Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utcs!tmsoft!mason
From: mason@tmsoft.UUCP
Newsgroups: comp.lang.c
Subject: Re: short circuit evaluation & side-effects
Message-ID: <107@tmsoft.UUCP>
Date: Wed, 4-Feb-87 11:23:48 EST
Article-I.D.: tmsoft.107
Posted: Wed Feb  4 11:23:48 1987
Date-Received: Wed, 4-Feb-87 20:39:51 EST
References: <425@bobkat.UUCP> <102600001@datacube> <7610@utzoo.UUCP>
Reply-To: mason@tmsoft.UUCP (Dave Mason)
Organization: TM Software Associates, Toronto
Lines: 23
Summary: Does ANSI change the statement re: side effects?

In K&R pg. 184 it says: "...Otherwise the order of evaluations of expressions
is undefined.  In particular the compiler considers itself free to compute
subexpressions in the order it believes most efficient, even if the
subexpressions involve side effects.  The order in which side effects take place
is undefined."
    As I understand it this means the results of:
(1)	x = *i++ * (1 - *i++);
is unpredicatable.  ANSI added the unary + to fix this (as well as for numerical
analysis (i.e. floating point) calculations) so that:
(2)	x = +(*i++ * (1 - *i++));
does what one would expect.  I have seen notes here that suggested that (1)
could be equivalent to:
(3)	x = *i * (1 - *i); i += 2;

Does the unary + force the side effects to happen where expected?  Is unary +
still in the ANSI standard proposal?

The above quote would appear to require side effects to take place (although
the order is undefined) even if short circuiting were to be done (except of
course in the case of || and &&)
-- 
	../Dave Mason,	TM Software Associates	(Compilers & System Consulting)
	..!{utzoo!ryesone seismo!mnetor utcsri!ryesone}!tmsoft!mason