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

Asked 2008-10-09T18:11:08.903

185

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.

coding-style switch-statement

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2 Answers

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).

answered 2011-09-10T21:07:43.357

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.

answered 2008-10-09T18:17:58.157

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

2 Answers

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