/*!

\page callbacks Using ns-3 callbacks

\anchor howtos-callbacks

\section null_callbacks Null Callbacks

<b>Question:</b> The API I am using calls for using a callback (in the 

function signature), but I do not

want to provide one.  Is there a way to provide a null callback?

<b>Answer:</b> Use the ns3::MakeNullCallback construct:

\code

template<typename R>

Callback< R, T1, T2, T3, T4, T5, T6 > ns3::MakeNullCallback (void)

\endcode

Example usage:  The ns3::Socket class uses callbacks to indicate completion

of events such as a successful TCP connect().  These callbacks are set

in the following function:

\code

  void Socket::SetConnectCallback (Callback<void, Ptr<Socket> > connectionSucceeded,

                        Callback<void, Ptr<Socket> > connectionFailed,

                        Callback<void, Ptr<Socket> > halfClose);

\endcode

But suppose you do not care about registering a callback for the 

halfClose event (but you want to register one for the 

connectionSucceeded and connectionFailed cases).  In that case, you

can pass a null callback as the third argument.  You just need to

pass a callback with the matching signature, as follows:

\code

  localSocket->SetConnectCallback (

  MakeCallback (&ConnectionSucceededCallback),

  MakeCallback (&ConnectionFailedCallback),

  MakeNullCallback<void, Ptr<Socket> > () );

\endcode

*/