# std::iterator_traits<std::counted_iterator>

From cppreference.com
< cpp‎ | iterator‎ | counted iterator

Defined in header `<iterator>` |  |
template< std::input_iterator I > requires /* see below */ struct iterator_traits<std::counted_iterator<I>> : std::iterator_traits<I> { using pointer = std::conditional_t<std::contiguous_iterator<I>, std::add_pointer_t<std::iter_reference_t<I>>, void>; }; |  | (since C++20)

Inherits the properties from customized (generated from either a standard partial specialization or a program-defined specialization) std::iterator_traits<I>, with the member type `pointer` adjusted, where `I` models `input_iterator`.

Notably, the `iterator_concept` (if present) and `iterator_category` are inherited from std::iterator_traits<I>.

The condition in the requires-clause is true if and only if std::iterator_traits<I> is not generated from the primary template.

### Note

Before P2259R1, this specialization is used even if std::iterator_traits<I> is generated from the primary template. As a result, when testing std::counted_iterator<I> against an iterator concept (e.g. `forward_iterator`), the determination of /*ITER_CONCEPT*/ does not take `I::iterator_concept` into account, and thus std::counted_iterator<I> sometimes erroneously behaves as if it cannot model that concept. This incorrect behavior is implemented in libstdc++ prior to 10.4, and in MSVC STL prior to VS 2022 17.0 Preview 3.

The standard library provides partial specializations of std::iterator_traits for pointer types, std::counted_iterator, and std::common_iterator.

### Example

Run this code

    #include <iterator>
    #include <type_traits>
    #include <list>
    #include <vector>

    int main() {
        std::vector v{1,2,3,4};
        std::list l{1,2,3,4};
        std::counted_iterator iv{v.begin(), 3};
        std::counted_iterator il{l.begin(), 3};
        static_assert(std::is_same<int*, std::iterator_traits<decltype(iv)>::pointer>());
        static_assert(std::is_same<void, std::iterator_traits<decltype(il)>::pointer>());
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2259R1 | C++20 | there's no requires-clause `pointer` is unconditionally defined as `void` | constraint added

### See also

iterator_traits | provides uniform interface to the properties of an iterator (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/counted_iterator/iterator_traits&oldid=133952"