foo.cc: In function 'int main()':
foo.cc:9:27: error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' to 'const char*' for argument '1' to 'size_t strlen(const char*)'
     for(int i=0;i<strlen(a);i++)
                           ^

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。