Assign to this object the value, hash function, and key‐equality comparator of the specified rhs object and return a reference offering modifiable access to this object. The entries of rhs are moved (in constant time) to this object if the two have the same allocator, otherwise entries from rhs are moved into this table. In either case, rhs is left in a valid but unspecified state. If an exception is thrown, this object is left in a valid but unspecified state.