how to use SIFT in opencv

Question

I am learning C++ and OpenCV these days. Given an image, I want to extract its SIFT features. From http://docs.opencv.org/modules/nonfree/doc/feature_detection.html, we can know

Answer 1

I was having the same question for opencv3 but i found this. It explains why SIFT and SURF removed from the default install of OpenCV 3.0 and how to use SIFT and SURF in OpenCV 3.

The algorithms and associated implementations in opencv_contrib are not installed by default and you need to explicitly enable them when compiling and installing OpenCV to obtain access to them.

They are move to xfeatures2d library.
#include