/in/foo.cc: In function 'std::__cxx11::string add(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:23:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<max(a.length(),b.length());i++){
              ~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:33:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=mm;i<mm+res.length();i++){
               ~^~~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:90:15: error: could not convert 'b' from 'int' to 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}'
  cout<<add(a,b);
               ^