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