[ROS2] Understanding get_msg_class

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Having seen this post, I have been trying to implement a generic subscriber using get_msg_class() from ros2topic.api.

However, I'm struggling to understand the conditions required for get_msg_class() to be able to determine the type of a message from another node.

This is my current best guess: 1. The publisher must be alive. 2. The publisher must have published at least one message.

Is this correct? Could someone help me understand runtime type introspection and the requirements of get_msg_class().