#include <boost/mpl/list.hpp>
    
    template <class List>
    struct foo{
        template <class T>
        static void* bar(T* obj, size_t size)
        {
            typedef typename boost::mpl::front<List>::type type;
            if (sizeof(type) == size) 
                return reinterpret_cast<type*>(obj);

            // Otherwise check the rest of the list
        return foo<typename List::next>::bar(obj, size);
      }
    };

    template <>
    struct foo<boost::mpl::list0<boost::mpl::na> >
    {
        template <class T>
        static void* bar(T*)
        {
            return NULL;
        }
    };

    #include <iostream>

    int main()
    {
        int n = 3;
        void *p = foo<boost::mpl::list<char, bool, double, long> >::bar(&n, 4);
        std::cout << p << std::endl;
    }

/usr/local/include/boost/mpl/list/aux_/front.hpp: In instantiation of 'boost::mpl::front_impl<boost::mpl::aux::list_tag>::apply<boost::mpl::l_end>':
/usr/local/include/boost/mpl/front.hpp:31:   instantiated from 'boost::mpl::front<boost::mpl::l_end>'
t.cpp:9:   instantiated from 'static void* foo<List>::bar(T*, size_t) [with T = int, List = boost::mpl::l_end]'
t.cpp:14:   instantiated from 'static void* foo<List>::bar(T*, size_t) [with T = int, List = boost::mpl::list1<long int>]'
t.cpp:14:   instantiated from 'static void* foo<List>::bar(T*, size_t) [with T = int, List = boost::mpl::list2<double, long int>]'
t.cpp:14:   instantiated from 'static void* foo<List>::bar(T*, size_t) [with T = int, List = boost::mpl::list3<bool, double, long int>]'
t.cpp:14:   instantiated from 'static void* foo<List>::bar(T*, size_t) [with T = int, List = boost::mpl::list<char, bool, double, long int, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>]'
t.cpp:33:   instantiated from here
Line 26: error: no type named 'item' in 'struct boost::mpl::l_end'
compilation terminated due to -Wfatal-errors.