[ns] A Q re. "data_" field in class Packet (fwd)

[Yan Yu <yanyu@ISI.EDU>]

Dear ns users, I have this Q:
  In class Packet, there is a field called " AppData* data_;"

IN class AppData, there is no real data field, AppData looks like a
virtual class, 
there is another class called PacketData, a subclass of AppData.

should I use PacketData instead when I want some data field in Packet?

Thanks  a LOT!
yan