Returns:;; The maximum number of elements the container can hold without rehashing, assuming that no further elements will be erased. Note:;; After construction, rehash or clearance, the container's maximum load is at least `max_load_factor() * bucket_count()`. This number may decrease on erasure under high-load conditions.
Attempt to "merge" two containers by iterating `source` and extracting any node in `source` that is not contained in `*this` and then inserting it into `*this`.