Switch statement fall-through...should it be allowed?

Question

For as long as I can remember I have avoided using switch statement fall-through. 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 fall-through.

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 fall-through. 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 very useful and safe. The second seems kind of dangerous.

Answer 1

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"

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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
}

Answer 6

It is powerful and dangerous. The biggest problem with fall-through is that it's not explicit. For example, if you come across frequently-edited code that has a switch with fall-throughs, how do you know that's intentional and not a bug?

Anywhere I use it, I ensure that it's properly commented:

switch($var) {
    case 'first':
        // Fall-through
    case 'second':
        i++;
        break;
 }

Answer 7

I'd love a different syntax for fallbacks in switches, something like, errr..

switch(myParam)
{
  case 0 or 1 or 2:
    // Do something;
    break;
  case 3 or 4:
    // Do something else;
    break;
}

Note: This would already be possible with enums, if you declare all cases on your enum using flags, right? It doesn't sound so bad either; the cases could (should?) very well be part of your enum already.

Maybe this would be a nice case (no pun intended) for a fluent interface using extension methods? Something like, errr...

int value = 10;
value.Switch()
  .Case(() => { /* Do something; */ }, new {0, 1, 2})
  .Case(() => { /* Do something else */ } new {3, 4})
  .Default(() => { /* Do the default case; */ });

Although that's probably even less readable :P

Answer 8

As with anything: if used with care, it can be an elegant tool.

However, I think the drawbacks more than justify not to use it, and finally not to allow it anymore (C#). Among the problems are:

• it's easy to "forget" a break
• it's not always obvious for code maintainers that an omitted break was intentional

Good use of a switch/case fall-through:

switch (x)
{
case 1:
case 2:
case 3:
 Do something
 break;
}

Baaaaad use of a switch/case fall-through:

switch (x)
{
case 1:
    Some code
case 2:
    Some more code
case 3:
    Even more code
    break;
}

This can be rewritten using if/else constructs with no loss at all in my opinion.

My final word: stay away from fall-through case labels as in the bad example, unless you are maintaining legacy code where this style is used and well understood.

Answer 9

Using fall-through like in your first example is clearly OK, and I would not consider it a real fall-through.

The second example is dangerous and (if not commented extensively) non-obvious. I teach my students not to use such constructs unless they consider it worth the effort to devote a comment block to it, which describes that this is an intentional fall-through, and why this solution is better than the alternatives. This discourages sloppy use, but it still makes it allowed in the cases where it is used to an advantage.

This is more or less equivalent to what we did in space projects when someone wanted to violate the coding standard: they had to apply for dispensation (and I was called on to advise about the ruling).

Answer 10

In some instances, using fall-throughs is an act of laziness on the part of the programmer - they could use a series of || statements, for example, but instead use a series of 'catch-all' switch cases.

That being said, I've found them to be especially helpful when I know that eventually I'm going to need the options anyway (for example in a menu response), but have not yet implemented all the choices. Likewise, if you're doing a fall-through for both 'a' and 'A', I find it substantially cleaner to use the switch fall-through than a compound if statement.

It's probably a matter of style and how the programmers think, but I'm not generally fond of removing components of a language in the name of 'safety' - which is why I tend towards C and its variants/descendants more than, say, Java. I like being able to monkey-around with pointers and the like, even when I have no "reason" to.

Answer 11

I don't like my switch statements to fall through - it's far too error prone and hard to read. The only exception is when multiple case statements all do exactly the same thing.

If there is some common code that multiple branches of a switch statement want to use, I extract that into a separate common function that can be called in any branch.

Answer 12

Fall-through should be used only when it is used as a jump table into a block of code. If there is any part of the code with an unconditional break before more cases, all the case groups should end that way.

Anything else is "evil".