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From: geoff@ism780c.UUCP (Geoff Kimbrough)
Newsgroups: net.lang.c
Subject: Re: swap() macro
Message-ID: <2595@ism780c.UUCP>
Date: Thu, 19-Jun-86 14:21:54 EDT
Article-I.D.: ism780c.2595
Posted: Thu Jun 19 14:21:54 1986
Date-Received: Sat, 21-Jun-86 10:34:32 EDT
References: <1201@brl-smoke.ARPA>
Reply-To: geoff@ism780c.UUCP (Geoff Kimbrough -- No One in Particular)
Followup-To: /dev/null
Organization: Interactive Systems Corp., Santa Monica, CA
Lines: 33
Keywords: macros, side effects, Obfuscation

In article <1201@brl-smoke.ARPA> Schauble@MIT-MULTICS.ARPA writes:
>Can someone construct a version that makes
>
>    int *a, *b;
>    swap (*a++,*b++);
>
>work right?

#define SWAP(a,b) {int *A= &(a),*B= &(b),temp; temp= *A; *A= *B; *B= temp;}
	/* Or, for more generality.  */
#define SWAP(type,a,b) {type *A = &(a),*B = &(b),temp; temp = *A; *A = *B; \
	*B = temp;}  /* invoked as SWAP( int, *a++, *b++ );     */

As has been pointed out, macros generally don't work when given arguments
with side effects, (arguments? parameters? formals? who cares, if you
understand what I mean)  However, if you're careful to have the arguments
appear ONLY ONCE in the expansion, you can get the results you're after.
This trick only works with macros that have pointers as arguments, of
course.

I *don't* think this macro should be *used*, since it's got to be slower to
do all the extra pointer chasing and such than to break up your statement
into  {  swap(*a,*b); a++; b++; }.  If you're concerned about having to
insert extra braces, I don't even want to tadc tg qgu   0V! !$ A bust ggtt`rgug` readafg af eftaredq ufcgeeefted prggrae wat`2w`ade   sgeet`afg )(/*(<--(no(kurly brace */
		if ( whatever ) {
			hundreds of statements on several pages;
			with 8 space tabs, the code dissappeared off the
			right margin, and gradually walked back, until:
		}
	/* <-- no curly here either */
Took me over an hour just to confirm that the braces matched up.