std::distance

 

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Defined in header <iterator>
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.

The behavior is undefined if last is not reachable from first by (possibly repeatedly) incrementing first.	(until C++11)
If InputIt is not LegacyRandomAccessIterator, the behavior is undefined if last is not reachable from first by (possibly repeatedly) incrementing first. If InputIt is LegacyRandomAccessIterator, the behavior is undefined if last is not reachable from first and first is not reachable from last.	(since C++11)

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 first is reachable from last (since C++11)

Complexity

Linear.

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

Possible implementation

See also the implementations in libstdc++ and libc++.

First version
// implementation via tag dispatch, available in C++98 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()); }
Second version
// implementation via constexpr if, available in C++17 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 C++11)
 
    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

See also

advance	advances an iterator by given distance (function template)
countcount_if	returns the number of elements satisfying specific criteria (function template)
ranges::distance (C++20)	returns the distance between an iterator and a sentinel, or between the beginning and end of a range (niebloid)