#include "TROOT.h"
#include "TSystem.h"
#include <TChain.h>
#include <TFile.h>
#include <memory>  
#include "TTree.h"
#include "TString.h"
#include "TClonesArray.h"
#include <stdio.h>

std::string filename1="pp_2j_NLO_H_T_35GeV_jetsreco_pTjetmin35GeV.root";
std::string filename2="pp_4j_LO_H_T_2_35GeV_jetsreco_pTjetmin35GeV.root";
std::string output_filename= "file1_w_file2_as_friend.root";
std::string output_filename2= "friend.root";

std::string treename="LHEF";

int func(std::string filename1,std::string filename2)
{
int max_allowed_entries = 100;
    std::unique_ptr<TFile> first{TFile::Open(filename1.c_str())};
    TTree * first_tree= (TTree*)first->Get<TTree>(treename.c_str());    

    TFile* output_file= TFile::Open( output_filename.c_str(), "RECREATE" );
    TTree *output_tree = first_tree->CloneTree(0);

    std::unique_ptr<TFile> second{TFile::Open(filename2.c_str())} ;
    TTree *second_tree = (TTree*)second->Get<TTree>(treename.c_str());

    TFile* output_file2= TFile::Open( output_filename2.c_str(), "RECREATE" );
    TTree *output_friend = second_tree->CloneTree(0);


auto nentries=max_allowed_entries;

for(Long64_t e = 0; e < nentries; e++) {

    first_tree->GetEntry(e); 
    output_tree->Fill();

    second_tree->GetEntry(e);
    output_friend->Fill();
}

output_friend->StartViewer();

output_tree->AddFriend(output_friend);

return 0;
}