struct DETECTOR
{
    Long64_t time;
    Double_t energy;
    Int_t number;
};

void member(string member_string)
{
    auto DETECTOR::*ptr;

    if(member_string == "time") ptr = &DETECTOR::time;
    if(member_string == "energy") ptr = &DETECTOR::energy;
    if(member_string == "number") ptr = &DETECTOR::number;

    //loop over a lot of DETECTOR objects and do some magic with them
}