Switch statement fall-through...should it be allowed? [closed]

Question

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 double-edged sword. It is sometimes very useful, but 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.

Have you heard of Duff's device? This is a great example of using switch fallthrough.

It's a feature that can be used and it can be abused, like almost all language features.

Fall-through 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.

It can be very useful a few times, but in general, no fall-through is the desired behavior. Fall-through should be allowed, but not implicit.

An example, to update old versions of some data:

switch (version) {
    case 1:
        // Update some stuff
    case 2:
        // Update more stuff
    case 3:
        // Update even more stuff
    case 4:
        // And so on
}