c switch default

Question:

Hello. While trying to answer one of the questions here, I was surprised to find that the following code

#include <stdio.h>
int main()
{
    int a = 123;
    const char *r = "initial value";
    switch (a) {
        case 1: r = "1"; break;
        if (0) {
            default: r = "not 1";
        }
    }
    puts(r);
    return 0;
}

GCC compiles normally and prints at runtime

not 1

It seems to me that the code is clearly with such an error that it should not even compile. What's the matter here?

Answer:

The body of a switch (like the body of while , do/while , for , etc.) is just a statement in which we place the case and default labels, and in which the use of break possible. No more.

The case and default labels can appear only inside a statement belonging to some switch , but otherwise the restrictions imposed on the arrangement of these labels are no different from any other labels.

In a similar way, the transfer of control to these labels occurs according to rules similar to goto rules, with the only difference that only the immediately enclosing switch can transfer control to the case and default label.

In your case, if you replace the case and default labels with "regular" labels, we get just a compound statement

{
    l1:
      r = "1"; break;
    if (0) {
      l2:
        r = "not 1";
    }
}

There is nothing unusual in this compound statement. The position of the labels in it does not violate any language rules. It is not unusual for the default: label to transfer control to a branch of some if .

Nobody forces you to even use the compound statement to implement the switch body, i.e. contrary to common practice, the switch body does not need to be enclosed in {} . You can use any other type of statament and place labels in it

int i = rand() % 4;
switch (i)
  if (0)      case 0:  printf("Privet!\n");
  else if (0) case 1:  printf("Hello!\n");     
  else if (0) case 2:  printf("World!\n");     
  else        default: printf("Ku-ku!\n");     

Note that this great switch technique saves you the trouble of remembering break at the end of each branch.