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From: Laurent Guerby <guerby@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Parameter Modes, In In Out and Out
Date: 09 Jan 2001 21:44:49 +0100
Organization: Club-Internet (France)
Message-ID: <86itno316m.fsf@acm.org>
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"i.a.mcleod" <i.a.mcleod@ntlworld.com> writes:
> What is the difference between In Out and Out parameters according
> to the way compilers handle these.  I can't see any difference.  An
> out parameter is still taken in.  I am confused by this anyone help?

To add some code to the discussion, here is a case where changing
"out" into "in out" changes the behaviour of a program from perfectly
ok to erroneous:

procedure T is
   Magic : constant := 12345;

   X : Integer := Magic;

   procedure P (Y : in out Integer) is
   begin
      null;
   end P;

begin
   P (X);
   if X /= Magic then
      raise Program_Error;
   end if;
end T;

If you compile and run T, you'll get no output as expected. Now if you
change the mode of Y to "out", if your compiler uses by copy passing
for integer, you'll probably get a program_error, because the
generated code for exiting P will set X with an unitialized stack
value (the place reserved for the Y value).

It is interesting to see that the P call "undo" the X initialization.
So if you have a coding standard that says that all "variables" must
be initialized, it's not enough in Ada, you also need to initialize
all "out parameters of type not determined to be by-reference" to be
safe from the unitialized variable syndrome in Ada.

PS: this is not theory, I just saw (and fixed) an instance of this
problem at work today (just with code a bit more convoluted ;-).

-- 
Laurent Guerby <guerby@acm.org>