{

auto f = new TFile("f.root", "update");
auto t_old = (TTree*)f->Get("t");
t_old->SetName("t_old");
t_old->SetBranchStatus("a", 0);

int a=0, b=0;

auto t = t_old->CloneTree(0);
t->SetName("t");
t->Branch("a", &a, "a/I");

a = 2;

t->Fill();

delete t_old;

t->Write("", TObject::kWriteDelete);
f->Close();

}