contact sensor has no member named contacts

edit

I was following this tutorial , but as i try to build the code i get theis error msg:

/usr/include/boost/asio/detail/impl/socket_ops.ipp:266:5: note:   ‘boost::asio::detail::socket_ops::bind’
 int bind(socket_type s, const socket_addr_type* addr,
     ^
/home/rob/Documents/gazebo_contact_tutorial/ContactPlugin.cc: In member function ‘virtual void gazebo::ContactPlugin::OnUpdate()’:
/home/rob/Documents/gazebo_contact_tutorial/ContactPlugin.cc:43:34: error: ‘class gazebo::sensors::ContactSensor’ has no member named ‘Contacts’
   contacts = this->parentSensor->Contacts();

How to remove it? I am using ros indigo.