bla.cpp

#include <boost/mpl/vector/vector10.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/at.hpp>
#include <boost/type_traits/is_integral.hpp>
#include <boost/type_traits/is_reference.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/static_assert.hpp>

#include <iostream>

struct choice_a {
  void operator()() const { std::cout << "a" << std::endl; }
};

struct choice_b {
  void operator()() const { std::cout << "b" << std::endl; }
};

struct choice_c {
  void operator()() const { std::cout << "c" << std::endl; }
};

typedef boost::mpl::vector3<choice_a, choice_b, choice_c> choices;


// \returns 0 when T is integral, 1 when T is a reference and 2 for
// everything else. Note 
template <typename T>
struct make_choice 
  : boost::mpl::eval_if< typename boost::is_integral<T>::type
                         , boost::mpl::int_<0>
                         , boost::mpl::eval_if< typename boost::is_reference<T>::type
                                                , boost::mpl::int_<1>
                                                , boost::mpl::int_<2>
                                                > 
                         >
{};

struct X {};

int main()
{
  //print<make_choice<int&>::type> adsf;
  BOOST_STATIC_ASSERT((boost::is_same< make_choice<long>::type, boost::mpl::int_<0> >::value));

  // int& is not an integral
  BOOST_STATIC_ASSERT((boost::is_same< make_choice<int&>::type, boost::mpl::int_<1> >::value));
  // const int is
  BOOST_STATIC_ASSERT((boost::is_same< make_choice<const int>::type, boost::mpl::int_<0> >::value));

  // the rest
  BOOST_STATIC_ASSERT((boost::is_same< make_choice<double>::type, boost::mpl::int_<2> >::value));
  BOOST_STATIC_ASSERT((boost::is_same< make_choice<X>::type, boost::mpl::int_<2> >::value));

  // now actually select a functor and call it based on our choice
  boost::mpl::at< choices, make_choice<long>::type >::type()(); // a
  boost::mpl::at< choices, make_choice<X&>::type >::type()(); // b
  boost::mpl::at< choices, make_choice<double>::type >::type()(); // c

  return 0;
}