Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!munnari.oz.au!bruce!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: macros and semicolons
Message-ID: <6531@goanna.cs.rmit.oz.au>
Date: 27 Jun 91 05:45:09 GMT
References: <1991Jun24.213932.595@otago.ac.nz> <160662@pyramid.pyramid.com>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 24

In article <160662@pyramid.pyramid.com>, markhall@pyrps5.pyramid.com (Mark Hall) writes:
> Forgive me for this unsufferable horn-tooting; I just couldn't resist
> (I also couldn't believe no one else sent this reply :-). . . .
> NO macro will work for a swap.  You suffer from the call-by-name rule
> which undid ALGOL in this case.  Consider the expansion of SWAP(i,a[i]):
> 
> 	int c; c = i; i = a[i]; a[i] = c;

 Er, this turns out not to be the case.  Consider
	#define swap(Type, This, That)				\
	    do {						\
		Type *ThisQZ = &(This), *ThatQZ = &(That), t;	\ 
		t = *ThisQZ, *ThisQZ = *ThatQZ, *ThatQZ = t;	\
	    } while (0)

What happens to swap(int, i, a[i])?
	do { int *ThisQZ = &i, *ThatQZ = &a[i], t;
	     t = *ThisQZ, *ThisQZ = *ThatQZ, *ThatQZ = t;
	} while (0);

This is essentially what you'd get from inlining a call to a swap
procedure with reference parameters.
-- 
I agree with Jim Giles about many of the deficiencies of present UNIX.