can not compile opencv programs in ubuntu - c++

Question

I have wrote a simple program like this
#include <opencv2/opencv.hpp>
using namespace cv;
int main(int argc, const char* argv[])
{
Mat input = imread(argv[1], 0); //Load as grayscale
//~ FAST detector;
//~ vector<KeyPoint> keypoints;
//~ FAST(input, keypoints, 0.2);
// Add results to image and save.
//~ Mat output;
//~ drawKeypoints(input, keypoints, output);
namedWindow ("Image", CV_WINDOW_FREERATIO);
imshow("Image", input);
//~ imwrite(argv[2], output);
return 0;
}
Then compile program like this:
g++ `pkg-config --libs opencv` main.cpp
And here is output of g++:
/tmp/ccK1Sbrw.o: In function `main':
main.cpp:(.text+0x66): undefined reference to `cv::imread(std::string const&, int)'
main.cpp:(.text+0xc2): undefined reference to `cv::namedWindow(std::string const&, int)'
main.cpp:(.text+0xf6): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)'
main.cpp:(.text+0x139): undefined reference to `cv::imshow(std::string const&, cv::_InputArray const&)'
/tmp/ccK1Sbrw.o: In function `cv::Mat::~Mat()':
main.cpp:(.text._ZN2cv3MatD2Ev[_ZN2cv3MatD5Ev]+0x39): undefined reference to `cv::fastFree(void*)'
/tmp/ccK1Sbrw.o: In function `cv::Mat::release()':
main.cpp:(.text._ZN2cv3Mat7releaseEv[_ZN2cv3Mat7releaseEv]+0x47): undefined reference to `cv::Mat::deallocate()'
collect2: error: ld returned 1 exit status
I have the library installed, I can see *.so and *.hpp files in their folders, and ld find them, but what does it complain about? there is nothing in .so files?!
Also, I do not have nonfree modules installed (I uset apt-get to install opencv), how can I get them? I need SIFT which is inside that module. Do I have to compile opencv myself?

Answer 1

It appears to me that you forgot to specify a module in
`pkg-config --libs MODULENAMEGOESHERE`