Using ns-3 callbacks

Null Callbacks

Question: 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?

Answer: Use the ns3::MakeNullCallback construct:

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

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:

  void Socket::SetConnectCallback (Callback<void, Ptr<Socket> > connectionSucceeded,
                        Callback<void, Ptr<Socket> > connectionFailed,
                        Callback<void, Ptr<Socket> > halfClose);

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:

  localSocket->SetConnectCallback (
  MakeCallback (&ConnectionSucceededCallback),
  MakeCallback (&ConnectionFailedCallback),
  MakeNullCallback<void, Ptr<Socket> > () );