Column.h

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#ifndef FWCore_SOA_Column_h
#define FWCore_SOA_Column_h
// -*- C++ -*-
//
// Package:     FWCore/SOA
// Class  :     Column
//
//
// Original Author:  Chris Jones
//         Created:  Thu, 24 Aug 2017 16:17:56 GMT
//

// system include files

// user include files

// forward declarations
namespace edm {
  namespace soa {

    template <typename COL, typename F>
    struct ColumnFillerHolder {
      using Column_type = COL;
      F m_f;
    };

    template <typename T, typename INHERIT>
    struct Column {
      using type = T;

      static constexpr const char* const& label() { return INHERIT::kLabel; }

      template <typename F>
      static ColumnFillerHolder<INHERIT, F> filler(F&& iF) {
        return {iF};
      }

      Column(const Column&) = delete;  // stop default

      const Column& operator=(const Column&) = delete;  // stop default

    private:
      Column() = default;
    };

  }  // namespace soa
}  // namespace edm

#define SOA_DECLARE_COLUMN(_ClassName_, _Type_, _String_)             \
  struct _ClassName_ : public edm::soa::Column<_Type_, _ClassName_> { \
    static constexpr const char* const kLabel = _String_;             \
  }

/*
 * SOA_DECLARE_DEFAULT macro to define a default way to retrieve the column
 * values from an arbitrary class, i.e. a templated value_for_column function.
 * A call to the macro should be placed in edm::soa namespace.
 *
 * This templated value_for_column function has the advantage that
 * value_for_column does not have to be redefined for multiple classes with a
 * similar interface, but it has one caveat: if a different value_for_column
 * function needs to be defined for a given class, it needs to be defined also
 * for all derived classes, otherwise the templated value_for_column function
 * will still be used for the derived classes.
 *
 * If this becomes a real problem, the SOA_DECLARE_DEFAULT can be changed to
 * define a value_for_column_default() function, and all callers of
 * value_for_column() check the existence of a value_for_column_default()
 * overload first.
 *
 * For now, it is encouraged to adapt the interface of the class to the generic
 * value_for_column function generated by this macro, rather than implementing
 * value_for_column overloads for each class.
 */
#define SOA_DECLARE_DEFAULT(_ClassName_, _Expression_)                \
  template <class Object>                                             \
  _ClassName_::type value_for_column(Object const& x, _ClassName_*) { \
    return x._Expression_;                                            \
  }
#endif