In the documentation for class ORB (http://opencv.itseez.com/trunk/modules/features2d/doc/feature_detection_and_description.html#orb), ([[http://opencv.itseez.com/trunk/modules/features2d/doc/feature_detection_and_description.html#orb]]), the constructor is as follows:
<pre><code class="cpp"> <pre>
// constructor that initializes all the algorithm parameters

ORB( const CommonParams detector_params );
</code></pre> </pre>
But in header (features2d.hpp, rev 7020), the constructor has 2 parametres:
<pre><code class="cpp">
<pre>
/** Constructor

* @param n_features the number of desired features

* @param detector_params parameters to use

*/

ORB(size_t n_features = 500, const CommonParams & detector_params = CommonParams());
</code></pre> </pre>