Undefined reference errors for OpenCV Hello World in Ubuntu - c++

Question

I'm trying the following code:
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>
using namespace cv;
int main(int argc, char** argv) {
namedWindow("Output",1);
Mat output = Mat::zeros( 120, 350, CV_8UC3 );
putText(output,"Hello World",cvPoint(15,70),
FONT_HERSHEY_PLAIN,3,cvScalar(0,255,0),4);
imshow("Output", output);
waitKey(0);
return 0;
}
I then tried g++ -I/usr/local/include -L/usr/local/lib -lopencv_core -lopencv_imgcodecs -lopencv_highgui opencv_hello.cpp -o opencv_hello
and g++pkg-config opencv cvblob --cflags --libsopencv_hello.cpp -o opencv_hello
But they both give the same undefined reference errors:
opencv_hello.cpp:(.text+0x132): undefined reference to `cv::namedWindow(cv::String const&, int)'
opencv_hello.cpp:(.text+0x15f): undefined reference to `cv::Mat::zeros(int, int, int)'
opencv_hello.cpp:(.text+0x26f): undefined reference to `cv::putText(cv::_InputOutputArray const&, cv::String const&, cv::Point_<int>, int, double, cv::Scalar_<double>, int, int, bool)'
opencv_hello.cpp:(.text+0x2d7): undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)'
opencv_hello.cpp:(.text+0x2ff): undefined reference to `cv::waitKey(int)'
/tmp/cctt8VGQ.o: In function `cv::String::String(char const*)':
opencv_hello.cpp:(.text._ZN2cv6StringC2EPKc[_ZN2cv6StringC5EPKc]+0x4d): undefined reference to `cv::String::allocate(unsigned long)'
/tmp/cctt8VGQ.o: In function `cv::String::~String()':
opencv_hello.cpp:(.text._ZN2cv6StringD2Ev[_ZN2cv6StringD5Ev]+0x14): undefined reference to `cv::String::deallocate()'
/tmp/cctt8VGQ.o: In function `cv::Mat::~Mat()':
opencv_hello.cpp:(.text._ZN2cv3MatD2Ev[_ZN2cv3MatD5Ev]+0x39): undefined reference to `cv::fastFree(void*)'
/tmp/cctt8VGQ.o: In function `cv::Mat::release()':
opencv_hello.cpp:(.text._ZN2cv3Mat7releaseEv[_ZN2cv3Mat7releaseEv]+0x4b): undefined reference to `cv::Mat::deallocate()'
How can I fix this?

Answer 1

If pkg-config opencv --cflags --libs command finds OpenCV include files and libraries below compilation works wituout any errors.
g++ opencv_hello.cpp -o opencv_hello $(pkg-config opencv --cflags --libs)
or
g++ opencv_hello.cpp -o opencv_hello `pkg-config opencv --cflags --libs`