Path: utzoo!utgpu!water!watmath!clyde!rutgers!lll-lcc!pyramid!decwrl!cookie.dec.com!devine
From: devine@cookie.dec.com (Bob Devine)
Newsgroups: comp.lang.c
Subject: swap macro -- beware
Message-ID: <8801200422.AA19420@decwrl.dec.com>
Date: 20 Jan 88 04:22:50 GMT
Organization: Digital Equipment Corporation
Lines: 16

> #define swap(a,b) ((a) = ((b) = ((a) = (a) ^ (b)) ^ (b)) ^ (a))
> He [the genius] says it will work for anything (int, char *, etc.).

  Well, yes and no.  This old trick will work for those types that are
defined for the ^ op.  That means structs, arrays, unions, bitfields,
and floats won't work the way you want.  Something else that
will bite you if you use this macro is sign-extension, examine what
happens when swapping an int with a signed char whose top bit is on.
It may not result in what you thought it would.  The preprocessor
does not know C and does not follow type rules.

  The endless discussion of a generic swap macro some time ago
usually ended with the person bemoaning the lack of a "typeof()"
operator to handle the non-word-sized types.

exit(Bob) -- the unique and naturally refreshing choice for an exit value