Normal ORB matcher in cpp and choosing good pair using stereo constraint

ORB_matching.cpp

// Look at https://github.com/vonzhou/opencv/blob/master/image-search/orb.cpp
std::vector<cv::DMatch> ratio_test(std::vector< std::vector<cv::DMatch> > matches12, double ratio, std::vector<cv::KeyPoint> kpl, std::vector<cv::KeyPoint> kpr){
    std::vector<cv::DMatch> good_matches;         
    for(int i = 0; i < matches12.size(); i++){  
        if(abs(kpl[matches12[i][0].queryIdx].pt.y - kpr[matches12[i][0].trainIdx].pt.y ) < 1){ // in the same line
            if(matches12[i][0].distance < ratio * matches12[i][1].distance){
                good_matches.push_back(matches12[i][0]);
            }
        }
    }
    return good_matches;                  
}

void show_keymatching(const cv::Mat left,const cv::Mat right)
{
    cv::Ptr<cv::FeatureDetector> detector = cv::ORB::create();
    std::vector<cv::KeyPoint> kpl,kpr;
    detector->detect(left, kpl);
    detector->detect(right, kpr);
    
    cv::Ptr<cv::DescriptorExtractor> extractor = cv::ORB::create();
    cv::Mat desl,desr;
    extractor->compute(left, kpl, desl);
    extractor->compute(right, kpr, desr);
    
    
    std::vector< std::vector<cv::DMatch> > matches12, matches21;
    cv::Ptr<cv::DescriptorMatcher> matcher = cv::DescriptorMatcher::create("BruteForce-Hamming"); 

    matcher->knnMatch( desl, desr, matches12, 2 );
    //matcher->knnMatch( desr, desl, matches21, 2 );
    
    std::vector<cv::DMatch> good_matches1, good_matches2;
    double ratio = 0.8;
    good_matches1 = ratio_test(matches12, ratio, kpl, kpr);
    //good_matches2 = ratio_test(matches21, ratio);
    
    
    cv::Mat img_matches;
    cv::drawMatches( left, kpl, right, kpr,
               good_matches1, img_matches, cv::Scalar::all(-1), cv::Scalar::all(-1),
               std::vector<char>(), cv::DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );
    
    imshow( "Good Matches", img_matches );
    cv::waitKey(1);
    
}