Question

I’ve read this question about the “jump to case label” error, but I still have some questions. I’m using g++ 4.7 on Ubuntu 12.04.

This code gives an error:

int main() {
  int foo = 1;
  switch(foo) {
  case 1:
    int i = 0;
    i++;
    break;
  case 2:
    i++;
    break;
  }
}

The error is

jump-to-case-label.cpp: In function ‘int main()’:
jump-to-case-label.cpp:8:8: error: jump to case label [-fpermissive]
jump-to-case-label.cpp:5:9: error:   crosses initialization of ‘int i’

However, this code compiles fine,

int main() {
  int foo = 1;
  switch(foo) {
  case 1:
    int i;
    i = 0;
    i++;
    break;
  case 2:
    i++;
    break;
  }
}

Is the second code any less dangerous than the first? I’m confused as to why g++ allows it.

Secondly, the fix for this problem is to scope the initialized variable. If the initialized variable is a large object, and the switch statement is in a while loop, won’t the constructor and destructor be called each time that scope is entered and left, causing a decrease in efficiency? Or will the compiler optimize this away?

Answer 1

Jumping past the initialization of an object, even if the object is of type int, is always undefined behavior. Note, that the switch-statement’s statement isn’t anything special: It is just a statement and people have [ab-]used this interesting ways, e.g., for Duff’s Device. The only thing which is special within the statement is that labels can take the form default: and case <const-integer-expr>:.

The statement int i; is a definition of the variable but no initialization. Thus, no initialization of a variable is by-passed. There is no bigger problem jumping past this definition than there is in the first place. Of course, the value gets assigned when jumping to case 1: and not when jumping to case 2: but this is no different than what happens in code outside of switch-statements if people only define variables.