Path: utzoo!utgpu!watserv1!watdragon!lion!smking
From: smking@lion.waterloo.edu (Scott M. King)
Newsgroups: comp.lang.c
Subject: Using macros as statements (Re: Typeof operator in C)
Keywords: macro statement
Message-ID: <19840@watdragon.waterloo.edu>
Date: 16 Jan 90 15:36:19 GMT
References: <16678@megaron.cs.arizona.edu> <7106@tank.uchicago.edu> <-K016ODxds13@ficc.uu.net> <721@eedsp.eedsp.gatech.edu>
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Reply-To: smking@lion.waterloo.edu (Scott M. King)
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In article <721@eedsp.eedsp.gatech.edu> baud@eedsp.UUCP (Kurt Baudendistel) writes:
:Be careful, however, of the nasty C preprocessor, which will make this
:nice looking and appealing definition:
:  #define SWAP(a,b) {typeof (a) tmp; tmp=a; a=b; b=tmp}
:fail in many cases, like
:  if (x < y)
:    SWAP (x, y);	// bracketing of SWAP makes the `;' extraneous
:  else			// and fatal
:    x = y;
:
:Kurt Baudendistel --- GRA
:Georgia Tech, School of Electrical Engineering, Atlanta, GA  30332
:internet: baud@eedsp.gatech.edu         uucp: gatech!gt-eedsp!baud

To make a macro behave exactly as a statement, replace the { and } in
the macro definition with START_MACRO and END_MACRO:
#define START_MACRO		do {
#define END_MACRO		} while ( 0 )

The compiler should catch uses with too few or too many semi-colons here.

(Note that another version I have seen has very unexpected effects if the
semi-colon is left off the macro usage:
#define START_MACRO		if ( 1 ) {
#define END_MACRO		} else
)
Scott M. King, Software Devlopment Group, University of Waterloo