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Showing content from https://en.cppreference.com/w/cpp/language/../regex/../algorithm/../iterator/distance.html below:

std::distance - cppreference.com

template< class InputIt > typename std::iterator_traits<InputIt>::difference_type distance( InputIt first, InputIt last ); (constexpr since C++17)

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.

[edit] 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. [edit] 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 first is reachable from last.

(since C++11) [edit] Complexity

Linear.

However, if InputIt additionally meets the requirements of LegacyRandomAccessIterator, complexity is constant.

[edit] 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;
    }
}

[edit] Example

#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

[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 LWG 940 C++98 the wording was unclear for the case where first is reachable from last made clear [edit] See also advances an iterator by given distance
(function template) [edit] returns the number of elements satisfying specific criteria
(function template) [edit] returns the distance between an iterator and a sentinel, or between the beginning and end of a range
(algorithm function object)[edit]

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