using namespace std;
namespace std
{
  template<class _CharT> struct char_traits;
  template<typename _CharT, typename _Traits = char_traits<_CharT> > class basic_ostream;
  typedef basic_ostream<char> ostream;
}
class A  {};
class B: public A {};
class C: public B
{
  virtual void f(std::ostream& os) const;
};
void C::f(ostream& os) const {}