#include <TComplex.h>
#include <TFile.h>
#include <TTree.h>

class Data {
    public:
        unsigned int          entry;
        TComplex              value;
        std::vector<TComplex> values;

    ClassDef(Data, 1)
};

void
write()
{
    TFile* file = TFile::Open("test.root", "RECREATE");

    TTree* tree = new TTree("tree", "tree");

    Data  data;
    Data* pData = &data;
    tree->Branch("data", &pData);

    for (unsigned int i=0; i<10; ++i) {
        data.entry = i;
        data.value = TComplex(i, TMath::TwoPi() * i/10., true);
        data.values.push_back(data.value);
        tree->Fill();
    }

    tree->Write();
    delete tree;

    file->Close();
    delete file;
}