c++ - 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

As I said in comments, the solutions presented here rely on non-portable properties of iterators_traits in some implementations. According to the C++03 and C++11 standards, iterator_traits is only defined for iterators (and the special case of pointers) so any other use is undefined behaviour. Specifically, using iterator_traits<T>::pointer in a SFINAE context isn't guaranteed to work, because instantiating iterator_traits<T> will refer to T::value_type, T::pointer, T::iterator_category etc. and that happens outside the "immediate context" where SFINAE doesn't apply.

C++14 will fix that was supposed to fix that (it happened post-C++14 with DR 2408), but for C++11 the safe way to define is_iterator is to write a trait that checks for all the required operations an iterator must define. The only operations that all iterators are required to support are operator* and pre- and post-increment. Unfortunately, there can be types that define those operations which are not valid iterators, so writing a correct trait is quite hard.