What Does ROOT TH1::SetDirectory Do?

[ChrisVer]

Can someone explain me what SetDirectory actually does?

I found the problem with a macro like that:

#include<iostream>
#include <TH1F.h>
#include <TCanvas.h>
#include <TFile.h>

int histoSum(){

//create histograms to use
TH1F* histo1 = new TH1F("histo1",";A", 100, 0,10);
TH1F* histo2 = new TH1F("histo2",";B", 100, 0,10);
TH1F* histoSUM = new TH1F("histoSUM",";C", 100, 0,10);

//call histograms from a root file
TFile f ("histograms.root");
f.GetObject("h_A_events",histo1);
f.GetObject("h_B_events",histo2);

//add the histograms of A,B
histo1->sumw2();
histo2->sumw2();
histoSUM->Add(histo1,histo2);

//print the histogram of SUM on canvas
TCanvas * c= new TCanvas("sum_histos",900,700);
histoSUM->Draw();

return 0;
}

The problem when I ran the program like this, is that the Canvas appeared empty...
On the other hand if I set for my histos histo1,histo2,histoSUM the ->SetDirectory(0)

[...]
f.GetObject("h_B_events",histo2);

histo1->SetDirectory(0);
histo2->SetDirectory(0);
histoSUM->SetDirectory(0);

//add the histograms of A,B
histo1->sumw2();
[...]

the histogram appeared in the canvas...
I only found this:
https://root.cern.ch/drupal/content/histograms-and-current-directory
but it doesn't help me understand how it works.

 

[jedishrfu]

My reading of it is that the Directory is a means to group histograms together for easier organized operations. Specifically, it says that if you delete the Directory all associated histograms are deleted as well.

It seems to work like a filesystem with histograms representing files and Directories representing filesystem subdirectories. In the filesystem paradigm, one can work with files as a group with a common subdirectory name ie one can copy files or on can delete the subdirectory to delete its files (some cautions may apply ie might have to force the operation).

This page talks about changing Directory to get a different set of histograms:

https://root.cern.ch/drupal/content/subdirectories-and-navigation