std::ranges::distance

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

#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

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)