const std::string columnName = "x";

ROOT::EnableImplicitMT(2);
auto rdf = ROOT::RDataFrame(5);
auto d = rdf.Define(columnName, "rdfentry_ + 1.");

auto aggregator = [](double acc, double x) { return acc * x; };

auto merger = [](std::vector<double> &accumulators) {
   auto size = accumulators.size();
   for (int i = 1; i < size; ++i) {
      accumulators[0] *= accumulators[i];
   }
};

double initValue = 1.;

auto result = d.Aggregate(aggregator, merger, columnName, initValue);

std::cout << *result << std::endl;

gROOT->GetListOfCanvases()->Draw()