Given a rosbag::MessageInstance, I do

rosbgag::View view (bag_file_instance, rosbag::TopicQuery(topic_list);
rosbag::View::iterator msg_isnt;
msg_inst = view.begin();
const auto ptr = msg_inst.instantiate<MyType>();

What's the inverse of this operation, i.e. given an instance of MyType create a message instance?