/in/foo.cc: In function 'int main()':
/in/foo.cc:13:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < str.size() - substr.size(); i++) {
                  ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:16:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (int j = 0; j < substr.size(); j++) {
                    ~~^~~~~~~~~~~~~~~