is subtype conformant to "not null access T" depending on T

!, for apparently no gain.

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