boost::mpl::identityとboost::type

#include <iostream>

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

#include <boost/mpl/for_each.hpp>

#include <boost/mpl/vector.hpp>

#include <boost/type.hpp>
namespace pst {

  namespace mpl = boost::mpl;
  struct my_print_type {

    // mpl-style

    template <typename T>

    void operator()(mpl::identity<T>) const {

      std::cout << typeid(T).name() << std::endl;

    }

    // []boost[]-style

    template <typename T>

    void operator()(boost::type<T>) const {

      std::cout << typeid(T).name() << std::endl;

    }

  };
  void test_print_type() {

    // mpl::for_each needs value-initialize-able types, but...

    typedef mpl::vector<int&, long&, char*&> t_nonvalue_types;
    // so wrap a type with mpl::identity<type> which is value-initialize-able

    mpl::for_each< t_nonvalue_types, mpl::make_identity<mpl::_1> >(my_print_type());

    // you can use boost::type which behaves like a Metafunction returning itself

    mpl::for_each< t_nonvalue_types, boost::type<mpl::_1> >(my_print_type());

  }

}

C++ Template Metaprogramming Section 9.1.1