Use extended parameter for Surf descriptor

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Hi, I want to use the extended parameter for Surf algorithm. Here is my code :

SurfFeatureDetector detector2(minHessian);
detector2.extended=1;

and then

extractor2.compute(img, keypoints_4, descriptors2);
cout<<"descriptors2   "<<descriptors2.rows<<" +  "<<descriptors2.cols<<endl;

and the result is:

descriptors2 168 + 64

but I want my Surf descriptor to have 128 columns with extendedparameter. How is it possible to do this? Thanks