The Class Version identifier (in ClassDef) can be used in
your class Streamer function to take into account different versions
of the same class. If you add/remove data members, you must modify
the Streamer function to take into account the changes.
Only the Read part of Streamer must be modified with statements like:
if (R__v > 1) {
R__b >> newdata; // if you add a data member
}
where R__v is the class version as specified in the ClassDef when
the class object was written to the file.
With this mechanism, you can always read your old files with
the latest version of your classes.
Rene Brun