std::ranges::distance

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Iterator library
Iterator concepts
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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< std::input_or_output_iterator 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< std::input_or_output_iterator I, std::sized_sentinel_for<I> S >
constexpr std::iter_difference_t<I> distance( const I& first, const 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 - 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.
2) 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<std::input_or_output_iterator 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<std::input_or_output_iterator I, std::sized_sentinel_for<I> S>
  constexpr std::iter_difference_t<I> operator()(const I& first, const 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 };
    namespace ranges = std::ranges;
    std::cout << "distance(first, last) = "
              << ranges::distance(v.begin(), v.end()) << '\n'
              << "distance(last, first) = "
              << ranges::distance(v.end(), v.begin()) << '\n'
              << "distance(v) = " << 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 distance takes iterator by value, thus rejecting move-only iterator lvalue with a sized sentinel by reference overload added

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)