problem with Orb matching

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Hi to all! i try to use Orb feature for object detection... i use this code:

int numKeyPoints = 200;
std::vector<KeyPoint> keypoints_1,keypoints_2;
OrbFeatureDetector detector(numKeyPoints);
detector.detect( img_1, keypoints_1 );
detector.detect( img_2, keypoints_2 );
OrbDescriptorExtractor extractor;
Mat descriptors_1,descriptors_2;
extractor.compute( img_1, keypoints_1, descriptors_1 );
extractor.compute( img_2, keypoints_2, descriptors_2 );
BFMatcher matcher(Hamming);
std::vector< DMatch > matches;
matcher.match( descriptor_1, descriptor_2, matches );

but i have this error:

request for member 'match' in 'matcher', which is of non-class type 'cv::BFMatcher ()(cv::Hamming)'

I don't understand where is the error....

thanks!!