How to define is_iterator type trait?

Question

• I'm trying to code a is_iterator<T> type trait. Where when T is an iterator type is_iterator<T>::value == true otherwise is is_iterator<T>::value == false.

• What I tried so far:

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template <class, class Enable = void> 
struct is_iterator : std::false_type {};

template <typename T> 
struct is_iterator<T, typename std::enable_if<std::is_pointer<typename
     std::iterator_traits<T>::pointer>::value>::type> : std::true_type {};

LIVE DEMO

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Q: Is there a more proper way to define a is_iterator type trait than the one displayed above?

Answer 1

Your check fails if std::iterator_traits<T>::pointer is a type that is not a pointer, for instance if T = std::ostream_iterator<U>.

I think a better test might be whether std::iterator_traits<T>::iterator_category is either std::input_iterator_tag or a derived type, or std::output_iterator_tag.

template <class, class Enable = void> struct is_iterator : std::false_type {};
template <typename T> 
struct is_iterator
<T, 
 typename std::enable_if<
    std::is_base_of<std::input_iterator_tag, typename std::iterator_traits<T>::iterator_category>::value ||
    std::is_same<std::output_iterator_tag, typename std::iterator_traits<T>::iterator_category>::value 
 >::type> 
 : std::true_type {};