CollectionCombiner.h

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#ifndef CollectionCombiner_H
#define CollectionCombiner_H

// system include files
#include <memory>

// user include files
#include "FWCore/Framework/interface/Frameworkfwd.h"
#include "FWCore/Framework/interface/global/EDProducer.h"

#include "FWCore/Framework/interface/Event.h"
#include "FWCore/Framework/interface/MakerMacros.h"

#include "FWCore/ParameterSet/interface/ParameterSet.h"

template <typename Collection>
class CollectionCombiner : public edm::global::EDProducer<> {
public:
  explicit CollectionCombiner(const edm::ParameterSet&);
  ~CollectionCombiner() override;

private:
  void produce(edm::StreamID, edm::Event&, const edm::EventSetup&) const override;

  // ----------member data ---------------------------
  std::vector<edm::InputTag> labels;
  std::vector<edm::EDGetTokenT<Collection> > collectionTokens;
};

template <typename Collection>
CollectionCombiner<Collection>::CollectionCombiner(const edm::ParameterSet& iConfig) {
  labels = iConfig.getParameter<std::vector<edm::InputTag> >("labels");
  produces<Collection>();
  for (unsigned int i = 0; i < labels.size(); ++i)
    collectionTokens.push_back(consumes<Collection>(labels.at(i)));
}
template <typename Collection>
CollectionCombiner<Collection>::~CollectionCombiner() {}

template <typename Collection>
void CollectionCombiner<Collection>::produce(edm::StreamID, edm::Event& iEvent, const edm::EventSetup& es) const {
  unsigned int i = 0, i_max = labels.size();
  edm::Handle<Collection> handle;
  auto merged = std::make_unique<Collection>();
  for (; i != i_max; ++i) {
    iEvent.getByToken(collectionTokens[i], handle);
    merged->insert(merged->end(), handle->begin(), handle->end());
  }
  iEvent.put(std::move(merged));
}

#endif