How to use iterator_category to indicate the tag of a customized iterator class?

Question

As titled, the following code snippet doesn't work even I defined the type "iterator_category" and made it visible to the outside in the class definition, the expectation of the output should be '1' rather than '0'. How should I fix that?

struct Iter {
  using iterator_category = std::random_access_iterator_tag;
};
int main(int argc, char **argv) {
  std::cout << std::__has_iterator_typedefs<Iter>::value << std::endl;  // output '0';
  // std::cout << (typeid(std::iterator_traits<Iter>::iterator_category) == typeid(std::random_access_iterator_tag)) << std::endl;
  return 0;
}

Answer 1

iterator_category is not the only member type that a standard library compatible iterator should provide. If you take look at how __has_iterator_typedefs is implemented , you'll see that it also checks for the presence of these member types: difference_type , value_type , reference , and pointer . Add all these, and then std::iterator_traits<Iter>::iterator_category will return the category of your iterator.

struct Iter {
  using iterator_category = std::random_access_iterator_tag;
  using value_type        = int;
  using difference_type   = std::ptrdiff_t;
  using reference         = value_type&;
  using pointer           = value_type*;
};

static_assert(std::is_same_v<std::iterator_traits<Iter>::iterator_category, 
                             std::random_access_iterator_tag>);