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From: dant@tekla.TEK.COM (Dan Tilque;1893;92-789;LP=A;60HC)
Newsgroups: comp.lang.c
Subject: Case fall throughs
Message-ID: <2201@zeus.TEK.COM>
Date: Thu, 13-Aug-87 21:58:47 EDT
Article-I.D.: zeus.2201
Posted: Thu Aug 13 21:58:47 1987
Date-Received: Sat, 15-Aug-87 11:39:38 EDT
Sender: news@zeus.TEK.COM
Reply-To: dant@tekla (Dan Tilque)
Organization: First National Security Trust Bar and Grill
Lines: 67


I (Dan Tilque) say that it's bad programming to have this kind of thing:
> 
> 	case 'a':
> 		a = b + count;
> 	case 'b':
> 		count += 2;
>
> Lint should definitely give warning since it's almost always the case that
> what's desired is a break before case 'b':

I expected a number of people to respond saying "I often write these kind of 
fall throughs".  So far only one person has done so.  I admire everyone
else's restraint (or else because of net lag I haven't seen the articles yet).

You might notice that, unlike Karl Heuer, I do not advocate removing this
feature completely from the language and substituting goto case labels
instead.  The reason is that a fall through is the fastest possible transfer
of control and there are some time critical routines which use fall through
for that very reason.

However, in the common ordinary program, I think it's cleaner to do something
like this:

 	case 'a':
 		a = b + count;
		common_statements();
		break;
 	case 'b':
		common_statements();
		break;

In those cases which have to have fall throughs, the /*FALLTHROUGH*/
comment to lint (as suggested by Karl Heuer) would also be a good thing to
put in even if lint didn't check for it.  It tells any maintenance
programmer that the break was deliberately left out and is not the cause
of the hard to track down bug.

) = Karl Heuer:
)In article <2182@zeus.TEK.COM> dant@tekla.UUCP (Dan Tilque) writes:
)>[If switch statements had automatic break instead of fall-through, the
)>current idiom "case CR: case LF: ..." wouldn't work.]  One possibility is
)>separating them with commas: "case CR, LF, TAB, ' ':" which changes the
)>meaning of the comma operator.  Probably not a good idea.
)
)The comma operator is neither necessary nor useful in a constant expression,
)and is explicitly disallowed by ANSI.

I checked in K&R and it's not allowed there either (by omission rather than
explicitely).


] = David Collier-Brown.
]	...
]	switch (foo) {
]	case 'f' | 'e':		-- one possible syntax...
]		/* whatever */
]	}

I thought of using | also.  Unfortunately, | is already legal here
and has a different meaning.  Looks like a comma is the better notation.
Actually, I like things the way they are except to have all lints check
for fall through.

---
Dan Tilque
dant@tekla.tek.com  or dant@tekla.UUCP