/in/foo.cc: In function 'void del(std::__cxx11::string&, int, int)':
/in/foo.cc:12:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(si>s1.length()) return;
     ~~^~~~~~~~~~~~
/in/foo.cc:13:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=si;i<si+len && i<s1.length();i++)
                       ~^~~~~~~~~~~~
/in/foo.cc: In function 'void output(std::__cxx11::string)':
/in/foo.cc:21:16: 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:31:6: warning: unused variable 'i' [-Wunused-variable]
  int i,si,len;
      ^