foo.cc: In function ‘std::string num(std::string)’:
foo.cc:7:22: 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<=s.size();i++)
      |                     ~^~~~~~~~~~
foo.cc:15:1: warning: control reaches end of non-void function [-Wreturn-type]
   15 | }
      | ^