How to get the descriptor value from "SiftDescriptorExtractor"?

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I use SiftDescriptorExtractor to compute the descriptors of keypoints. I'm trying to get the descriptor value. Does someone know how to do it?

(PS:My opencv version is 2.4.2)

This is the part of the code

//-- Step 2: Calculate descriptors (feature vectors)
SiftDescriptorExtractor extractor;
Mat descriptors_object, descriptors_scene;
extractor.compute( img_object, keypoints_object, descriptors_object );
extractor.compute( img_scene, keypoints_scene, descriptors_scene );