View source for HOWTO determine the path of an attribute or trace source

==Aggregated Objects==
$-prefixed items in a path are used to perform an aggregation lookup using the TypeId of the Object you are looking for.
Since almost all TypeIds have a ns3:: prefix, you will usually see the compound prefix $ns3:: 

Example: 
<pre>/NodeList/*/$ns3::TcpL4Protocol</pre> 
the above path works because ns3::TcpL4Protocol instances are aggregated to the node in which they exist.
On the other hand see this one:
<pre>/NodeList/0/DeviceList/0/$ns3::WifiRemoteStationManager</pre> 
the above path is getting device 0 in node 0, and, then, doing GetObject<WifiRemoteStationManager>
on it but it gets null because the remote station manager is not
aggregated to the device.

==Objects of a specific type==
$-prefixed items in a path are also used to do a ''type cast'' to an object of a particular type. 

For example: 
<pre>/NodeList/*/DeviceList/*/$ns3::WifiNetDevice</pre>
will match only with devices of type WifiNetDevice. 

Conversely, the following:
<pre>/NodeList/*/DeviceList/*</pre>
will match with any device, regardless of the type.

==Objects referenced by attributes of other Objects==
2) non-$ prefixed non-terminating items in a path are used to perform an
attribute lookup on an attribute of type Pointer. This is often used as an alternative to object aggregation.

As an example, when using a wifi device:
<pre>/NodeList/0/DeviceList/0/RemoteStationManager/MacTxDataFailed</pre> 
the above path works, because devices with TypeId ns3::WifiNetDevice have a RemoteStationManager attribute which is of type Ptr<WifiRemoteStationManager>. On the other hand, see this one:
<pre>/NodeList/0/TcpL4Protocol</pre> 
the above path doesn't work, <pre>/NodeList/0</pre> gets you to a Node Object, and there is no Attribute named TcpL4Protocol. All this in spite of the fact that TcpL4Protocol is aggregated with Node -- remember that for querying aggregated objects you need the $ prefix.


==Hints==
* A nice way to figure out a path is to [[HOWTO use the ConfigStore | use the ConfigStore]]: if you use it to dump all attributes in a text file, you can see a valid path for each attribute (this, arguably, does not work for trace sources).
* Also, note that there is a 'Config' LogComponent: just activate it by stating <code>LogComponentEnable ("Config", LOG_LEVEL_ALL);</code> somewhere in your simulation script.


==References==
*http://groups.google.com/group/ns-3-users/browse_thread/thread/c162f5a8d13fe9d1/67b21278f7abf925?lnk=gst&q=config#67b21278f7abf925
*http://mailman.isi.edu/pipermail/ns-developers/2009-August/006350.html