Issues with having both subscriber and publisher in the same node

Question

Currently, I have a node that has to have both the subscriber and publisher. However, I am having certain errors when I catkin build.

#include <geometry_msgs/Twist.h>
#include <ros/ros.h>
#include <sensor_msgs/LaserScan.h>
#include <std_msgs/Float32.h>

void laserCallBack(const sensor_msgs::LaserScan::ConstPtr &msg) {

  geometry_msgs::Twist reply;

  if (msg.ranges[360] >= 1.0) {
    reply.linear.x = 0.5;
    reply.angular.z = 0.0;
    pub.publish(reply);
  } else if (msg.ranges[360] < 1.0) {
    reply.linear.x = 0.0;
    reply.angular.z = 0.5;
    pub.publish(reply);
  }
}

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

  ros::init(argc, argv, "topics_quiz_node");
  ros::NodeHandle nh;
  ros::Publisher pub = nh.advertise<geometry_msgs::Twist>("/cmd_vel", 1000);
  ros::Subscriber sub = nh.subscribe("/kobuki/laser/scan", 1000, laserCallBack);

  ros::spin();

  return 0;
}

The errors are as follows: Errors

Any help in debugging these error would be appreciated. Thanks

Answer 1

For msg you should use:

msg->ranges[360]

And since "pub" is declared in your main function you cannot call it in a different function. You can first declare it globally and initialize it in the main function.

eg:

#include <geometry_msgs/Twist.h>
#include <ros/ros.h>
#include <sensor_msgs/LaserScan.h>
#include <std_msgs/Float32.h>

ros::Publisher pub;

void laserCallBack(const sensor_msgs::LaserScan::ConstPtr &msg) {

  geometry_msgs::Twist reply;

  if (msg->ranges[360] >= 1.0) {
    reply.linear.x = 0.5;
    reply.angular.z = 0.0;
    pub.publish(reply);
  } else if (msg->ranges[360] < 1.0) {
    reply.linear.x = 0.0;
    reply.angular.z = 0.5;
    pub.publish(reply);
  }
}

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

  ros::init(argc, argv, "topics_quiz_node");
  ros::NodeHandle nh;
  pub = nh.advertise<geometry_msgs::Twist>("/cmd_vel", 1000);
  ros::Subscriber sub = nh.subscribe("/kobuki/laser/scan", 1000, laserCallBack);

  ros::spin();

  return 0;
}

Also, check your package.xml and CMakeLists.txt

Refer to section 3 (Building your Nodes) from http://wiki.ros.org/ROS/Tutorials/WritingPublisherSubscriber%28c%2B%2B%29