undefined reference to image_transport - c++

Question

I am developing a ROS Qt GUI application and I face a problem on ROS Hydro (I had the same problem while working on ROS Fuerte). My project does not recognize my library like image_transport.h. I added it to the beginning of the qnode.hpp file but it did not solve the problem.
My major problem:
/home/attila/catkin_ws/src/arayuz/src/qnode.cpp:-1: error: undefined reference to `image_transport::ImageTransport::ImageTransport(ros::NodeHandle const&)'
This is the code that generates the error:
#include "ros/ros.h"
#include "ros/network.h"
#include "string"
#include "std_msgs/String.h"
#include "sstream"
#include "../include/arayuz/qnode.hpp"
namespace enc=sensor_msgs::image_encodings;
static const char WINDOW[ ]="Kinect";
namespace arayuz {
QNode::QNode(int argc, char** argv ) :
init_argc(argc),
init_argv(argv)
{}
QNode::~QNode() {
if(ros::isStarted()) {
ros::shutdown(); // explicitly needed since we use ros::start();
ros::waitForShutdown();
}
cv::destroyWindow(WINDOW);
wait();
}
bool QNode::init() {
ros::init(init_argc,init_argv,"arayuz");
if ( ! ros::master::check() ) {
return false;
}
ros::start(); // explicitly needed since our nodehandle is going out of scope.
ros::NodeHandle n;
// Add your ros communications here.
image_transport::ImageTransport it(n);
imagesub = it.subscribe("/kinectCamera", 1,&QNode::chatterimage,this);
start();
return true;
}
bool QNode::init(const std::string &master_url, const std::string &host_url) {
std::map<std::string,std::string> remappings;
remappings["__master"] = master_url;
remappings["__hostname"] = host_url;
ros::init(remappings,"arayuz");
if ( ! ros::master::check() ) {
return false;
}
ros::start(); // explicitly needed since our nodehandle is going out of scope.
ros::NodeHandle n;
// Add your ros communications here.
image_transport::ImageTransport it(n);
imagesub = it.subscribe("/kinectCamera",1,&QNode::chatterimage,this);
start();
return true;
}
void QNode::chatterimage(const sensor_msgs::ImageConstPtr& msg)
{
rgbimage=cv_bridge::toCvCopy(msg,enc::BGR8);
Q_EMIT chatterimageupdate();
}
void QNode::run() {
while ( ros::ok() ) {
ros::spin();
}
std::cout << "Ros shutdown, proceeding to close the gui." << std::endl;
Q_EMIT rosShutdown(); // used to signal the gui for a shutdown (useful to roslaunch)
}
}

Answer 1

In order to link against ROS libraries you need to add the dependency to your package.xml file:
<build_depend>image_transport</build_depend>
<run_depend>image_transport</run_depend>
and to your CMakeLists.txt:
find_package(catkin REQUIRED COMPONENTS
...
image_transport
...
)

Answer 2

Just adding the header to your compilation isn't sufficient: the error message states that you successfully compiled the code but it failed to link the executable. You will also need to find the library implementing the image_transport code and link it to your executable. I have no idea of ros but here is a link which seems to describe how to build code using this library.