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std::iterator_traits<std::counted_iterator>

From cppreference.com
 
 
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Utilities
(C++20)
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
 
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.

Contents

[edit] 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.

[edit] Example

#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>());
}

[edit] 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

[edit] See also

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