/in/foo.cc: In function 'int main()':
/in/foo.cc:12:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (si >= 0 && si < s.length() && si + len >= 0 && si + len <= s.length()) {
                    ~~~^~~~~~~~~~~~
/in/foo.cc:12:65: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (si >= 0 && si < s.length() && si + len >= 0 && si + len <= s.length()) {
                                                        ~~~~~~~~~^~~~~~~~~~~~~