SOA/interface/Column.h

0001 #ifndef FWCore_SOA_Column_h
0002 #define FWCore_SOA_Column_h
0003 // -*- C++ -*-
0004 //
0005 // Package:     FWCore/SOA
0006 // Class  :     Column
0007 //
0008 /**\class Column Column.h "Column.h"
0009
0010  Description: Column describes a column in a Table
0011
0012  Usage:
0013     Class instances inheriting from the Column class are not intended to be used.
0014  Instead, the specific Column type is used as a template argument
0015  to a edm::soa::Table<> to describe a column in the table.
0016
0017  A column is defined by a name and a C++ type.
0018
0019 Classes inheriting from Column must declare a 'constexpr const char' array named 'kLabel'.
0020  \code
0021  namespace edm {
0022  namespace soa {
0023    struct Eta : public Column<double,Eta> {
0024      static constexpr const char * const kLabel = "eta";
0025    };
0026  }
0027  }
0028  \endcode
0029 Alternatively, one can use the macro SOA_DECLARE_COLUMN
0030 \code
0031 namespace edm {
0032 namespace soa {
0033
0034 SOA_DECLARE_COLUMN(Eta,double, "eta");
0035
0036 }
0037 }
0038
0039 \endcode
0040
0041 */
0042 //
0043 // Original Author:  Chris Jones
0044 //         Created:  Thu, 24 Aug 2017 16:17:56 GMT
0045 //
0046
0047 // system include files
0048
0049 // user include files
0050
0051 // forward declarations
0052 namespace edm {
0053   namespace soa {
0054
0055     /**Helper class used to fill a column of a table
0056   in a 'non standard' way
0057 */
0058     template <typename COL, typename F>
0059     struct ColumnFillerHolder {
0060       using Column_type = COL;
0061       F m_f;
0062     };
0063
0064     template <typename T, typename INHERIT>
0065     struct Column {
0066       using type = T;
0067
0068       static constexpr const char* const& label() { return INHERIT::kLabel; }
0069
0070       template <typename F>
0071       static ColumnFillerHolder<INHERIT, F> filler(F&& iF) {
0072         return {iF};
0073       }
0074
0075       Column(const Column&) = delete;  // stop default
0076
0077       const Column& operator=(const Column&) = delete;  // stop default
0078
0079     private:
0080       Column() = default;
0081     };
0082
0083   }  // namespace soa
0084 }  // namespace edm
0085
0086 #define SOA_DECLARE_COLUMN(_ClassName_, _Type_, _String_)             \
0087   struct _ClassName_ : public edm::soa::Column<_Type_, _ClassName_> { \
0088     static constexpr const char* const kLabel = _String_;             \
0089   }
0090
0091 /*
0092  * SOA_DECLARE_DEFAULT macro to define a default way to retrieve the column
0093  * values from an arbitrary class, i.e. a templated value_for_column function.
0094  * A call to the macro should be placed in edm::soa namespace.
0095  *
0096  * This templated value_for_column function has the advantage that
0097  * value_for_column does not have to be redefined for multiple classes with a
0098  * similar interface, but it has one caveat: if a different value_for_column
0099  * function needs to be defined for a given class, it needs to be defined also
0100  * for all derived classes, otherwise the templated value_for_column function
0101  * will still be used for the derived classes.
0102  *
0103  * If this becomes a real problem, the SOA_DECLARE_DEFAULT can be changed to
0104  * define a value_for_column_default() function, and all callers of
0105  * value_for_column() check the existence of a value_for_column_default()
0106  * overload first.
0107  *
0108  * For now, it is encouraged to adapt the interface of the class to the generic
0109  * value_for_column function generated by this macro, rather than implementing
0110  * value_for_column overloads for each class.
0111  */
0112 #define SOA_DECLARE_DEFAULT(_ClassName_, _Expression_)                \
0113   template <class Object>                                             \
0114   _ClassName_::type value_for_column(Object const& x, _ClassName_*) { \
0115     return x._Expression_;                                            \
0116   }
0117 #endif