Explanation, please! - comp.lang.c

Message from discussion Explanation, please!

Doug Gwyn

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More options Aug 25 1988, 12:16 pm

Newsgroups: comp.lang.c

From: g...@smoke.ARPA (Doug Gwyn )

Date: 25 Aug 88 16:16:29 GMT

Local: Thurs, Aug 25 1988 12:16 pm

Subject: Re: Explanation, please!

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In article <6...@paris.ICS.UCI.EDU> schm...@bonnie.ics.uci.edu (Douglas C. Schmidt) writes:
- switch(count % 8) {
- case 0: do { *to++ = *from++;
- case 7: *to++ = *from++;
- case 6: *to++ = *from++;
- case 5: *to++ = *from++;
- case 4: *to++ = *from++;
- case 3: *to++ = *from++;
- case 2: *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?

Could you explain why it shouldn't be? "case" labels are just a
special form of label. You can stick a label on most statements.

-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

?? "Thus"? Why would the labels go out of scope? They're
definitely within the body of the switch.

-(should a break statement exit both the switch and the loop, or just one?!?!)

A "break" applies to whatever the innermost container is. A "break"
between two of the "*to++ = *from++;" statements would exit the
do-while loop.

-Finally, Stroustrup asks the rhetorical question ``why would anyone
-want to write something like this.'' Any guesses?!

It's about the fastest way to move arbitrarily-aligned data in
portable C with a guarantee as to what happens in the case that the
data overlaps. memcpy() doesn't guarantee anything for overlaps.
memmove() does, but that's a recent X3J11 invention that probably
doesn't exist on your system yet.

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