boost::mpl::inherit_linearlyとLoki::Gen***Hierarchyの関係

• Modern C++ Design Section 3.13をboost::mplで書ける
• mpl::inherit_"linearly"は同書の"LinearHierarchy"を示しているのではないことに注意

#include <[]boost[]/mpl/inherit.hpp>

#include <[]boost[]/mpl/inherit_linearly.hpp>

#include <[]boost[]/mpl/assert.hpp>

#include <[]boost[]/mpl/vector.hpp>

namespace pst {

  namespace date_2005_02_13 {

    // Modern []C++[] Design Section 3.13

    namespace mpl = []boost[]::mpl;

    using mpl::_1; using mpl::_2;
    typedef double window;

    typedef int button;

    typedef char scrollbar;

    typedef mpl::vector< window, button, scrollbar > t_widgets;
    // scattered version

    template< typename T >

    []class[] event_handler {

    public:

      virtual void on_event(T& obj, int event_id) = 0;

      virtual ~event_handler() { }

      typedef event_handler type; // you []can[] omit

    };
    template< typename Sequence >

    struct widget_event_handler_scattered

      : mpl::inherit_linearly< Sequence, mpl::inherit2< _1, event_handler<_2> > >::type { };
    []class[] the_handler_scattered : public widget_event_handler_scattered<t_widgets> {

    public:

      virtual void on_event(window& w, int event_id) { }

      virtual void on_event(button& b, int event_id) { }

      virtual void on_event(scrollbar& s, int event_id) { }

    };
    // better version

    template< typename T, typename Base >

    []class[] event_handler_linearly : public Base {

    public:

      virtual void on_event(T& obj, int event_id) = 0;

      virtual ~event_handler_linearly() { }

      typedef event_handler_linearly type; // you []can[] omit

    };
    // easier than inherit2, which represents "scatter"!

    template< typename Sequence >

    struct widget_event_handler_linearly

      : mpl::inherit_linearly< Sequence, event_handler_linearly<_2, _1> >::type { };

    

    []class[] the_handler_linearly : public widget_event_handler_linearly<t_widgets> {

    public:

      virtual void on_event(window& w, int event_id) { }

      virtual void on_event(button& b, int event_id) { }

      virtual void on_event(scrollbar& s, int event_id) { }

    };
    // 4bytes * 3  > 4bytes * 1

    BOOST_MPL_ASSERT_RELATION( sizeof(the_handler_scattered), >, sizeof(the_handler_linearly) );
  } // date_2005_02_13

} // pst