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From: dewar@cs.nyu.edu (Robert Dewar)
Subject: Re: Uninitialized "out" parameters
Date: 1996/07/18
Message-ID: <dewar.837740222@schonberg>#1/1
X-Deja-AN: 168943703
references: <31EEACDA.64880EEB@sage.inel.gov>
organization: Courant Institute of Mathematical Sciences
newsgroups: comp.lang.ada
Date: 1996-07-18T00:00:00+00:00
List-Id: <comp.lang.ada>


Paul asked

"In the following Ada program, should either the compiler or the run-time
get some kind of error because the out parameter is not initialized?

procedure Testit is

  procedure SubP (op : out Integer) is
  begin
    op := op+1;
  end SubP;

  I : Integer;

begin
  SubP (I);
end Testit;



The answer is that this program is perfectly legal, so it would be incorrect
for a compiler to reject it. At execution time, if subp is called, then the
addition operation references an uninitialized variable and renders the
program execution erroneous, which means anything might happen, but you
certainly cannot expect/require a runtime error. The execution might raise
Program_Error, but it also might do anything else.

A compiler could give a warning for this program. For example, GNAT, using
the option -Wuninitialized (warn on uninitialized variable use), gives

testit.adb: In function `subp':
testit.adb:3: warning: `op' might be used uninitialized in this function

But certainly it is impossible for a compiler to give warnings in all
cases of uninitialized variable use.