Segmentation fault(core dumped) -ROS error - c++

Question

Trying to write a cpp code to print out messages from camera image using darknet. I built a class in which there is mutex method which I use for utilizing callback message in multiple threads. Although catkin_make builds the file successfully, it gives segmentation error when I run the ros command with rosrun . The code is as follows:
#include "ros/ros.h"
#include "darknet_ros_msgs/BoundingBoxes.h"
#include "darknet_ros_msgs/BoundingBox.h"
#include<string>
#include<thread>
#include<iostream>
#include <mutex>
#include "geometry_msgs/Twist.h"
class Firstolo
{
private:
std::mutex yolo_mtx;
darknet_ros_msgs::BoundingBoxes last_yolo_msg;
public:
void callback(const darknet_ros_msgs::BoundingBoxes& msg)
{
std::lock_guard<std::mutex> lck(yolo_mtx);
last_yolo_msg = msg;
}
const darknet_ros_msgs::BoundingBoxes getYoloLastMsg()
{
std::lock_guard<std::mutex> lck(yolo_mtx);
return last_yolo_msg;
}
void dothejob()
{
std:: cout << "Here it goes: " << getYoloLastMsg().bounding_boxes[0].xmin << std::endl;
std:: cout << "Here it goes: " << getYoloLastMsg().bounding_boxes[0].xmax << std::endl;
std:: cout << "\033[2J\033[1;1H";
}
Firstolo()
{
}
Firstolo(Firstolo&)
{
std::mutex yolo_mtx;
}
~Firstolo()
{
}
};
int main( int argc, char **argv)
{
ros::init(argc,argv,"cood_subscriber");
Firstolo nc;
ros::NodeHandle nh;
ros::Subscriber sub;
sub = nh.subscribe("/darknet_ros/bounding_boxes", 100, &Firstolo::callback, &nc);
nc.dothejob();
ros::spin();
return 0;
}
Edit: It turns out that the problem is in the void dothejob(). I added std::lock_guardstd::mutex lck(yolo_mtx); to the void dothejob() and Segmentation error no longer shows up. Now the only remaining problem is that std:: cout << "Here it goes: " << getYoloLastMsg().bounding_boxes[0].xmin << std::endl; line keeps waiting for messages rather than printing them out. In fact, messages naturally should appear since there is darknet running in the background and generating messages.