How to call a function in C++ file

[Leyond]

Hello,All,
  I want to calculate the signal strength received from two access points in free space. And I find there is a function called
double FreeSpace::Pr(PacketStamp *t, PacketStamp *r, WirelessPhy *ifp)
{...}[ns-2.31/mobile/propagation.cc]
But, I don't know how to call this function to calculate the value of SS, so, could you do me a favor?
Actually I don't know if this function works or not? if not, is there any way ?
Thanks.
Leyond