std::ranges::distance

 

C++

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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)
Iterator primitives
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_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)
Algorithm concepts and utilities
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indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)   indirect_unary_predicate (C++20)	indirect_binary_predicate (C++20) indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20) indirectly_copyable_storable (C++20)	indirectly_swappable (C++20) indirectly_comparable (C++20) permutable (C++20) mergeable (C++20) sortable (C++20)
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indirect_result_t (C++20)	projected (C++20)
Iterator adaptors
reverse_iterator move_iterator (C++11) move_sentinel (C++20) common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) 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) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
Stream iterators
istream_iterator ostream_iterator	istreambuf_iterator ostreambuf_iterator
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ranges::iter_move (C++20)
ranges::iter_swap (C++20)
Iterator operations
advance distance prev (C++11) next (C++11)	ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)
Range access
begincbegin (C++11)(C++14) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) empty (C++17) data (C++17)

 

Defined in header <iterator>
Call signature
template< class I, std::sentinel_for<I> S >     requires (!std::sized_sentinel_for<S, I>) constexpr std::iter_difference_t<I>     distance( I first, S last );	(1)	(since C++20)
template< class I, std::sized_sentinel_for<std::decay_t<I>> S > constexpr std::iter_difference_t<std::decay_t<I>>     distance( I&& first, S last );	(2)	(since C++20)
template< ranges::range R > constexpr ranges::range_difference_t<R>     distance( R&& r );	(3)	(since C++20)

1,2) Returns the number of hops from first to last.

3) Returns the size of r as a signed integer.

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

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

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

1) The number of increments needed to go from first to last.

2) last - static_cast<const std::decay_t<I>&>(first).

3) If R models ranges::sized_range, returns ranges::size(r); otherwise ranges::distance(ranges::begin(r), ranges::end(r)).

Complexity

1) Linear.

2) Constant.

3) If R models ranges::sized_range or if std::sized_sentinel_for<ranges::sentinel_t<R>, ranges::iterator_t<R>> is modeled, complexity is constant; otherwise linear.

Possible implementation

struct distance_fn { template<class I, std::sentinel_for<I> S> requires (!std::sized_sentinel_for<S, I>) constexpr std::iter_difference_t<I> operator()(I first, S last) const { std::iter_difference_t<I> result = 0; while (first != last) { ++first; ++result; } return result; }   template<class I, std::sized_sentinel_for<std::decay<I>> S> constexpr std::iter_difference_t<I> operator()(const I& first, S last) const { return last - first; }   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 (*this)(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};
 
    std::cout << "distance(first, last) = "
              << std::ranges::distance(v.begin(), v.end()) << '\n'
              << "distance(last, first) = "
              << std::ranges::distance(v.end(), v.begin()) << '\n'
              << "distance(v) = "
              << std::ranges::distance(v) << '\n';
}

Output:

distance(first, last) = 3
distance(last, first) = -3
distance(v) = 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 3392	C++20	overload (1) takes iterator by value, thus move-only iterator lvalue with a sized sentinel was rejected	added overload (2)
LWG 3664	C++20	the resolution of LWG issue 3392 made ranges::distance reject array arguments	accepts them

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)