How to check if an iterator is an output_iterator in c++?

Question

template<typename Iterator>
void put_value(Iterator pos, int n)
{
    static_assert(IsOutputIterator<Iterator>); 
    //
    // How to implement IsOutputIterator?
    //

    *pos = n;
}

std::iterator_traits<Iterator>::iterator_category doesn't help. For example: vector<int>::iterator is obvious an output_iterator, but std::iterator_traits<vector<int>::iterator>::iterator_category will returns random_access_iterator, which might not be an output_iterator, say a const_iterator.

Is there any viable way to check if an iterator is an output_iterator in c++?

Answer 1

My first response is to ask "Output iterator for what?" C++ output iterators don't specify a value type, because the same iterator may be able to output multiple value types. The only way to determine if you can write a given expression E through a given iterator o is to see if *o = std::declval<decltype((E))>() is a valid expression.

In C++14, I'd define a trait to do so:

template <class...> using void_t = void;

template <class, class, class = void>
constexpr bool is_output_iterator = false;

template <class I, class E>
constexpr bool is_output_iterator<I, E, void_t<
    typename std::iterator_traits<I>::iterator_category,
    decltype(*std::declval<I>() = std::declval<E>())>> = true;

In C++ with concepts - which I assume you are interested in since you tagged this question with c++-concepts - I would grab the sample Ranges TS implementation from github and use its OutputIterator<I, E>() concept instead.