/in/foo.cc: In function 'bool cmp(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:15:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < a.length(); ++i) {
                 ~~^~~~~~~~~~~~
/in/foo.cc:18:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < b.length(); ++i) {
                 ~~^~~~~~~~~~~~