ROS node subscription not connected

I am using a rosbag to publish on various topics and i am trying to get my sample program to allow one node to subscribe to those topics via class method functions. But nothing is being printed out on console for the subscribers. I tried roswtf and i got

 WARNING The following node subscriptions are unconnected:
 * /roscpp_pcl_example:
 * /camera/depth/points

Here is my program code and i am not sure where the problem lies. These are my reference for the API https://wiki.ros.org/roscpp_tutorials/Tutorials/UsingClassMethodsAsCallbacks http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29


// Include the ROS library
#include <ros/ros.h>

// Include pcl
#include <pcl_conversions/pcl_conversions.h>
#include <pcl/point_cloud.h>
#include <pcl/point_types.h>

// Include PointCloud2 message
#include <sensor_msgs/PointCloud2.h>
#include <std_msgs/String.h>

// Topics
static const std::string SUB_TOPIC_1 = "/ecu_pcl";
static const std::string SUB_TOPIC_2 = "/velodyne_back/velodyne_points";
static const std::string SUB_TOPIC_3 = "/velodyne_center/velodyne_points";
static const std::string SUB_TOPIC_4 = "/velodyne_front/velodyne_points"


//define class with member functions for callback usage
//allows for subscription of various topics in the same node
class callback_node{
    public:

    void callback1(const std_msgs::String::ConstPtr& msg)
    {
      ROS_INFO_STREAM(msg->data.c_str());
    }

    void callback2(const std_msgs::String::ConstPtr& msg)
    {
      ROS_INFO_STREAM(msg->data.c_str());
    }

    void callback3(const std_msgs::String::ConstPtr& msg)
    {
      ROS_INFO_STREAM(msg->data.c_str());
    }

    void callback4(const std_msgs::String::ConstPtr& msg)
    {
      ROS_INFO_STREAM(msg->data.c_str());
    }

    

};

int main (int argc, char** argv)
{
    // Initialize the ROS Node "roscpp_pcl_example"
    ros::init (argc, argv, "roscpp_pcl_example");
    ros::NodeHandle nh;
    callback_node callback_obj;
    // Print "Hello" message with node name to the terminal and ROS log file
    ROS_INFO_STREAM("Hello from ROS Node: " << ros::this_node::getName());

    // Create ROS Subscribers to SUB_TOPIC with a queue_size of 1000 and a callback function via class methods
    ros::Subscriber sub1 = nh.subscribe(SUB_TOPIC_1, 1000, &callback_node::callback1, &callback_obj);
    ros::Subscriber sub2 = nh.subscribe(SUB_TOPIC_2, 1000, &callback_node::callback2, &callback_obj);
    ros::Subscriber sub3 = nh.subscribe(SUB_TOPIC_3, 1000, &callback_node::callback3, &callback_obj);
    ros::Subscriber sub4 = nh.subscribe(SUB_TOPIC_4, 1000, &callback_node::callback4, &callback_obj);


    // Spin
    ros::spin();

    // Success
    return 0;
}

Solution

One thing that immediately jumps to my eye is that you have actually put std_msgs::String::ConstPtr as the topic type but have topics /ecu_pcl and /velodyne_points which should not be of type std_msgs::String but instead should have a different data types, point clouds I assume, so something like sensor_msgs::PointCloud2. Therefore you would have to modify the callbacks to their correct data types, e.g. sensor_msgs::PointCloud2::ConstPtr.

What surprises me as well is the message roswtf gives you:

WARNING The following node subscriptions are unconnected:
  * /roscpp_pcl_example:
    * /camera/depth/points

This means that your node roscpp_pcl_example is subscribed to a topic /camera/depth/points nobody is actually publishing. Yet your code snippet does not mention /camera/depth/points. (Are you only showing part of the node's source code or have you forgotten to recompile the code with catkin build?)

Anyways in order to debug it try the following:

Make sure you have sourced the correct workspace by executing $ source devel/setup.bash in each console you open for the following steps
Open a console and browse the rosbag that you want to play and execute $ rosbag info <filename.bag> and check in the output. Make sure that all the topics you need for your node have actually been recorded!
If that is the case continue to play it with $ rostopic play <filename.bag>.
Now check if all topics are published correctly by opening a new console, sourcing the workspace and typing $ rostopic list.
Then check out the type of each individual topic from this list with $ rostopic info <topicname>.
Finally make sure that you use the correct topic type for your subscribers. You might need to update your CMakeLists.txt and package.xml accordingly!