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std::distance(3) C++ Standard Libary std::distance(3) NAME std::distance - std::distance Synopsis Defined in header <iterator> template< class InputIt > typename std::iterator_traits<InputIt>::difference_type (constexpr since C++17) distance( InputIt first, InputIt last ); Returns the number of hops from first to last. If InputIt is not LegacyRandomAccessIterator, the behavior is unde- fined if last is not reachable from first. If InputIt is LegacyRandomAccessIterator, the behavior is undefined if first and last are neither reachable from each other. Parameters first - iterator pointing to the first el- ement last - iterator pointing to the end of the range Type requirements - InputIt must meet the requirements of LegacyInputIterator. The oper- ation is more efficient if InputIt additionally meets the requirements of LegacyRandomAccessIterator. Return value The number of increments needed to go from first to last. The value may be negative if random-access iterators are used and (since C++11) first is reachable from last. Complexity Linear. However, if InputIt additionally meets the requirements of LegacyRandomAccessIterator, complexity is constant. Possible implementation See also the implementations in libstdc++ and libc++. C++98 implementation via tag dispatch, with constexpr removed namespace detail { template<class It> constexpr // required since C++17 typename std::iterator_traits<It>::difference_type do_distance(It first, It last, std::input_iterator_tag) { typename std::iterator_traits<It>::difference_type result = 0; while (first != last) { ++first; ++result; } return result; } template<class It> constexpr // required since C++17 typename std::iterator_traits<It>::difference_type do_distance(It first, It last, std::random_access_itera- tor_tag) { return last - first; } } // namespace detail template<class It> constexpr // since C++17 typename std::iterator_traits<It>::difference_type distance(It first, It last) { return detail::do_distance(first, last, typename std::itera- tor_traits<It>::iterator_category()); } C++17 implementation via if constexpr template<class It> constexpr typename std::iterator_traits<It>::difference_type distance(It first, It last) { using category = typename std::iterator_traits<It>::itera- tor_category; static_assert(std::is_base_of_v<std::input_iterator_tag, cate- gory>); if constexpr (std::is_base_of_v<std::random_access_iterator_tag, category>) return last - first; else { typename std::iterator_traits<It>::difference_type result = 0; while (first != last) { ++first; ++result; } return result; } } Example // Run this code #include <iostream> #include <iterator> #include <vector> int main() { std::vector<int> v{3, 1, 4}; std::cout << "distance(first, last) = " << std::distance(v.begin(), v.end()) << '\n' << "distance(last, first) = " << std::distance(v.end(), v.begin()) << '\n'; // the behavior is undefined (until LWG940) static constexpr auto il = {3, 1, 4}; // Since C++17 `distance` can be used in constexpr context. static_assert(std::distance(il.begin(), il.end()) == 3); static_assert(std::distance(il.end(), il.begin()) == -3); } Output: distance(first, last) = 3 distance(last, first) = -3 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 940 C++98 the wording was unclear for the case where first made clear is reachable from last See also advance advances an iterator by given distance (function template) count returns the number of elements satisfying specific criteria count_if (function template) ranges::distance returns the distance between an iterator and a sen- tinel, or between (C++20) the beginning and end of a range (niebloid) http://cppreference.com 2024.06.10 std::distance(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Possible implementation | Example | Output: | See also
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