subscriber and publisher in class

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Hello, I am doing simulation in gazebo. And I have two topics, by writing a subscriber for the first topic I can get the position of my mobile robot, and by writing a publisher for the second topic I can control the velocity of my robot.

But I want to control the velocity of the robot through the position information. So I wonder how I can publish the information I got from the subscriber to a topic.

Any help as to what could be done would be really appreciated