os::TransportHints are used to specify hints about how you want the transport layer to behave for this topic. For example, if you wanted to specify an "unreliable" connection (and not allow a "reliable" connection as a fall back):

transport_hints The transport hints allow you to specify hints to roscpp's transport layer. This lets you specify things like preferring a UDP transport, using tcp nodelay, etc. This is explained in more detail later.

there is an implicit contract between you and roscpp: you may not modify the message you've sent after you send it, since that pointer will be passed directly to any intraprocess subscribers. If you want to send another message, you must allocate a new one and send that.

ros::init_options::AnonymousName);

Separating out callbacks into different queues can be useful for a number of reasons. Some examples include: Long-running services. Assigning a service its own callback queue that gets serviced in a separate thread means that service is guaranteed not to block other callbacks. Threading specific computationally expensive callbacks. Similar to the long-running service case, this allows you to thread specific callbacks while keeping the simplicity of single-threaded callbacks for the rest your application.

roslaunch creates a new .launch file that contains the nodes you wish to launch on the remote machine. It then sshes into the remote machine and launches a roslaunch child process with that new .launch file.