operator<<

template<class E, class T, size_t N>
    basic_ostream<E, T>& operator<<(basic_ostream<E, T>& os,
        const bitset<N>& x);

The template function overloads operator<< to insert a text representation of the bit sequence in os. It effectively executes os << x.to_string<E, char_traits<E>, allocator<E> >(), then returns os.