foo.cc: In function 'int main()':
foo.cc:16:34: error: no match for 'operator*' (operand types are 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' and 'int')
   cout << "快递费为：" << a * 5;
                                ~~^~~

自豪的采用jd5进行评测(github.com/masnn/jd5)