Detect output iterator value type

iterator_value.hpp

#include <iterator>

template <typename T>
struct output_iterator_value {
    using type = void;
};

template <typename Container>
struct output_iterator_value<std::insert_iterator<Container>> {
    using type = typename Container::value_type;
};

template <typename Container>
struct output_iterator_value<std::back_insert_iterator<Container>> {
    using type = typename Container::value_type;
};

template <typename Container>
struct output_iterator_value<std::front_insert_iterator<Container>> {
    using type = typename Container::value_type;
};

template <typename T, typename CharT, typename CharTraits>
struct output_iterator_value<std::ostream_iterator<T, CharT, CharTraits>> {
    using type = T;
};

template <typename T>
struct iterator_value {
    using iterator_traits = std::iterator_traits<T>;
    using type = typename std::conditional<
            std::is_same<typename iterator_traits::iterator_category,
                std::output_iterator_tag>::value,
            typename output_iterator_value<T>::type,
            typename iterator_traits::value_type
        >::type;
};