using const_iterator = implementation-defined;

constexpr const_iterator begin() const noexcept; constexpr const_iterator cbegin() const noexcept;

Returns: An iterator such that

• if !empty(), addressof(*begin()) == data_,
• otherwise, an unspecified value such that [begin(), end()) is a valid range.

constexpr const_iterator end() const noexcept; constexpr const_iterator cend() const noexcept;

Returns: begin() + size().

constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept;

Returns: const_reverse_iterator(end()).

constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crend() const noexcept;

Returns: const_reverse_iterator(begin()).