Path: utzoo!utgpu!water!watmath!clyde!bellcore!faline!ulysses!allegra!mit-eddie!husc6!cmcl2!brl-adm!brl-smoke!gwyn
From: gwyn@brl-smoke.UUCP
Newsgroups: comp.lang.c
Subject: Re: ! operator in constant expressions
Message-ID: <7047@brl-smoke.ARPA>
Date: 12 Jan 88 00:28:30 GMT
References: <3813@hoptoad.uucp> <7027@brl-smoke.ARPA>
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 25
Posted: Mon Jan 11 19:28:30 1988

In article <7027@brl-smoke.ARPA>, I wrote:
> In article <3813@hoptoad.uucp> gnu@hoptoad.uucp (John Gilmore) writes:
> >int bar = !3.14156;
> >int pno = ! &bar;
> >int pyes = !(char *)0;
> I don't see how you could expect any of these three to work; the
> operand of ! has to be an integer ...

My only excuse is that I was very ill when I wrote that!
Of course, one all the time sees code like
	type *p;
	if (!p) ...
My personal practice is to apply ! only to "Boolean expressions",
which is not something that C officially understands, but it's
easy to self-impose the "Boolean" discipline, and beneficial to
code quality and maintenance.  However, just because I never apply
! to a non-integer (which is a superset of Boolean in C), doesn't
mean that it is prohibited by the language spec.

Now that I can check the dpANS, I find that an initializer for a
scalar is constrained to be a constant-expression, and otherwise
the same type constraints and conversions apply as for simple
assignment.  Also, !E is equivalent to E==0.  Thus, all the above
examples are supposedly legal in ANSI C.  I apologize for my
earlier mistake.