std::ranges::distance

 

C++

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Feature test macros (C++20)

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Ranges library (C++20)

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Symbols index

External libraries

 

Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (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)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
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)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter 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) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points 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) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) 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 <cassert>
#include <forward_list>
#include <iterator>
#include <vector>
 
int main() 
{
    std::vector<int> v{3, 1, 4};
    assert(std::ranges::distance(v.begin(), v.end()) == 3);
    assert(std::ranges::distance(v.end(), v.begin()) == -3);
    assert(std::ranges::distance(v) == 3);
 
    std::forward_list<int> l{2, 7, 1};
    // auto size = std::ranges::size(l); // error: not a sizable range
    auto size = std::ranges::distance(l); // OK, but aware O(N) complexity
    assert(size == 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)