unordered_map::begin

iterator begin();
    const_iterator begin() const;
    local_iterator begin(size_type nbucket);
    const_local_iterator begin(size_type nbucket) const;

The first two member functions return a forward iterator that points at the first element of the sequence (or just beyond the end of an empty sequence). The last two member functions return a forward iterator that points at the first element of bucket nbucket (or just beyond the end of an empty bucket).