Reading STL container in RDataFrame branch

ROOT
1

Dear Users,

using RDataFrame and Foreach tools to read data from a TTree I wrote the following code:

ROOT::RDataFrame particles("INPUT_particle", input_data);
std::vector<std::vector<ExpParticle>> allParticles;

particles.Foreach( [&] (int event, int track, std::string particle)
	{
		ExpParticle current_particle(event, track, particle);			
                    // doing stuff to get correct index to place in allParticles from outside...
                    int index;
		allParticles[index].push_back(current_particle);			

	}, {"event", "track", "particle"}
);

I get the following error:

Error in TTreeReaderValueBase::GetBranchDataType(): Must use TTreeReaderArray to read branch particle: it contains an array or a collection.
Error in TTreeReaderValueBase::CreateProxy(): The branch particle contains data of type {UNDETERMINED TYPE}, which does not have a dictionary.
terminate called after throwing an instance of ‘std::runtime_error’
what(): An error was encountered while processing the data. TTreeReader status code is: 6
Aborted (core dumped)

My question is how to create dictionary of a type std::string.
I tried to workout at the LinkDef.h file through #pragma directive but it did not work. Maybe I did not provide the correct instructions of maybe the issue is elsewhere.

Please let me know, Thank you in advance

_ROOT Version: 6.32.00
_Platform: Ubuntu 22.04
_Compiler: gcc 11.4.0

2

Hello,
What are the types stored in the Tree? It cannot just be a std::string since ROOT provides the dictionary for string.

Lorenzo