Check for std::iostream input/output operators

has_io_operator.hpp

template < typename T >
class has_output_operator {
    template < typename U >
    using helper = decltype( ::std::declval<::std::ostream&>() << ::std::declval<U>(), void() );

    template < typename U = T>
    static ::std::true_type  test(helper<U>*);
    static ::std::false_type test(...);
public:
    using type = decltype(test(nullptr));
    static constexpr bool value = type::value;
};

template < typename T >
class has_input_operator {
    template < typename U >
    using helper = decltype( ::std::declval<::std::istream&>() >> ::std::declval<U&>(), void() );

    template < typename U = T>
    static ::std::true_type  test(helper<U>*);
    static ::std::false_type test(...);
public:
    using type = decltype(test(nullptr));
    static constexpr bool value = type::value;
};