TH1::SetDirectory(TDirectory*)

ROOT
1

Hello!
What is the meaning of TH1::SetDirectory(TDirectory*)

I had created some TH1’s and called TH1::SetDirectory(NULL) on some and not on others. This caused my program to crash. The problem was resolved only when I called TH1::SetDirectory(NULL) on all of the histograms.

Cheers,
Viesturs

2

Hi Viesturs,

this is not expected: maybe this is related to another issue?
The documentation of this method can be found here:
o root.cern.ch/doc/master/classTH1.html
o root.cern.ch/root/htmldoc/guide … gdirectory

Cheers,
Danilo

3

Hello Danilo,
The problem was caused by closing a file after opening a histogram as in the following scheme:
TFile *input_file = TFile::Open(“input_file.root”, “READ”);
TH1F * h = new TH1F(“test”, “test”, 5, 0, 5);
h-> SetDirectory(0);
TH1F *h1 = new TH1F(“test1”, “test1”, 5, 0, 5);
input_file -> Close();
TFile * output_file = TFile::Open(“output_file.root”, “RECREATE”);
h-> SetDirectory(output_file);
h1-> SetDirectory(output_file); //CRASH
h-> Write(); h1 -> Write();
output_file -> Close();

It is not the first time that I stumble upon this problem spending great effort in debugging.

Cheers,
Viesturs

4

Hi viesturs,

That would explain it. Glad you fixed your issue!

D

5

Hi Viesturs,

We are keenly aware of these kind of pitfalls and we are currently prototyping new interfaces (nicknamed v7) that would (amongst other things) prevents this kind of ambiguities.

Cheers,
Philippe.