Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!mit-eddie!uw-beaver!uw-june!uw-entropy!dataio!bright
From: bright@dataio.Data-IO.COM (Walter Bright)
Newsgroups: comp.lang.c
Subject: Re: typeof isn't enough to define swap correctly
Message-ID: <1365@dataio.Data-IO.COM>
Date: Fri, 11-Sep-87 14:23:54 EDT
Article-I.D.: dataio.1365
Posted: Fri Sep 11 14:23:54 1987
Date-Received: Sat, 12-Sep-87 18:52:20 EDT
References: <557@rocky.STANFORD.EDU> <1880@sol.ARPA> <109@umigw.MIAMI.EDU> <2019@sfsup.UUCP> <2105@sol.ARPA>
Reply-To: bright@dataio.UUCP (Walter Bright)
Organization: Data I/O Corporation; Redmond, WA
Lines: 34

In article <2105@sol.ARPA> crowl@cs.rochester.edu (Lawrence Crowl) writes:
#/* the mechanism to construct new identifiers */
##define concat(n,m) n/**/m
#
##define swap(a,b) { \
#/* construct new type with new name */
#typedef struct { char c[ sizeof(a) ] ; } concat(swap_T,__LINE__) ; \
#/* pointers to sources allow evaluating arguments only once */ \
#concat(swap_T,__LINE__) *concat(swap_P,__LINE__) = &(a) ; \
#concat(swap_T,__LINE__) *concat(swap_Q,__LINE__) = &(b) ; \
#/* do actual swap */
#concat(swap_T,__LINE__) concat(swap_R,__LINE__) = *concat(swap_P,__LINE__) ; \
#*concat(swap_P,__LINE__) = *concat(swap_Q,__LINE__) ; \
#*concat(swap_Q,__LINE__) = concat(swap_R,__LINE__) ; \
#}
#Have I forgotten anything?

I know that
	#define swap(a,b) { typeof(a) _tmp; _tmp = a; a = b; b = _tmp; }
is not completely generic. But it is at least readable, simple, compiles
efficiently, and works for variables, register variables, structs and unions.

As far as swap(a++,b++) goes, who cares? Good coding style should
preclude that occurrence.

If you have a name collision with _tmp, your program is probably a
candidate for the Obfuscated C Contest anyway.

Arrays are a special beast anyway. Very little else works with arrays.
I'd rather have swap work with register variables than with arrays.

To summarize, I would prefer a simple macro with obvious limitations to
a complex macro with very obscure limitations. A completely foolproof
swap() would have to be implemented within the compiler anyway.