C++ Error: 'x' is not a constant expression, how to fix?

Question

To make it short, here is a minimal example:

struct C {
    const int X = 2;
    int y = 2;
};
void f(C* x) {
    switch(x->y) {
        case x->X: printf("%d", 42); break;
        case 123: printf("foo");
    }
}
int main()
{
    C c;
    f(&c);
    return 0;
}

Why is the compiler complaining error: 'x' is not a constant expression and how to fix it?

Answer 1

Case labels inside switch accept only compile-time constant expressions. x->X is not a constant expression; hence it could not be used as a case label.

Replace the switch statement with an if to fix this problem:

if (x->y == x->X) {
    printf("%d", 42);
} else if (x->y == 123) {
    printf("foo");
}