TCanvas deleted unwantedly

ROOT
1

Dear friends,

When I run this code twice:

TCanvas *c = new TCanvas("plot", "plot"); c->BuildLegend(); c->Modified(); c->Update();

It gives a warning the second time:

Naturally, I tried to encapsulate the code within an if-statement:

if (!gROOT->GetListOfCanvases()->FindObject("plot")) { TCanvas *c = new TCanvas("plot", "plot"); }
so that it wouldn’t create a new TCanvas the second time. But this only makes things worse:

: error: use of undeclared identifier 'c' c->BuildLegend(); ^ error: use of undeclared identifier 'c' c->Modified(); c->Update(); ^ error: use of undeclared identifier 'c' c->Modified(); c->Update(); ^

Now I completely fail to see how these warnings and errors are consistent.

My question is how do I modify the ‘TCanvas *c = new TCanvas’-statement so that it works properly the second time I run it (either within the if-statement or without)?

2

Hi,

the issue is that you are declaring the canvas pointer within a scope:

if (!gROOT->GetListOfCanvases()->FindObject("plot")) {
  TCanvas *c = new TCanvas("plot", "plot");
   }

upon closing the curly brace, all variables local to the scope, e.g. your “c”, are forgotten by the compiler.
You could do this:

TCanvas *c = nullptr;
if (!gROOT->GetListOfCanvases()->FindObject("plot")) {
   c = new TCanvas("plot", "plot");
   }

Cheers,
Danilo

3

Try: TCanvas *c = (TCanvas *)gROOT->GetListOfCanvases()->FindObject("plot"); if (c) c->Clear(); else c = new TCanvas("plot", "plot");

4

[quote=“dpiparo”]Hi,

the issue is that you are declaring the canvas pointer within a scope:

upon closing the curly brace, all variables local to the scope, e.g. your “c”, are forgotten by the compiler.
You could do this:

Cheers,
Danilo[/quote]
The comment about variables local to the scope was insightful. But the code gave a crash after the second run:

[code] *** Break *** segmentation violation

There was a crash.
This is the entire stack trace of all threads:

[/code]

[quote=“Pepe Le Pew”]Try: TCanvas *c = (TCanvas *)gROOT->GetListOfCanvases()->FindObject("plot"); if (c) c->Clear(); else c = new TCanvas("plot", "plot");[/quote]

This keeps working fine. I’ll study on why/how the first line works. The second line seems clear to me.

Thanks a lot to all!