The random_access_iterator concept adds support for
constant-time advancement with +=, +, -=, and -,
as well as the computation of distance in constant time with -.

Random access iterators also support array notation via subscripting.

template<class I>
concept random_access_iterator =
bidirectional_iterator<I> &&
derived_from<ITER_CONCEPT(I), random_access_iterator_tag> &&
totally_ordered<I> &&
sized_sentinel_for<I, I> &&
requires(I i, const I j, const iter_difference_t<I> n) {
{ i += n } -> same_as<I&>;
{ j + n } -> same_as<I>;
{ n + j } -> same_as<I>;
{ i -= n } -> same_as<I&>;
{ j - n } -> same_as<I>;
{ j[n] } -> same_as<iter_reference_t<I>>;
};
Let a and b be valid iterators of type I
such that b is reachable from a
after n applications of ++a,
let D be iter_difference_t<I>,
and let n denote a value of type D.

I models random_access_iterator only if

- For any two positive values x and y of type D, if (a + D(x + y)) is valid, then (a + D(x + y)) is equal to ((a + x) + y).