/in/foo.cc: In function 'std::__cxx11::string my_add(std::__cxx11::string, std::__cxx11::string, int)':
/in/foo.cc:35:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.length(); i++)
                  ~~^~~~~~~~~~~~
/in/foo.cc: In function 'bool check(std::__cxx11::string)':
/in/foo.cc:50:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a.length(); i++)
                  ~~^~~~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string xx(std::__cxx11::string)':
/in/foo.cc:58:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < x.length(); i++)
                  ~~^~~~~~~~~~~~