/in/foo.cc: In function 'int main()':
/in/foo.cc:7:17: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    7 |   for (int i=0;i<a.length();i++)
      |                ~^~~~~~~~~~~