ROS Robot speed change

robot_speed_change.cpp

#include <ros/ros.h> 
#include <geometry_msgs/Twist.h> 
int main(int argc, char** argv) {
  ros::init(argc, argv, "topic_publisher"); ros::NodeHandle nh;

    ros::Publisher pub = nh.advertise("cmd_vel",1000);
    ros::Rate loop_rate(2);

    geometry_msgs::Twist twist;
    twist.linear.x=0.5;
    twist.linear.z=0.5;

    while (ros::ok())
    {
        pub.publish(twist);
        ros::spinOnce();
        loop_rate.sleep();

    }

    return 0;
}