/in/foo.cc: In function 'int main()':
/in/foo.cc:18:9: warning: unused variable 'num' [-Wunused-variable]
  int n, num, ch, ma, en, all;
         ^~~
/in/foo.cc:18:26: warning: unused variable 'all' [-Wunused-variable]
  int n, num, ch, ma, en, all;
                          ^~~
/in/foo.cc: In function 'bool cmp(grade, grade)':
/in/foo.cc:16:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^