std::ranges::distance

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Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
Utilities
(C++20)
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
ranges::distance
(C++20)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(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:

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 <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

advances an iterator by given distance or to a given bound
(niebloid)
returns the number of elements satisfying specific criteria
(niebloid)
returns the distance between two iterators
(function template)