TGraph2D::Build function may leave the newly created object in inconsistent non-initialised state

[SergueiEK]

Check duplicate issues.

• Checked for duplicates

Description

If I create a new TGraph2D object with 0 points the new object is not properly initialized. The issue is that the TGraph2D::Build(n) function that is called by the constructors returns immediately if the number of points equals to zero. As a result the new TGraph2D object has most of its member attributes non-initialized and an attempt to use it (for example by calling SetDirectory() function) causes Segmentation Fault.

Reproducer

TGraph2D * g = new TGraph2d(0);
g->SetDirectory(0);

ROOT version

6.38.00

Installation method

CERN LCG_109 standard installation

Operating system

Alma 9 Linux

Additional context

No response

[couet]

TGraph2D::Build(n) function that is called by the constructors returns immediately if the number of points equals to zero

That's sounds quite sensible. What is the use case for a TGraph2D with 0 points ?

[SergueiEK]

I have some graphs that are supposed to be dynamically updated and displayed in the GUI application. Initially they are created with zero points because at the time of the GUI initialisation there is no data to be displayed. When the GUI receives new data from an external source it adds them to the graphs and updates the display. This approach works fine for the TGraph objects but crashes with the TGraph2D.
Just for the record, here is how the TGraph2D::Build function is implemented in 6.38.00

[couet]

Thanks for your feed back. I'll check.

[couet]

PR here: #21368