I'm reading testRTSPClient.cpp at live555 library for read RTSP stream At a constructor's implementation of DummySink, i meet a strange syntax

    DummySink::DummySink(UsageEnvironment& env, MediaSubsession& subsession, char const* streamId)
      : MediaSink(env),
        fSubsession(subsession) {
      fStreamId = strDup(streamId);
      fReceiveBuffer = new u_int8_t[DUMMY_SINK_RECEIVE_BUFFER_SIZE];
    }

I don't understand what

: MediaSink(env),  
fSubsession(subsession)

that after function prototype means, because common function defining syntax is

functionName(args)
//without :
{
//implement
}

If anyone know about it, please tell me know. Thank you so much