ClassDef(TTestClass,1);
};[/code]which should be handle just fine by the default streamer. Alternatively you could use:
class TTestClass : public TObject {
public:
std::vector<std::vector<Double_t> > fData;
ClassDef(TTestClass,1);
}; which would be much easier to use ; if you use this, you will need to explicitly generate the dictonary for TTestClass and for std::vector<std::vector<Double_t> >.