TTree: Creating a branch with std::vector can raise an internal error when mixing I/O systems.

[fweig]

• Checked for duplicates

Describe the bug

Creating a branch in TTree with type std::vector<A> raises an internal error in ROOT, when class A inherits from a class that uses the old I/O system.

Expected behavior

Branch should be created successfully (I don't know if this behavior is supposed to be supported) or ROOT should abort with a more descriptive error message.

To Reproduce

Compile and run the following macro with root -l -b -q T.C+:

#include <vector>
#include "TTree.h"

#pragma link C++ class A;
#pragma link C++ class B+;
#pragma link C++ class std::vector<B>+;

struct A {
    int x;
    ClassDef(A, 1);
};
ClassImp(A);

struct B : A {
    int y;
    ClassDef(B, 1);
};
ClassImp(B);

void T() {
   TTree* tree = new TTree("T", "T");
   std::vector<B> bvec;
   tree->Branch("B", &bvec);
}

Macro aborts with the error:
Fatal in <TBranchElement::InitializeOffsets>: Could not find the real data member 'B' when constructing the branch 'B' [Likely an internal error, please report to the developers].

Setup

1. ROOT version: 6.22.08 (built myself), 6.26.00 (binary download)
2. Operating System: Ubuntu 18.04

Additional context

None.

[pcanal]

Did you intentionally used:

#pragma link C++ class A;

instead of

#pragma link C++ class A+;

?

[fweig]

Yes, that's the bug. The second variant should work fine.

[pcanal]

This is indeed a problem but should be a rare occurrence as the 'without the +' case has a lot of disadvantage.

Out of curiosity, what lead you to use that form?

[fweig]

I discovered this by accident when working on CbmRoot. Basically I created a branch with the setup from the reproducer and was greeted with that error message. The base class in that case is pretty old code. I assume nobody ever bothered to update the link def and that's why the '+' was missing in the first place. However it took me some time to figure out that this was the problem and caused quite a bit of headache. That's why I wrote it would be nice if Root would abort with a better error message here. Becomes trivial to fix then.