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Path: utzoo!mnetor!seismo!mcvax!botter!sjoerd
From: sjoerd@botter.UUCP (sjoerd)
Newsgroups: net.lang
Subject: Re: null statements
Message-ID: <593@botter.UUCP>
Date: Fri, 25-Jul-86 06:54:26 EDT
Article-I.D.: botter.593
Posted: Fri Jul 25 06:54:26 1986
Date-Received: Fri, 25-Jul-86 21:55:57 EDT
References: <800015@ccvaxa> <463@opus.nbires.UUCP> <442@sunybcs.UUCP> <484@valid.UUCP> <2565@umcp-cs.UUCP>
Reply-To: sjoerd@vu44.UUCP (Sjoerd Mullender)
Organization: VU Informatica, Amsterdam
Lines: 29

In article <2565@umcp-cs.UUCP> chris@maryland.UUCP (Chris Torek) writes:
[ About null statements in #defines ]
>In many cases the function is more complex:
>
>	#define	SaveB(b) \
>		if ((b)->b_AfterGroupRst == NULL || \
>		    (b)->b_AfterGroupRst->sv_level != CurrentGroup) \
>			DoSave(b); \
>		else \
>			/* do nothing */
>
And continues to suggest a few solutions since the above will wreak havoc
if you forget a semicolon in the call.
Another solution (which does not require a null statement) is:
#define SaveB(b) \
	(/* if */ ((b)->b_AfterGroupRst == NULL || \
	 (b)->b_AfterGroupRst->sv_level != CurrentGroup) /* then */ && \
		DoSave(b))
I agree that this may be bit confusing, but this has the great advantage
that you can use it everywhere where you can use an expression (like in a
for statement).
Another use for this construct is the assert macro:
#define assert(e)	(!(e) && printf("Assertion failed ..."))
You can write this differently:
#define assert(e)	((e) || printf("Assertion failed ..."))
I like this much better than the if statement that is usually used.
>In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
>UUCP:	seismo!umcp-cs!chris
>CSNet:	chris@umcp-cs		ARPA:	chris@mimsy.umd.edu