std::ranges::distance(3)

NAME
std::ranges::distance - std::ranges::distance

Synopsis
   Defined in header <iterator>
   Call signature
   template< class I, std::sentinel_for<I> S >
       requires (!std::sized_sentinel_for<S, I>) (1) (since C++20)
   constexpr std::iter_difference_t<I>
       distance( I first, S last );
   template< class I, std::sized_sentinel_for<std::decay_t<I>> S >
   constexpr std::iter_difference_t<std::decay_t<I>> (2) (since C++20)
       distance( I&& first, S last );
   template< ranges::range R >
   constexpr ranges::range_difference_t<R> (3) (since C++20)
       distance( R&& r );
   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
// 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);
 }
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
                       overload (1) takes iterator by value, thus
   LWG 3392 C++20 move-only added overload (2)
                       iterator lvalue with a sized sentinel was
                       rejected
   LWG 3664 C++20 the resolution of LWG issue 3392 made accepts them
                       ranges::distance reject array arguments

See also
   ranges::advance advances an iterator by given distance or to a given bound
   (C++20) (niebloid)
   ranges::count
   ranges::count_if returns the number of elements satisfying specific criteria
   (C++20) (niebloid)
   (C++20)
   distance returns the distance between two iterators
                    (function template)