Persistency & Polymorphism

Hello,

I have thew following problem I want to solve with ROOT:

I want to make an instance of class called "Track" persistent to a ROOT file.

"Track" contains a collection of TrackReps (which are track
representations used with a Kalman filter). There is an abstract base class "AbsTrackRep" (which inherits from TObject), and derived from that are e.g. "HelixTrackRep" and "StraightLineTrackRep" (or in general
"ConcreteTrackRep").

I figure that the right collection is a "TObjArray", since it contain a collection of whatever, as long as everything inherits from TObject. Please note, that the Array will never contain any instances of
"AbsTrackRep" but only different "ConcreteTrackReps".

However, when trying to persist, I get an error message:

No dictionary for AbsTrackRep

This is completely true, because this abstract base class doesn't have one, it can't even be instanciated. The "ConcreteTrackRep"s however have dictionaries, and these are the ones which should be used.

Here is what I could track down what happens when the TObjArray Streamer is called:

TObjArray has a method Streamer which is used for writing to the IO. It uses the << operator of TBuffer to do so. This operator uses the

Int_t TBuffer::WriteObjectAny(const void *obj, const TClass *ptrClass)

method. In this method something strange seems to happen:

  temp -= clActual->GetBaseClassOffset(ptrClass);   WriteObject(temp, clActual);

This code somehow gets the adress of the base class and tries to call the dictionary for the base class. And this is exactly where the "no dictionary" error comes from. I tried this one the CINT command line after I loaded my shared lib. The clActual points to a HelixTrackRep and temp to AbsTrackRep.

Finally my question: Does anyone know how to work around this. E.g. is there some pragma for the linkdef to tell root not to look for the base class in the WriteObjectAny method?

Cheers, Christian Received on Mon Feb 12 2007 - 13:48:18 CET