std::random_access_iterator

Since C++20

Header: <iterator>

The concept random_access_iterator refines bidirectional_iterator by adding support for constant time advancement with the +=, +, -=, and - operators, constant time computation of distance with -, and array notation with subscripting [].

# Declarations

template< class I >
concept random_access_iterator =
std::bidirectional_iterator<I> &&
std::derived_from</*ITER_CONCEPT*/<I>, std::random_access_iterator_tag> &&
std::totally_ordered<I> &&
std::sized_sentinel_for<I, I> &&
requires(I i, const I j, const std::iter_difference_t<I> n) {
{ i += n } -> std::same_as<I&>;
{ j + n } -> std::same_as<I>;
{ n + j } -> std::same_as<I>;
{ i -= n } -> std::same_as<I&>;
{ j - n } -> std::same_as<I>;
{ j[n] } -> std::same_as<std::iter_reference_t<I>>;
};

(since C++20)

# Notes

Unlike the LegacyRandomAccessIterator requirements, the random_access_iterator concept does not require dereference to return an lvalue.

# Example

#include <iterator>
 
namespace cxx20
{
    template<std::input_or_output_iterator Iter>
    constexpr std::iter_difference_t<Iter> distance(Iter first, Iter last)
    {
        if constexpr(std::random_access_iterator<Iter>)
            return last - first;
        else
        {
            std::iter_difference_t<Iter> result{};
            for (; first != last; ++first)
                ++result;
            return result;
        }
    }
}
 
int main()
{
    static constexpr auto il = {3, 1, 4};
 
    static_assert(std::random_access_iterator<decltype(il.begin())> &&
                  cxx20::distance(il.begin(), il.end()) == 3 &&
                  cxx20::distance(il.end(), il.begin()) == -3);
}

# See also