/in/foo.cc:100:22: warning: multi-character character constant [-Wmultichar]
   cout<<"class "<<i<<': '<<endl;
                      ^
/in/foo.cc: In function 'bool cmp(stu, stu)':
/in/foo.cc:14:26: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
  if(a.ji>b.ji||a.ji==b.ji&&a.name<b.name){
                          ^