Invalid memory access in RooArgSet::contentsString() with an empty RooArgSet

[TomasDado]

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Description

While debugging a crash in TRExFitter, I found out that the following code:

void myString() {
    RooArgSet s;
    std::string myString = s.contentsString();
    std::cout << "my string: " << myString << "\n";
}

Results in an invalid string, in my case:

tomas@tomas-pc:~/TRExFitter$ root -l myString.cxx 
root [0] 
Processing myString.cxx...
my string: ���@Y ]��@Yȃ}��f|���e|&��@Y���@Y��}�3p�f|p�}��	\_|P^��@Y��
                                                                           )*h�F�@Yp�}�@�}� ]��@Y0�}���T_|h�F�@Y�g�@YG]_|��F�@Y��
                                                                                                                                 )* ]��@Yp�}���}��}���}�H�T_|�[��@�,����� ]��@Y��}��e|P���@Ygn7��
                                                                                                                                                                                                 )*f| ]��@Yp�N�@YP�}�0�}��e|�}b_|��}�p�}�@�}���}��}����@Y@�}� �}��}� �}����@Y�e|@�}��}myString0�}myStringP�␦�@Y7V()}�������}�0�}��0c_|��
                                                                                )*����%p�}�
                                                                                            �}�0�}�
                                                                                                   X@c_|@�}��[��@Y
                                                                                                                  |%%@�}�()}� �}�root [1] .q

This is caused by this line: https://github.com/root-project/root/blob/master/roofit/roofitcore/src/RooAbsCollection.cxx#L1157 which has no protection for an empty string.

Reproducer

See above

ROOT version

ROOT Version: 6.37.01
Built for linuxx8664gcc on Oct 24 2025, 06:58:40
From heads/master@v6-37-01-8567-gfd42da9fb

Installation method

built from source

Operating system

Ubuntu 25.04

Additional context

No response