Why can't variables be declared in a switch statement?

Question

I\'ve always wondered this - why can\'t you declare variables after a case label in a switch statement? In C++ you can declare variables pretty much anywhere (and declaring

Answer 1

A switch block isn't the same as a succession of if/else if blocks. I'm surprised no other answer explains it clearly.

Consider this switch statement :

switch (value) {
    case 1:
        int a = 10;
        break;
    case 2:
        int a = 20;
        break;
}

It may be surprising, but the compiler will not see it as a simple if/else if. It will produce the following code :

if (value == 1)
    goto label_1;
else if (value == 2)
    goto label_2;
else
    goto label_end;

{
label_1:
    int a = 10;
    goto label_end;
label_2:
    int a = 20; // Already declared !
    goto label_end;
}

label_end:
    // The code after the switch block

The case statements are converted into labels and then called with goto. The brackets create a new scope and it is easy to see now why you can't declare two variables with the same name within a switch block.

It may look weird, but it is necessary to support fallthrough (that is, not using break to let execution continue to the next case).