error C2065: 'SiftFeatureDetector' : undeclared identifier

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I'm trying to use SiftFeatureDetector but i'm having undeclared identifier error. But i imported the libraries as follows. Any help is appreciated. ( i followed article -> (http://www.mattsheckells.com/opencv-a...)

imported libraries

#include <opencv2/features2d.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/features2d/features2d.hpp>

I'm using opencv 2.4

SiftDescriptorExtractor extractor; // ERROR undeclared identifier
extractor.compute(timg, kps, desc); // ERROR undeclared identifier