/in/foo.cc: In function 'std::__cxx11::string times(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:35:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i <= b.size() - 1; i++)
                         ~~^~~~~~~~~~~~~~~
/in/foo.cc:37:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j <= a.size() - 1; j++)
                             ~~^~~~~~~~~~~~~~~
/in/foo.cc:52:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = start; i <= start_digit - 2 + a.size(); i++)
                             ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc: In function 'int comparison(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:66:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i <= a.size() - 1; i++)
                         ~~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string minusstr(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:104:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = start; i <= a.size(); i++)
                         ~~^~~~~~~~~~~