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 undefined 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 element
   last - iterator pointing to the end of the range

Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator. The operation 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_iterator_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::iterator_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>::iterator_category;
       static_assert(std::is_base_of_v<std::input_iterator_tag, category>);
       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 retroactively 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 sentinel, or between
   (C++20) the beginning and end of a range
                    (niebloid)