OpenCV 2.42 FeatureDetector FREAK - c++

Question

I want to try the new class FREAK in OpenCV 2.4.2.
I tried to use common interface of feature detector to construct FREAK, but,of course, it doesn't work. How should I revise my code to get result?
#include <stdio.h>
#include <iostream>
#include <opencv\cxcore.h>
#include <opencv2\nonfree\features2d.hpp>
#include <opencv\highgui.h>
#include <opencv2\features2d\features2d.hpp>
#include <vector>
using namespace std;
using namespace cv;
int main(){
Mat mat1;
mat1 = imread("Testimg06.jpg",0);
vector<KeyPoint> P1;
Ptr<FeatureDetector> freakdes;
Ptr<DescriptorExtractor> descriptorExtractor;
freakdes = FeatureDetector::create("FREAK");
freakdes->detect(mat1,P1);
Mat keypoint_img;
drawKeypoints( mat1, P1, keypoint_img, Scalar::all(-1), DrawMatchesFlags::DEFAULT );
imshow("Keypoints 1", keypoint_img );
cvWaitKey(0);
}

Answer 1

FREAK is descriptor only. There is no corresponding feature detector.
So you need to combine it with one of the available detectors: FAST, ORB, SIFT, SURF, MSER or use goodFeaturesToTrack function.

Answer 2

There is an OpenCV example that shows how to use FREAK combined with FAST.
The basic instructions are:
FREAK extractor;
BruteForceMatcher<Hamming> matcher;
std::vector<KeyPoint> keypointsA, keypointsB;
Mat descriptorsA, descriptorsB;
std::vector<DMatch> matches;
FAST(imgA,keypointsA,10);
FAST(imgB,keypointsB,10);
extractor.compute( imgA, keypointsA, descriptorsA );
extractor.compute( imgB, keypointsB, descriptorsB );
matcher.match(descriptorsA, descriptorsB, matches);