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Newsgroups: comp.lang.c
From: ch...@mimsy.UUCP (Chris Torek)
Date: 25 Aug 88 07:16:15 GMT
Local: Thurs, Aug 25 1988 3:16 am
Subject: Re: Explanation, please!
In article <6...@paris.ICS.UCI.EDU> schm...@bonnie.ics.uci.edu (Douglas
C. Schmidt) writes: `break' exits the innermost switch or loop, hence a `break' in cases >The following piece of wonderful obscurity comes from Stroustup's >C++ Programming Language book, page 100: > switch(count % 8) { > case 0: do { *to++ = *from++; > case 7: *to++ = *from++; ... > case 1: *to++ = *from++; > } while (--n > 0); > } >Now, much to my surprise, this is not only valid C++, it is also valid C! >Could some one please explain to me why this is so? It seems like >the case 7-1 labels are actually nested inside the do {} while loop, >and thus not in the scope of the switch (should a break statement exit >both the switch and the loop, or just one?!?!). 7 through 1 exits the do-while, not the switch. >Finally, Stroustrup asks the rhetorical question ``why would anyone This has been called `Duff's device' (after Tom Duff, who probably did >want to write something like this.'' Any guesses?! not invent it first), and it looks exactly like what an optimising compiler generates when it does loop unrolling. It works because case labels are just that---labels. As long as there is a switch in scope, a case label is legal; the case applies to the closest switch. (And if C had used a separate keyword for `break switch', the whole thing could be consistent :-) .) Why? To quote a certain infamous C language hack :-) , `It looks exactly Incidentally, there are some compilers that choke on that form. I You must Sign in before you can post messages.
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