rfile003_ttree.C File Reference

Detailed Description

Demonstrate how to read a TTree with RFile.

Note that, due to how TTree handles lifetime and its relationship with TDirectory, writing TTree through RFile is currently unsupported.

#include <iostream>
#include <TSystem.h>
#include <TTree.h>
#include <ROOT/RDataFrame.hxx>
#include <ROOT/RFile.hxx>
 
void read_tree_from_rfile(const char *fileName)
{
   // Open an existing ROOT file for reading (will throw an exception if `fileName` cannot be read).
   // IMPORTANT! This file must remain open while reading data from the tree.
   auto file = ROOT::Experimental::RFile::Open(fileName);
 
   // Get back the TTree from the file
   auto tree = file->Get<TTree>("tree");
   R__ASSERT(tree); // we just wrote the tree so we expect to find it in the file
   int intVal;
   tree->SetBranchAddress("intBranch", &intVal);
 
   for (auto i = 0; i < tree->GetEntries(); ++i) {
      tree->GetEntry(i);
      std::cout << "value[" << i << "] = " << intVal << "\n";
   }
}
 
void rfile003_ttree()
{
   const char *const fileName = "rfile_ttree.root";
 
   // Note that TTree cannot be written to RFile, so we use RDataFrame to write it instead.
   ROOT::RDataFrame(10).Define("intBranch", "static_cast<int>(rdfentry_)").Snapshot("tree", fileName);
 
   read_tree_from_rfile(fileName);
 
   gSystem->Unlink(fileName);
}

value[0] = 0
value[1] = 1
value[2] = 2
value[3] = 3
value[4] = 4
value[5] = 5
value[6] = 6
value[7] = 7
value[8] = 8
value[9] = 9

Warning

This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback is welcome!

Date

2026-10-12

Author

Giacomo Parolini giaco.nosp@m.mo.p.nosp@m.aroli.nosp@m.ni@c.nosp@m.ern.c.nosp@m.h

Definition in file rfile003_ttree.C.