foo.cc: In function ‘bool f(std::string)’:
foo.cc:6:17: warning: comparison of integer expressions of different signedness: ‘ll’ {aka ‘long long int’} and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    6 |     for(ll i=0;i<x.length();++i){
      |                ~^~~~~~~~~~~
foo.cc: In function ‘std::string add(std::string, ll)’:
foo.cc:14:17: warning: comparison of integer expressions of different signedness: ‘ll’ {aka ‘long long int’} and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   14 |     for(ll i=0;i<x.length();++i){
      |                ~^~~~~~~~~~~
foo.cc:23:17: warning: comparison of integer expressions of different signedness: ‘ll’ {aka ‘long long int’} and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   23 |     for(ll i=0;i<x.length();++i){
      |                ~^~~~~~~~~~~