NAME

std::iterator_traits - std::iterator_traits

Synopsis

Defined in header <iterator>
template< std::input_iterator I >

requires /* see below */
struct iterator_traits<std::counted_iterator<I>> :
std::iterator_traits<I> { (since C++20)
using pointer = std::conditional_t<std::contiguous_iterator<I>,
std::add_pointer_t<std::iter_reference_t<I>>,
void>;

};

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 constraint added
pointer is unconditionally defined as void

See also

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