component.h

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#ifndef Candidate_component_h
#define Candidate_component_h

#include "FWCore/Utilities/interface/Exception.h"

namespace reco {

  class Candidate;

  struct DefaultComponentTag {};

  namespace componenthelper {

    struct SingleComponentTag {};

    struct MultipleComponentsTag {};

    template <typename C, typename T, T (C::*F)() const>
    struct SingleComponent {
      static T get(const Candidate &c) {
        const C *dc = dynamic_cast<const C *>(&c);
        if (dc == nullptr)
          return T();
        return (dc->*F)();
      }
    };

    template <typename C, typename T, T (C::*F)(size_t) const, size_t (C::*S)() const>
    struct MultipleComponents {
      static size_t numberOf(const Candidate &c) {
        const C *dc = dynamic_cast<const C *>(&c);
        if (dc == nullptr)
          return 0;
        return (dc->*S)();
      }
      static T get(const Candidate &c, size_t i) {
        const C *dc = dynamic_cast<const C *>(&c);
        if (dc == nullptr)
          return T();
        if (i < (dc->*S)())
          return (dc->*F)(i);
        else
          throw cms::Exception("Error") << "index " << i << " out ot range";
      }
    };

  }  // namespace componenthelper

  template <typename T, typename M, typename Tag = DefaultComponentTag>
  struct component {};

  template <typename T>
  inline T get(const Candidate &c) {
    return component<T, componenthelper::SingleComponentTag>::type::get(c);
  }

  template <typename T, typename Tag>
  inline T get(const Candidate &c) {
    return component<T, componenthelper::SingleComponentTag, Tag>::type::get(c);
  }

  template <typename T>
  inline T get(const Candidate &c, size_t i) {
    return component<T, componenthelper::MultipleComponentsTag>::type::get(c, i);
  }

  template <typename T, typename Tag>
  inline T get(const Candidate &c, size_t i) {
    return component<T, componenthelper::MultipleComponentsTag, Tag>::type::get(c, i);
  }

  template <typename T>
  inline size_t numberOf(const Candidate &c) {
    return component<T, componenthelper::MultipleComponentsTag>::type::numberOf(c);
  }

  template <typename T, typename Tag>
  inline size_t numberOf(const Candidate &c) {
    return component<T, componenthelper::MultipleComponentsTag, Tag>::type::numberOf(c);
  }

}  // namespace reco

#define GET_CANDIDATE_COMPONENT(CAND, TYPE, FUN, TAG)                      \
  template <>                                                              \
  struct component<TYPE, componenthelper::SingleComponentTag, TAG> {       \
    typedef componenthelper::SingleComponent<CAND, TYPE, &CAND::FUN> type; \
  }

#define GET_DEFAULT_CANDIDATE_COMPONENT(CAND, TYPE, FUN)                             \
  template <>                                                                        \
  struct component<TYPE, componenthelper::SingleComponentTag, DefaultComponentTag> { \
    typedef componenthelper::SingleComponent<CAND, TYPE, &CAND::FUN> type;           \
  }

#define GET_CANDIDATE_MULTIPLECOMPONENTS(CAND, TYPE, FUN, SIZE, TAG)                       \
  template <>                                                                              \
  struct component<TYPE, componenthelper::MultipleComponentsTag, TAG> {                    \
    typedef componenthelper::MultipleComponents<CAND, TYPE, &CAND::FUN, &CAND::SIZE> type; \
  }

#define GET_DEFAULT_CANDIDATE_MULTIPLECOMPONENTS(CAND, TYPE, FUN, SIZE)                    \
  template <>                                                                              \
  struct component<TYPE, componenthelper::MultipleComponentsTag, DefaultComponentTag> {    \
    typedef componenthelper::MultipleComponents<CAND, TYPE, &CAND::FUN, &CAND::SIZE> type; \
  }

#endif