foo.cc: In function ‘int main()’:
foo.cc:10:13: warning: zero-length gnu_printf format string [-Wformat-zero-length]
   10 |         Q(4,"");
      |             ^~
foo.cc:2:30: note: in definition of macro ‘Q’
    2 | #define Q(A,B) case A:printf(B);break;
      |                              ^