std::ranges::distance

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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)

(C++20)

(C++20)

(C++26)

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
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istreambuf_iterator
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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)

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

1,2) Returns the number of hops fromfirst tolast.

3) Returns the size ofr as a signed integer.

The function-like entities described on this page arealgorithm function objects (informally known asniebloids), that is:

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

[edit]Parameters

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

[edit]Return value

1) The number of increments needed to go fromfirst tolast.

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

3) IfR modelsranges::sized_range, returnsranges::size(r); otherwiseranges::distance(ranges::begin(r),ranges::end(r)).

[edit]Complexity

1) Linear.

2) Constant.

3) IfR modelsranges::sized_range or ifstd::sized_sentinel_for<ranges::sentinel_t<R>,ranges::iterator_t<R>> is modeled, complexity is constant; otherwise linear.

[edit]Possible implementation

struct distance_fn{template<class I,std::sentinel_for<I> S>        requires(!std::sized_sentinel_for<S, I>)constexprstd::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>constexprstd::iter_difference_t<I> operator()(const I& first, S last)const{return last- first;} template<ranges::range R>constexprranges::range_difference_t<R> operator()(R&& r)const{ifconstexpr(ranges::sized_range<std::remove_cvref_t<R>>)returnstatic_cast<ranges::range_difference_t<R>>(ranges::size(r));elsereturn(*this)(ranges::begin(r),ranges::end(r));}}; inlineconstexprauto distance= distance_fn{};

[edit]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 rangeauto size= std::ranges::distance(l);// OK, but aware O(N) complexityassert(size==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 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 ofLWG issue 3392 made `ranges::distance` reject array arguments	accepts them

[edit]See also

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