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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: Out parameters and unconstrained variant types
Date: Thu, 26 Jun 2008 19:40:44 -0700 (PDT)
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Date: 2008-06-26T19:40:44-07:00
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On Jun 26, 5:22 pm, Gene <gene.ress...@gmail.com> wrote:
> Hello friends,
>
> I would like to do something like:
>
> type Conditional_Result (Exists : Boolean := False) is
>   record case Exists is
>     when True =>
>       Value : Natural;
>     when False => null;
>   end record;
>
> procedure Do_Something(x : in Natural; rtn : out Conditional_Result)
> is
> begin
>   if x = 0 then
>     rtn := (Exists => False);
>   else
>     rtn := (Exists => True; Value => x + 1);
>   end if;
> end;
>
> procedure Foo is
>   rtn : Conditional_Result;
> begin
>   Do_Something(0, rtn);
>   if rtn.Exists then
>     Put_Line("Yes!");
>   end if;
>   Do_Something(1, rtn);
>   if rtn.Exists then
>     Put_Line("Yes!");
>   end if;
> end;
>
> ALRM 2005 and Barnes seem to say the procedure Do_Something will know
> that rtn in foo is unconstrained and allow both calls to succeed.  In
> GNAT 2007, the second call fails with a constraint error at the
> assignment to rtn.  Can you tell me what I'm missing?

It should work, and "anon" says he did get it to work with GNAT 2007.
But your example isn't a complete program, so perhaps if you post the
complete program you tried (assuming it's not much larger than this
example), maybe we can see something additional that you did that
would cause a problem.

                            -- Adam