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From: dewar@cs.nyu.edu (Robert Dewar)
Subject: Re: Uninitialized "out" parameters
Date: 1996/07/20
Message-ID: <dewar.837899732@schonberg>#1/1
X-Deja-AN: 170151786
references: <31EEACDA.64880EEB@sage.inel.gov> <dewar.837740222@schonberg>
 <4sq614$kai@mulga.cs.mu.OZ.AU>
organization: Courant Institute of Mathematical Sciences
newsgroups: comp.lang.ada
Date: 1996-07-20T00:00:00+00:00
List-Id: <comp.lang.ada>


Fergus said

"No, certainly it is possible.  There are languages in which any code
that might attempt to use an uninitialized variable is a compile-time
error.  It would certainly be possible for an Ada compiler to give
warnings in all cases where a variable might be used before it was
initialized."

Well it depends what you mean. If we read what you say literally, it is
obviously incorrect, since it requires the halting problem to be solved:

    procedure x is
        m,n : integer;
    begin
       (big chunk of code not referencing m,n)
       m := n;
    end;

this code references the uninitialized variable n if and only if the big
chunk of code halts. That's an easy proof.

If you don't mind getting bogus warnings, i.e. warnings that might be
false, then your statement is trivially true, just post a warning on
every use (of course your compiler might be more clever than this, but
this is again a simple proof.