What is the difference between iterator_category and iterator_concept?

C++20 brings a more powerful iterator system, one of them is to introduce iterator_concept on the basis of iterator_category.

I found that the iterator_concept and iterator_category of many iterators in C++20 are inconsistent. Take the most famous iota_view as an example:

using R = decltype(views::iota(0));
static_assert(random_access_range<R>);

using I = ranges::iterator_t<R>;
static_assert(same_as<typename I::iterator_category, input_iterator_tag>);
static_assert(same_as<typename I::iterator_concept,  random_access_iterator_tag>);

Although R models random_access_range, the iterator_category of its iterator is just an input_iterator_tag, which is inconsistent with the iterator_concept.

Why does C++20 introduce iterator_concept? What is its purpose? If I implement my own iterator, how do I define iterator_concept and iterator_category correctly? Does iterator_category still have a meaning in C++20?

There are differences between the C++17 (C++98) iterator model and the C++20 Ranges iterator model that are not backwards compatible. The two big ones are:

1. The C++98 model requires that forward iterators have a reference that is either value_type& or value_type const&.
2. The C++98 model does not allow for contiguous iterators. The strongest category was random_access.

The consequence of (1) is pretty significant - it means that if you have an iterator that returns a prvalue (whether proxy reference or not), it can never be stronger than an input iterator. So, views::iota(1, 10), despite easily being able to support random access, is only, at best, a C++98 input range.

However, you can't just... remove this requirement. Existing code that assumes C++98 iterators and uses iterator_category to make judgements is perfectly within its rights to assume that if iterator_category is, say, bidirectional_iterator_tag, that its reference is some kind of lvalue reference to value_type.

What iterator_concept does is add a new C++20 layer that allows an iterator to both advertise its C++98/17 category and, distinctly, advertise its C++20 category. So going back to the iota_view<int, int> example, that view's iterator has iterator_category set to input_iterator_tag (because the reference is a prvalue and so it does not satisfy the old requirements for even forward) but its iterator_concept is set to random_access_iterator_tag (because once we drop that restriction, we can easily support all the random access restrictions).

In [iterator.concepts.general], we have this magic function ITER_CONCEPT(I) which helps us figure out what tag to use in C++20.

The issue with (2) is that it was hard to just add a new contiguous_iterator_tag before due to the way that various C++98/17 code would check for that tag (lots of code might check for exactly random_access_iterator_tag). The iterator_concept approach avoids this problem by also introducing concepts that directly check the right thing for you (i.e. the random_access_iterator concept checks that ITER_CONCEPT(I) derives from random_access_iterator_tag, not that it simply is that).

Guidelines:

• If you're using an iterator in C++17, use std::iterator_traits<I>::iterator_category.
• If you're using an iterator in C++20, use the std::meow_iterator concepts
• If you're writing an iterator in C++17, add the iterator_category alias and make sure you follow the forward iterator/reference restriction (or... don't, but it's on you)
• If you're writing an iterator in C++20, follow the guidance in P2259 which has a good description of the problem and how and when to provide the iterator_category and iterator_concept type aliases.