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std::ranges::distance

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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>)
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 algorithm function objects (informally known as 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.

Contents

[edit] 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

[edit] 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)).

[edit] 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.

[edit] 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{};

[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 range
 auto size = std::ranges::distance(l); // OK, but aware O(N) complexity
 assert (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 of LWG 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]

returns the distance between two iterators
(function template) [edit]

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