title: Why is there no definition for MaskPredicate::operator=(const MaskPredicate&) in OpenCV?
body: <p><a href="https://github.com/Itseez/opencv/blob/master/modules/features2d/src/keypoint.cpp" rel="nofollow">https://github.com/Itseez/opencv/blob/master/modules/features2d/src/keypoint.cpp</a></p>
  
  <p>The declaration of the function is in line 154.</p>