/// Returns the maximum possible number of elements
 size_type max_size() const
 {
     return partition_vector_.max_size();
 }
 size_type max_size() const { return data_.max_size(); }
 /// Returns the maximum possible number of elements
 size_type max_size() const
 {
     return partition_unordered_map_.max_size();
 }
 size_type max_size() const noexcept { return data_.max_size(); }