23 Iterators library [iterators]

23.4 Iterator primitives [iterator.primitives]

23.4.4 Range iterator operations [range.iter.ops] ranges​::​distance [range.iter.op.distance]

template<input_­or_­output_­iterator I, sentinel_­for<I> S> constexpr iter_difference_t<I> ranges::distance(I first, S last);
Preconditions: [first, last) denotes a range, or [last, first) denotes a range and S and I model same_­as<S, I> && sized_­sentinel_­for<S, I>.
Effects: If S and I model sized_­sentinel_­for<S, I>, returns (last - first); otherwise, returns the number of increments needed to get from first to last.
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]