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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Passing the same actual as both in and out formal parameters?
Date: Tue, 17 Nov 2009 01:50:04 -0800 (PST)
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Date: 2009-11-17T01:50:04-08:00
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Consider:

   type T is tagged private;
   procedure P (A : in T; B : out T) is separate;
   Object : T;
begin
   P (A => Object, B => Object);

This seems legal but I suspect the execution might lead to bugs if P
reads and writes components of A and B in arbitrary order, e.g.

type T is tagged record
   L, M : Integer;
end record;

procedure P (A : in T; B : out T) is
begin
   B.L := A.M; -- does this change A.L too?
   B.M := A.L; -- bug: A.L has been clobbered, now B.M = B.L?
end P;

My concern stems from the fact that T is tagged (I cannot change
that), so Object is passed by reference as both A and B.

Am I right to be concerned?

--
Ludovic Brenta.