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From: Gene <gene.ressler@gmail.com>
Newsgroups: comp.lang.ada
Subject: Out parameters and unconstrained variant types
Date: Thu, 26 Jun 2008 17:22:29 -0700 (PDT)
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Date: 2008-06-26T17:22:29-07:00
List-Id: <comp.lang.ada>

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?

Thanks!