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i am directly running the code given on opencv documentation. my version of opencv is opencv 2.4.9

    #include <stdio.h>
    #include <iostream>
    #include "opencv2/core/core.hpp"
    #include "opencv2/features2d/features2d.hpp"
    #include "opencv2/highgui/highgui.hpp"
    //#include "opencv2/nonfree/features2d.hpp"
    #include "opencv2/legacy/legacy.hpp"

using namespace cv;

int main( int argc, char** argv )
{
  Mat img_1 = imread( argv[1], CV_LOAD_IMAGE_GRAYSCALE );
  Mat img_2 = imread( argv[2], CV_LOAD_IMAGE_GRAYSCALE );

  if( !img_1.data || !img_2.data )
  { std::cout<< " --(!) Error reading images " << std::endl; return -1; }

  //-- Step 1: Detect the keypoints using SURF Detector
  int minHessian = 400;

  SurfFeatureDetector detector( minHessian );

  std::vector<KeyPoint> keypoints_1, keypoints_2;

  detector.detect( img_1, keypoints_1 );
  detector.detect( img_2, keypoints_2 );

  //-- Step 2: Calculate descriptors (feature vectors)
  SurfDescriptorExtractor extractor;

  Mat descriptors_1, descriptors_2;

  extractor.compute( img_1, keypoints_1, descriptors_1 );
  extractor.compute( img_2, keypoints_2, descriptors_2 );

  //-- Step 3: Matching descriptor vectors using FLANN matcher
  FlannBasedMatcher matcher;
  std::vector< DMatch > matches;
  matcher.match( descriptors_1, descriptors_2, matches );

  double max_dist = 0; double min_dist = 100;

  //-- Quick calculation of max and min distances between keypoints
  for( int i = 0; i < descriptors_1.rows; i++ )
  { double dist = matches[i].distance;
    if( dist < min_dist ) min_dist = dist;
    if( dist > max_dist ) max_dist = dist;
  }

  printf("-- Max dist : %f \n", max_dist );
  printf("-- Min dist : %f \n", min_dist );

  //-- Draw only "good" matches (i.e. whose distance is less than 2*min_dist,
  //-- or a small arbitary value ( 0.02 ) in the event that min_dist is very
  //-- small)
  //-- PS.- radiusMatch can also be used here.
  std::vector< DMatch > good_matches;

  for( int i = 0; i < descriptors_1.rows; i++ )
  { if( matches[i].distance <= max(2*min_dist, 0.02) )
    { good_matches.push_back( matches[i]); }
  }

  //-- Draw only "good" matches
  Mat img_matches;
  drawMatches( img_1, keypoints_1, img_2, keypoints_2,
               good_matches, img_matches, Scalar::all(-1), Scalar::all(-1),
               vector<char>(), DrawMatchesFlags::NOT_DRAW_SINGLE_POINTS );

  //-- Show detected matches
  imshow( "Good Matches", img_matches );

  for( int i = 0; i < (int)good_matches.size(); i++ )
  { printf( "-- Good Match [%d] Keypoint 1: %d  -- Keypoint 2: %d  \n", i, good_matches[i].queryIdx, good_matches[i].trainIdx ); }

  waitKey(0);

  return 0;
}

i am getting this error message

/home/sumit/feature_detection/flann/flann.cpp: In function ‘int main(int, char**)’:
/home/sumit/feature_detection/flann/flann.cpp:33:3: error: ‘SurfFeatureDetector’ was not declared in this scope
   SurfFeatureDetector detector( minHessian );
   ^
/home/sumit/feature_detection/flann/flann.cpp:33:23: error: expected ‘;’ before ‘detector’
   SurfFeatureDetector detector( minHessian );
                       ^
/home/sumit/feature_detection/flann/flann.cpp:37:3: error: ‘detector’ was not declared in this scope
   detector.detect( img_1, keypoints_1 );
   ^
/home/sumit/feature_detection/flann/flann.cpp:41:3: error: ‘SurfDescriptorExtractor’ was not declared in this scope
   SurfDescriptorExtractor extractor;
   ^
/home/sumit/feature_detection/flann/flann.cpp:41:27: error: expected ‘;’ before ‘extractor’
   SurfDescriptorExtractor extractor;
                           ^
/home/sumit/feature_detection/flann/flann.cpp:45:3: error: ‘extractor’ was not declared in this scope
   extractor.compute( img_1, keypoints_1, descriptors_1 );
   ^
make[2]: *** [CMakeFiles/a.dir/flann.cpp.o] Error 1
make[1]: *** [CMakeFiles/a.dir/all] Error 2
make: *** [all] Error 2

also in my "/usr/include/opencv2" directory there isn't any "nonfree" directory!!

please help me out