traffic-control: Fix the size of the RootQueueDiscList attribute

examples/traffic-control/traffic-control.cc

@@ -130,6 +130,9 @@ main (int argc, char *argv[])
 
   Ptr<QueueDisc> q = qdiscs.Get (1);
   q->TraceConnectWithoutContext ("PacketsInQueue", MakeCallback (&TcPacketsInQueueTrace));
+  // Alternatively:
+  // Config::ConnectWithoutContext ("/NodeList/1/$ns3::TrafficControlLayer/RootQueueDiscList/0/PacketsInQueue",
+  //                                MakeCallback (&TcPacketsInQueueTrace));
 
   Ptr<NetDevice> nd = devices.Get (1);
   Ptr<PointToPointNetDevice> ptpnd = DynamicCast<PointToPointNetDevice> (nd);

src/traffic-control/model/traffic-control-layer.cc

@@ -185,8 +185,6 @@ TrafficControlLayer::GetRootQueueDiscOnDevice (Ptr<NetDevice> device) const
 
   if (ndi == m_netDevices.end ())
     {
-      NS_LOG_WARN ("GetRootQueueDiscOnDevice should not be called before the "
-                   << "device is setup or a queue disc is installed on the device.");
       return 0;
     }
   return ndi->second.rootQueueDisc;
@@ -241,7 +239,7 @@ TrafficControlLayer::NotifyNewAggregate ()
 uint32_t
 TrafficControlLayer::GetNDevices (void) const
 {
-  return m_netDevices.size();
+  return m_node->GetNDevices ();
 }