%%cpp -d
void classicWay()
{
   TFile f("df009_FromScratchVSTTree_classic.root", "RECREATE");
   TTree t("treeName", "treeName");
   double b1;
   int b2;
   t.Branch("b1", &b1);
   t.Branch("b2", &b2);
   for (int i = 0; i < 10; ++i) {
      b1 = i;
      b2 = i * i;
      t.Fill();
   }
   t.Write();
   f.Close();
}

%%cpp -d
void RDFWay()
{
   ROOT::RDataFrame df(10);
   auto b = 0.;
   df.Define("b1", [&b]() { return b++; })
     .Define("b2", "(int) b1 * b1")  This can even be a string
     .Snapshot("treeName", "df009_FromScratchVSTTree_df.root");
}

classicWay();
RDFWay();