/in/foo.cc: In function 'std::string subtract(std::string, std::string)':
/in/foo.cc:23:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   23 |     for (int i = 0; i < a.length(); ++i) {
      |                     ~~^~~~~~~~~~~~
/in/foo.cc:25:25: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   25 |         int digitB = (i < b.length()) ? (b[i] - '0') : 0;
      |                       ~~^~~~~~~~~~~~