function member as a TTree leaf

Hallo rooters,

I notice that some root-defined classes, like TVector3, can be saved in a TTree in such a way that its member functions, like TVector3::Eta() , can also be viewed and drawed like normal  variable members.

How can I do this in my own class?

For example, I have defined a class Test, and it has a function member
double Test::X() const { return 3.4;}

How can I view X() in a TTree, if it is saved by: Test *aa=new Test;
myTree->Branch("aa","Test",&a); ?

Thanks a lot !!

Cheers!
Xianguo

-- 
LU, Xianguo

DESY-HERMES
Notkestrasse 85
Hamburg, 22607
Germany