BSONObj obj = BSONObjBuilder().gen(); // create an empty BSON object BSONElement element = obj["_id"]; // get the "_id" field from the object __oid id = element.__oid(); // get the ObjectId from the elementThis code creates an empty BSON object, then retrieves the "_id" field from the object as a BSONElement. Finally, the ObjectId value is obtained from the element using the __oid() method. The package/library for this code is likely the MongoDB C++ driver, which provides a native C++ interface to MongoDB. The BSONElement and __oid classes are part of the driver's BSON library, which is used to encode and decode BSON data.