[span.iterators]

using iterator = implementation-defined;

The type models contiguous_iterator ([iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]), and meets the requirements for constexpr iterators ([iterator.requirements.general]), whose value type is value_type and whose reference type is reference.

All requirements on container iterators ([container.reqmts]) apply to span::iterator as well.

constexpr iterator begin() const noexcept;

Returns: An iterator referring to the first element in the span.

If empty() is true, then it returns the same value as end().

constexpr iterator end() const noexcept;

Returns: An iterator which is the past-the-end value.

constexpr reverse_iterator rbegin() const noexcept;

Effects: Equivalent to: return reverse_iterator(end();

constexpr reverse_iterator rend() const noexcept;

Effects: Equivalent to: return reverse_iterator(begin();

23 Containers library [containers]