/in/foo.cc: In function 'void del(std::__cxx11::string&, std::__cxx11::string&)':
/in/foo.cc:12:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (j = 0; j < s2.length(); j++)
              ~~^~~~~~~~~~~~~
/in/foo.cc:14:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < s1.length(); i++)
               ~~^~~~~~~~~~~~~
/in/foo.cc: In function 'void output(std::__cxx11::string)':
/in/foo.cc:26:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < s1.length(); i++)
              ~~^~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:36:6: warning: unused variable 'i' [-Wunused-variable]
  int i;
      ^