/in/foo.cc: In function 'int main()':
/in/foo.cc:12:22: error: 'print' was not declared in this scope
  print(zhongxu, houxu);
                      ^
/in/foo.cc: In function 'void print(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:27:23: error: 'std::__cxx11::string {aka class std::__cxx11::basic_string<char>}' has no member named 'substring'; did you mean 'substr'?
    string a = zhongxu.substring(0, i);
                       ^~~~~~~~~
/in/foo.cc:28:21: error: 'std::__cxx11::string {aka class std::__cxx11::basic_string<char>}' has no member named 'substring'; did you mean 'substr'?
    string b = houxu.substring(0, i);
                     ^~~~~~~~~
/in/foo.cc:31:16: error: 'std::__cxx11::string {aka class std::__cxx11::basic_string<char>}' has no member named 'substring'; did you mean 'substr'?
    a = zhongxu.substring(i + 1, length - i - 1);
                ^~~~~~~~~
/in/foo.cc:32:14: error: 'std::__cxx11::string {aka class std::__cxx11::basic_string<char>}' has no member named 'substring'; did you mean 'substr'?
    b = houxu.substring(i, length - i - 1);
              ^~~~~~~~~