[hist] Fix segfault when calling `GetNumberOfBins()` on an empty TH2Poly

[hqucms]

This Pull request:

This pull request fixes a segfault when calling GetNumberOfBins() on an empty TH2Poly by first checking whether fBins is null.

To reproduce the original segfault:

TH2Poly h; h.GetNumberOfBins();

Changes or fixes:

Checklist:

• [ ] tested changes locally
• [ ] updated the docs (if necessary)

This PR fixes #

[ferdymercury]

I'd say it's better:

If(!fBins) { Warning() return; } else if(nbins...)

[hqucms]

I changed it to

if (nbins != (fBins ? fBins->GetSize() : 0))

which should cover both cases. Let me know what you think.

[dpiparo]

Hi @hqucms . Thanks for this fix! Do you think you could also add a unit test (https://github.com/root-project/root/tree/master/hist/hist/test)?

[guitargeek]

How much sense does it actually make to use these constructors without arguments of the hist classes? I thought they were reserved for IO. Maybe this is a documentation issue? @lmoneta

[dpiparo]

That would be useful. Do we also maybe want to cure crashes which might be occurring for example in presence of default values?

[hqucms]

I added a unittest in https://github.com/root-project/root/pull/16370/commits/daa74274b51e4174bfdf23ecd26248e5f400a8e7.

[hqucms]

How much sense does it actually make to use these constructors without arguments of the hist classes? I thought they were reserved for IO. Maybe this is a documentation issue? @lmoneta

It's not just the default constructor. For TH2Poly, one needs to call AddBin() explicitly to create the bins, otherwise it will crash regardless of which constructor is called.

[github-actions[bot]]

Test Results

    18 files      18 suites   4d 2h 8m 18s :stopwatch:  2 716 tests  2 714 :white_check_mark: 0 :zzz: 2 :x: 46 047 runs  46 045 :white_check_mark: 0 :zzz: 2 :x:

For more details on these failures, see this check.

Results for commit 76760e7b.

:recycle: This comment has been updated with latest results.

[hqucms]

Thanks a lot @hageboeck ! Is it possible to backport this to 6.32 and 6.30?

[hageboeck]

Thanks a lot @hageboeck ! Is it possible to backport this to 6.32 and 6.30?

Sure! See the PRs referenced above.

[hqucms]

Thanks a lot @hageboeck ! Is it possible to backport this to 6.32 and 6.30?

Sure! See the PRs referenced above.

Thanks a lot @hageboeck !