how to synchronize two topics publishing PointCloud2 data

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Hi, I am new to ROS and I am trying to do a simple example of Approximate Time Synchronization. I have following two nodes and each publishes a simple single PointCloud2 data (I am using the PointCloud2 data type because it has header of time stamp), and I have a node for time synchronization. I am following the ROS Tutorial for Approximate Time Policy. All nodes are compiled without any error but it seems that the callback function never get called. Please advise.

Thank you!

First node

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

    ros::init(argc, argv, "talker");

    ros::NodeHandle n;

    ros::Publisher chatter_pub = n.advertise<sensor_msgs::PointCloud2>("chatter", 1000);

        ros::Rate loop_rate(10);

    int speed = 100;

        while (ros::ok()){

                sensor_msgs::PointCloud2 msg;

                msg.height = speed;

                chatter_pub.publish(msg);

                loop_rate.sleep();

        }

    return 0;

}

Second node is:

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

    ros::init(argc, argv, "friend");

    ros::NodeHandle n;

    ros::Publisher chatterfriend_pub = n.advertise<sensor_msgs::PointCloud2>("chatter_friend", 1000);

        ros::Rate loop_rate(20);

    int speed = 500;

        while (ros::ok()){

                sensor_msgs::PointCloud2 msg;

                msg.height = speed;

                chatterfriend_pub.publish(msg);

                loop_rate.sleep();

        }

    return 0;

}

And my node for Time Synchronizer is

using namespace sensor_msgs;

using namespace message_filters;

ros::Publisher data1_pub;

ros::Publisher data2_pub;


void callback(const sensor_msgs::PointCloud2::ConstPtr& data1, const sensor_msgs::PointCloud2::ConstPtr& data2)
{

  ROS_INFO("Sync_Callback");

  data1_pub.publish(data1);

  data2_pub.publish(data2);

}

int main(int argc, char** argv)

{
  ros::init(argc, argv, "TimeSynchronizer");

  std::string data1_topic = "/chatter";

  std::string data2_topic = "/chatter_friend";


  ros::NodeHandle nh;

  message_filters::Subscriber<sensor_msgs::PointCloud2> data1_sub(nh, data1_topic, 100);

  message_filters::Subscriber<sensor_msgs::PointCloud2> data2_sub(nh, data2_topic, 100);

  data1_pub = nh.advertise<sensor_msgs::PointCloud2>("/synchronized" + data1_topic, 1000);

  data2_pub = nh.advertise<sensor_msgs::PointCloud2>("/synchronized" + data2_topic, 1000);

  typedef sync_policies::ApproximateTime<PointCloud2, PointCloud2> MySyncPolicy;

  Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), data1_sub, data2_sub);

  sync.registerCallback(boost::bind(&callback, _1, _2));

  ros::spin();

  return 0;
}