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From: "James A. Squire" <m193884@CSEHP3.MDC.COM>
Subject: Re: Uninitialized "out" parameters
Date: 1996/07/19
Message-ID: <31EF9DFC.6FB4@csehp3.mdc.com>#1/1
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sender: Ada programming language <INFO-ADA@LISTSERV.NODAK.EDU>
references: <4smh9i$gp5@krusty.irvine.com> <4smmhe$9cs@goanna.cs.rmit.edu.au>
comments: Gated by NETNEWS@AUVM.AMERICAN.EDU
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Date: 1996-07-19T00:00:00+00:00
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Adam Beneschan wrote:
>
> Dale Stanbrough <dale@goanna.cs.rmit.edu.au> writes:
>
>  >Adam Beneschan writes:
>  >"Well, you can't read an "out" parameter at all, so it's illegal to use
>  > "op" in the right-hand side of your assignment.  So the compiler
>  > should give you an error.  Also, in Ada83, you'll get an error because
>  > I needs to be declared before the code for SubP appears.
>  >
>  > If you change op to an "in out" parameter, you'll be reading an
>  > uninitialized variable, but typically neither the compiler nor the
>  > runtime will complain."
>  >
>  >
>  >Ada95 does allow you to read out parameters. The code written simply
>  >uses an uninitialized variable, and I would imagine it would be declared
>  >erroneous by the LRM.
>
> Sorry, I wasn't aware of this change.
>
> This rule change frightens me a little.  In the posted example:
>
>    procedure SubP (op : out integer) is
>    begin
>       op := op + 1;
>    end SubP;
>
> I've been known to accidentally type "out" when I mean "in out".  In
> Ada 83, the compiler would catch me, but now it won't, and the effect
> will be very different because here the "op" on the right side of the
> assignment is always uninitialized.  However, I guess a smart compiler
> would let me know that I'm using an uninitialized parameter, and
> perhaps the advantages of letting you read an OUT parameter after
> you've written to it outweigh this disadvantage.

I tried to find out from the LRM if this compiler behavior is required,
but I don't see it anywher in chapter 6.  I do know that in 6.1.1 of the
Ada95 Rationale, it has the following paragraph:

"For Ada 95, we have removed the restrictions on the use of out
parameters.  Specifying that a formal parameter is of mode out indicates
that the caller need not initialize it prior to the call. However,
within the procedure, once the
                                                                    ^^^^^^^^
parameter has been initialized, it may be read and updated like any
other
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
variable. As with a normal variable, it is an error to depend on the
value of an out parameter prior to its being initialized."

but I don't know if that's a compiler error or a run-time error.  From
the context, I'd guess a run-time error.
--
James Squire                             mailto:ja_squire@csehp3.mdc.com
MDA Avionics Tools & Processes
McDonnell Douglas Aerospace              http://www.mdc.com/
Opinions expressed here are my own and NOT my company's
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