25 Iterators library [iterators]

25.4 Iterator primitives [iterator.primitives]

template<class I, sentinel_for<I> S>
  requires (!sized_sentinel_for<S, I>)
  constexpr iter_difference_t<I> ranges::distance(I first, S last);

Preconditions: [first, last) denotes a range.

Effects: Increments first until last is reached and returns the number of increments.

template<class I, sized_sentinel_for<decay_t<I>> S>
  constexpr iter_difference_t<decay_t<I>> ranges::distance(I&& first, S last);

Effects: Equivalent to: return last - static_cast<const decay_t<I>&>(first);

template<range R>
  constexpr range_difference_t<R> ranges::distance(R&& r);

Effects: If R models sized_range, equivalent to: return static_cast<range_difference_t<R>>(ranges::size(r)); // [range.prim.size]

Otherwise, equivalent to: return ranges::distance(ranges::begin(r), ranges::end(r)); // [range.access]