How to use the subscribed cmd_vel?

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Hi, i calculate the geometry_msgs::Twist cmd_vel and can subscribe to this topic /myrobot/cmd_vel successfully. But i don't know how to use it, e.g. send it directly to my robot? I mean I can call back with velCallback, but how to use the velocity, in the main() function or in the velCallback() function if i want to send it to the robot? I hope i express my question clearly. Below is the simple subscriber node.

#include "ros/ros.h"

#include "std_msgs/String.h"

#include "geometry_msgs/Twist.h"

using namespace std;

void velCallback(const geometry_msgs::Twist& vel) {

cout<<"velocity\n"<<vel<<"\n";

}

int main(int argc, char **argv) {

ros::init(argc, argv, "myrobot_cmd_vel");
ros::NodeHandle n;
ros::Subscriber sub = n.subscribe("/myrobot/cmd_vel", 10, velCallback);

ros::spin();
return 0;

}