Empty canvas when creating TFile on stack

[tlatorre]

I think this question was already answered here: Empty canvas, but I am curious as to what is happening under the hood.

When creating a ROOT macro which opens a TFile on the stack, ROOT is unable to draw a histogram:

void foo(void)
{
    TFile f("some_file.root");
    TH1D *h = new TH1D("h","blah",100,0,100);
    h->Fill(1.0);
    h->Draw();
}

produces an empty canvas whereas

void foo(void)
{
    TFile *f = new TFile("some_file.root");
    TH1D *h = new TH1D("h","blah",100,0,100);
    h->Fill(1.0);
    h->Draw();
}

works.

Is the TH1D somehow being associated with the TFile and then being deleted when it goes out of scope?

Thanks,

Tony

[dastudillo]

https://root.cern/root/htmldoc/guides/users-guide/ObjectOwnership.html

[couet]

Object ownership

[tlatorre]

Thanks for the link. For future readers, the documentation above doesn’t describe directly why a TFile affects the macro, but at ROOT files - ROOT it mentions that when you create a TFile it becomes the current directory, and then the rules stated in the above links show that a new histogram is now owned by this directory. Therefore, when the TFile object goes out of scope, it deletes the histogram.