Switch Statement Fallthrough...should it be allowed?
For as long as I can remember I have avoided using switch statement fallthrough. Actually, I can't remember it ever entering my consciousness as a possible way to do things as it was drilled into my head early on that it was nothing more than a bug in the switch statement. However, today I ran across some code that uses it by design, which got me immediately wondering what everyone in the community thinks about switch statement fallthrough.
Is it something that a programming language should explicitly not allow (like C# does, though it supplies a workaround) or is it a feature of any language that is powerful enough to leave in the programmer's hands?
Edit: I wasn't specific enough to what I meant by fallthrough. I use this type a lot:
switch(m_loadAnimSubCt){
case 0:
case 1:
// Do something
break;
case 2:
case 3:
case 4:
// Do something
break;
}
However, I'm concerned about something like this.
switch(m_loadAnimSubCt){
case 0:
case 1:
// Do something but fall through to the other cases
// after doing it.
case 2:
case 3:
case 4:
// Do something else.
break;
}
This way whenever the case is 0, 1 it will do everything in the switch statement. I've seen this by design and I just don't know if I agree that switch statements should be used this way. I think the first code example is a very useful and safe. The second seems kind of dangerous.
It may depend on what you consider fallthrough. I'm ok with this sort of thing:
switch (value)
{
case 0:
result = ZERO_DIGIT;
break;
case 1:
case 3:
case 5:
case 7:
case 9:
result = ODD_DIGIT;
break;
case 2:
case 4:
case 6:
case 8:
result = EVEN_DIGIT;
break;
}
But if you have a case label followed by code that falls through to another case label, I'd pretty much always consider that evil. Perhaps moving the common code to a function and calling from both places would be a better idea.
And please note that I use the C++ FAQ definition of "evil"
It's a two-edged sword. Sometimes very useful, often dangerous.
When is it good? When you want 10 cases all processed the same way...
switch (c) {
case 1:
case 2:
... do some of the work ...
/* FALLTHROUGH */
case 17:
... do something ...
break;
case 5:
case 43:
... do something else ...
break;
}
The one rule I like is that if you ever do anything fancy where you exclude the break, you need a clear comment /* FALLTHROUGH */ to indicate that was your intention.
Fallthrough is really a handy thing, depending on what you're doing. Consider this neat and understandable way to arrange options:
switch ($someoption) {
case 'a':
case 'b':
case 'c':
// do something
break;
case 'd':
case 'e':
// do something else
break;
}
Imagine doing this with if/else. It would be a mess.
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