std::ranges::distance

 

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Technical Specifications

 

Iterator library

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indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
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iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (C++20) iter_value_titer_difference_titer_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++20)(C++20)
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reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) back_inserter front_inserter inserter
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ranges::iter_move (C++20)
ranges::iter_swap (C++20)
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advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
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Defined in header <iterator>
Call signature
template< std::input_or_output_iterator I, std::sentinel_for<I> S > constexpr std::iter_difference_t<I> distance( I first, S last );	(1)	(since C++20)
template< ranges::range R> constexpr ranges::range_difference_t<R> distance(R&& r);	(2)	(since C++20)

1) Returns the number of hops from first to last.

2) Same as (1), but uses r as the source range, using ranges::size(r) if R models ranges::sized_range; otherwise as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists may not be specified when calling any of them.
• None of them is visible to argument-dependent lookup.
• When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first	-	iterator pointing to the first element
last	-	sentinel denoting the end of the range first is an iterator to
r	-	range to calculate the distance of

Return value

The number of increments needed to go from first to last. The value may be negative if I and S model std::same_as<S, I> && std::sized_sentinel_for<S, I> are used and first is reachable from last.

Complexity

1) If S models std::sized_sentinel_for<S, I>, complexity is constant; otherwise linear.

2) If R models ranges::sized_range, complexity is constant; otherwise same as (1).

Possible implementation

struct distance_fn { template<std::input_or_output_iterator I, std::sentinel_for<I> S> constexpr std::iter_difference_t<I> operator()(I first, S last) const { if constexpr (std::sized_sentinel_for<S, I>) { return last - first; } else { std::iter_difference_t<I> result = 0; while (first != last) { ++first; ++result; } return result; } }   template<ranges::range R> constexpr ranges::range_difference_t<R> operator()(R&& r) const { if constexpr (ranges::sized_range<std::remove_cvref_t<R>>) { return static_cast<ranges::range_difference_t<R>>(ranges::size(r)); } else { return ranges::distance(ranges::begin(r), ranges::end(r)); } } };   inline constexpr auto distance = distance_fn{};

Example

Run this code

#include <iostream>
#include <iterator>
#include <vector>
 
int main() 
{
    std::vector<int> v{ 3, 1, 4 };
    namespace ranges = std::ranges;
    std::cout << "distance(first, last) = "
              << ranges::distance(v.begin(), v.end()) << '\n'
              << "distance(last, first) = "
              << ranges::distance(v.end(), v.begin()) << '\n'
              << "distance(v) = " << ranges::distance(v) << '\n';
}

Output:

distance(first, last) = 3
distance(last, first) = -3
distance(v) = 3

See also

ranges::advance (C++20)	advances an iterator by given distance or to a given bound (niebloid)
ranges::countranges::count_if (C++20)(C++20)	returns the number of elements satisfying specific criteria (niebloid)
distance	returns the distance between two iterators (function template)