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Path: utzoo!linus!philabs!cmcl2!seismo!mcvax!boring!steven
From: steven@boring.UUCP
Newsgroups: net.lang
Subject: Re: Nil dereferencing protection
Message-ID: <6299@boring.UUCP>
Date: Wed, 30-Jan-85 09:45:48 EST
Article-I.D.: boring.6299
Posted: Wed Jan 30 09:45:48 1985
Date-Received: Sun, 3-Feb-85 02:05:53 EST
References: <2340@hplabsc.UUCP> <4948@utzoo.UUCP> <6292@boring.UUCP> <536@mako.UUCP> <338@harvard.ARPA>
Reply-To: steven@boring.UUCP (Steven Pemberton)
Distribution: net
Organization: CWI, Amsterdam
Lines: 21
Xref: seismo net.lang:1382
Apparently-To: rnews@mcvax.LOCAL

Jim Davies@uiucdcsb.UUCP writes:
> boring!steven states that it is possible to statically insure that nil
> pointers are never dereferenced.  The following program provides a simple
> counterexample to this claim: [a Pascal program followed]

and macrakis@harvard.ARPA (Stavros Macrakis) writes:
> > > For instance, it is possible to statically guarantee that nil
> > > pointers are never dereferenced  --Pemberton
> > Ada supports this...  --Steinman
> 
> Unfortunately, this is not and cannot be true.  (consider "a := if
> program P halts, then nil else new foo; deref(a)")
Of course you can't do it without extra syntax! But strong typing is also
impossible (consider int a; a:= if P halts then "hello" else 123).

The point is, that it is possible to define a language so that it is
possible statically to guarantee that nil is never dereferenced (Stavros's
Ada example contains the germ of the idea). You can't do it in Vanilla
Pascal or C.

Steven Pemberton, CWI, Amsterdam; steven@mcvax.