nh.subscribe works but message_filters::Subscriber not

edit

Thanks to the sharp observation of @gvdhoorn, I had to look it back to create a minimal reproducible example.

It is an interesting problem. In summary, in the second case, the subscriber, i.e., rgb_sub, is in local scope. Therefore after the construction call, it does not exist. So we must make it a class member to make it work.

Please see a minimal reproducible example below:

#include <ros/ros.h>
#include <message_filters/subscriber.h>
#include <sensor_msgs/Image.h>

using namespace sensor_msgs;
using namespace message_filters;

class ClientHandler
{
public:
  ClientHandler(ros::NodeHandle& nh, std::string& topic_name);
  void CamImgCb(const ImageConstPtr& image);

private:
  ros::NodeHandle n;
  std::string topic;
  message_filters::Subscriber<Image> rgb_sub;
};

ClientHandler::ClientHandler(ros::NodeHandle& nh, std::string& topic_name) : n(nh), topic(topic_name)
{
  // the following declaration does not work. hence it is commented.
  // the subscriber is in a local scope
  // message_filters::Subscriber<Image> rgb_sub(n, topic, 1);

  // let's make it a class member and then call subscribe as shown below
  rgb_sub.subscribe(n, topic, 1);

  // now bind a callback
  rgb_sub.registerCallback(boost::bind(&ClientHandler::CamImgCb, this, _1));
}

void ClientHandler::CamImgCb(const ImageConstPtr& image)
{
  ROS_INFO("Sequence: [%d]", image->header.seq);
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "image_listener");
  ros::NodeHandle n;

  std::string topic = "/usb_cam/image_raw";
  ClientHandler ClientHandler(n, topic);

  ros::spin();
  return 0;
}

I used the usb_cam package to publish my webcam and used the above node to subscribe, as shown below:

$ rosrun usb_cam usb_cam_node 
$ rosrun beginner_tutorials listener_class_filter 
[ INFO] [1664192571.776300573]: Sequence: [0]
[ INFO] [1664192571.808083556]: Sequence: [1]
[ INFO] [1664192571.839569528]: Sequence: [2]
[ INFO] [1664192571.875559602]: Sequence: [3]
[ INFO] [1664192571.907563678]: Sequence: [4]
[ INFO] [1664192571.943604144]: Sequence: [5]
[ INFO] [1664192571.975550851]: Sequence: [6]
[ INFO] [1664192572.007569647]: Sequence: [7]
[ INFO] [1664192572.043564526]: Sequence: [8]
[ INFO] [1664192572.075685918]: Sequence: [9]
[ INFO] [1664192572.107556296]: Sequence: [10]