Code made by MakeClass(), how can I execute by macro?

ROOT
5

With ROOT 6 this can be a bit tricky.
I guess the best idea (which should work with ROOT 5 and 6) is to create a simple local “rootlogon.C” file in your “current working directory”:

void rootlogon(void) { // rootlogon.C ...
  // std::cout << "... rootlogon.C ..." << std::endl;
  gROOT->LoadMacro("HistoMaker.C++");
} // ... rootlogon.C

and then the “run_HistoMaker.cxx” could look like this:

#include "HistoMaker.h"
#include "TSystem.h"
#include <iostream>

void run_HistoMaker()
{
  std::cout << "Running HistoMaker" << std::endl;
  gSystem->mkdir("./histos/", kTRUE);
  
  TChain *tree = new TChain("Events");
  tree->Add("../../skimmer/python/output_skim_Summer16MC_merged/Summer16.g1800*.root");
  HistoMaker t(tree);
  t.Loop();
  delete tree;
}