Hi, happy that you are using the mqtt_client!

You are right, the primitive message support only covers the primitives (int, float, bool, ...) from std_msgs. What is happening in your case is that the sending mqtt_client serializes the sensor_msgs/JointStates message, but your receiving client apparently has no way of deserializing the incoming data.

I'm wondering how the target state would look like to you? Receiving a stringified JSON of the message?

Hello, thank you for the quick reply!

Yes, my goal is to receive messages in a JSON string, ideally. Right now, my MQTT client (Mosquitto) receives essentially gibberish and I presumed it was due to /JointStates not being a primitive message type (after reading some more, I don't believe that was the reason).

Is there support for mqtt_client to de/serialize ROS messages as JSON strings?

There is no support of such functionality as of now.

Unfortunately, it's not that easy to do from C++, without having access to proper type introspection at runtime. Python would be easy for this (and could also simply use rosbridge_suite).

ros_msg_parser could potentially help to implement such a feature in our C++ node. We are already using a topic_tools::ShapeShifter to subscribe to generic message types. I would also like to try this out, but cannot yet guarantee that I will be getting to this anytime soon.

I'll look into these topics in order to implement this functionality. Thank you for your assistance and I'd like to apologize for not being very clear in my request -- clearly I didn't have a solid grasp on ROS-MQTT communications.

Nothing to apologize for! Feel free to open a pull request if you get this implemented in the mqtt_client!

This function could be used:

https://github.com/facontidavide/ros_msg_parser/blob/f874a6f98e0d1ed72e6d23f3671d26ed96a49986/include/ros_msg_parser/ros_parser.hpp#L160