The types concurrent_unordered_multiset::iterator and concurrent_unordered_multiset::const_iterator meet the requirements of ForwardIterator from the [forward.iterators] ISO C++ Standard section.

begin and cbegin

iterator begin();

const_iterator begin() const;

const_iterator cbegin() const;

Returns: an iterator to the first element in the container.

end and cend

iterator end();

const_iterator end() const;

const_iterator cend() const;

Returns: an iterator to the element that follows the last element in the container.