/in/foo.cc: In function 'int Diff(std::__cxx11::string)':
/in/foo.cc:8:24: error: invalid use of member function 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size() const [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>; std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::size_type = long unsigned int]' (did you forget the '()' ?)
  for (int i = 0; i < s.size; i++)
                      ~~^~~~