non-member function overloading an operator. Example defined in ostream to work with primitive normal types, which be idea to expand for user-defined object to output it into output stream. Excerpted from ostream header. See https://www.quora.com/What-are-the-differences-between-a-member-operator-function-and-a-friend-operator-function.

gistfile1.txt

    //@{
    /**
     *  @brief  String inserters
     *  @param  __out  An output stream.
     *  @param  __s  A character string.
     *  @return  out
     *  @pre  @p __s must be a non-NULL pointer
     *
     *  Behaves like one of the formatted arithmetic inserters described in
     *  std::basic_ostream.  After constructing a sentry object with good
     *  status, this function inserts @c traits::length(__s) characters starting
     *  at @p __s, widened if necessary, followed by any required padding (as
     *  determined by [22.2.2.2.2]).  @c __out.width(0) is then called.
     */
    template<typename _CharT, typename _Traits>
        inline basic_ostream<_CharT, _Traits>&
        operator<<(basic_ostream<_CharT, _Traits>& __out, const _CharT* __s)
        {
            if (!__s)
                __out.setstate(ios_base::badbit);
            else
                __ostream_insert(__out, __s,
                        static_cast<streamsize>(_Traits::length(__s)));
            return __out;
        }

    template<typename _CharT, typename _Traits>
        basic_ostream<_CharT, _Traits> &
        operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s);

    // Partial specializations
    template<class _Traits>
        inline basic_ostream<char, _Traits>&
        operator<<(basic_ostream<char, _Traits>& __out, const char* __s)
        {
            if (!__s)
                __out.setstate(ios_base::badbit);
            else
                __ostream_insert(__out, __s,
                        static_cast<streamsize>(_Traits::length(__s)));
            return __out;
        }

    // Signed and unsigned
    template<class _Traits>
        inline basic_ostream<char, _Traits>&
        operator<<(basic_ostream<char, _Traits>& __out, const signed char* __s)
        { return (__out << reinterpret_cast<const char*>(__s)); }

    template<class _Traits>
        inline basic_ostream<char, _Traits> &
        operator<<(basic_ostream<char, _Traits>& __out, const unsigned char* __s)
        { return (__out << reinterpret_cast<const char*>(__s)); }
    //@}