const_iterator_type cbegin() const
 {
     return partition_vector_.cbegin();
 }
 const_iterator cbegin() const { return data_.cbegin(); }
 const_iterator_type cbegin() const
 {
     return partition_unordered_map_.cbegin();
 }
 const_iterator cbegin() const noexcept { return data_.cbegin(); }