/// Returns the number of elements that the container has currently
 /// allocated space for.
 size_type capacity() const
 {
     return partition_vector_.capacity();
 }
 size_type capacity() const { return data_.capacity(); }
 /// Returns the number of elements that the container has currently
 /// allocated space for.
 size_type capacity() const
 {
     return partition_unordered_map_.capacity();
 }
 size_type capacity() const noexcept { return data_.capacity(); }