Declaring variables inside C switch/case

Question

Well, this is not actually a question..

I have just occasionally found out that there's an interesting way to declare local variables inside a switch/case block. Instead of using braces inside every case block, you can write:

switch (action) {
  int res;
  int value;
case ACTION_OPEN:
  res = open(...);
  ...
  break;
case  ...
}

So, I just wonder what C/C++ compilers besides gcc support this construction? It looks like a common fall-through. Any comments on this construction are welcome!

Answer 1

The switch body is just a normal statement (in your case a compound statement, looking like { ... }) which can contain any crap. Including case labels.

This switch philosophy is abused by Duffs device.

Many people don't realize that even something like switch(0) ; is a valid statement (instead of having a compound statement, it has a null statement as body), although quite useless.

Answer 2

Any standards-conforming C or C++ compiler will allow this . Even an old-fashioned (pre-ISO C99) C compiler will allow this, but only because the variable declarations are at the start of a block/compound statement (denoted by {}).

Note that what follows a switch is almost a normal statement, except for the possibility of case labels:

int main(int argc, char* argv[])
{
    switch (argc)
      default:
        puts("Hello, world!");

    return 0;
}

So in ANSI C89, it's the braces that do the magic here.