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Hi,

1. You can add a member variable in your class to save the last time when you get to msg.

   class moving_robot{
   public:
       moving_robot() : last_time_(ros::Time::now())
       {
           sub = nh.subscribe("/cmd_vel", 10, &moving_robot::callback, this);
       }
   private:
       void callback(const geometry_msgs::Twist::ConstPtr &msg)
       {
           ros::Time curr_time = ros::Time::now();
           double dt = (curr_time - last_time_).toSec();
           ROS_INFO_STREAM("dt: " << dt);
           last_time_ = curr_time;
           // other works
       }
       ros::NodeHandle nh;
       ros::Subscriber sub;
       ros::Time last_time_;  };
   

2. Yes, it depends on the topic frequency which you subscribe to.

Hi,

1. You can add a member variable in your class to save the last time when you get to msg.

   class moving_robot{
   public:
       moving_robot() : last_time_(ros::Time::now())
       {
           sub = nh.subscribe("/cmd_vel", 10, &moving_robot::callback, this);
       }
   private:
       void callback(const geometry_msgs::Twist::ConstPtr &msg)
       {
           ros::Time curr_time = ros::Time::now();
           double dt = (curr_time - last_time_).toSec();
           ROS_INFO_STREAM("dt: " << dt);
           last_time_ = curr_time;
           // other works
       }
       ros::NodeHandle nh;
       ros::Subscriber sub;
       ros::Time last_time_;  };
   

2. Yes, it depends on the topic frequency which you subscribe to. to and the callback function running time