compile time typelists and fold metafunction

gistfile1.cpp

#include <stdio.h>

/* an empty type to mark the empty list */
struct null_typelist {};

/* list representation */
template <class H, class T>
struct typelist {
    typedef typelist<H, T> type;
    typedef H head;
    typedef T tail;
};

/* constructor for typelists of length 0-4 */
template <
    class T1=null_typelist, class T2=null_typelist,
    class T3=null_typelist, class T4=null_typelist
    >
struct make_typelist { 
private:
    // tail
    typedef typename make_typelist<T2,T3,T4>::type tail_type;
public:
    // cons [H|T]
    typedef typelist<T1, tail_type> type;
};

/* specialization to stop recursion */	
template <>
struct make_typelist<> { 
    typedef null_typelist type;
};

/* simple helper class that creates unique types from integer constants */
template <int i>
struct int2type { 
    typedef int2type<i> type;
    static const int value = i;
};

/* some int-typedefs */
typedef int2type<0> zero;
typedef int2type<1> one;
typedef int2type<2> two;
typedef int2type<3> three;
typedef int2type<4> four;

/* has inner "apply" metafunction that "returns" the result 
   of adding two int2types */
struct adder { 
    template <class Elem, class Acc>
    struct apply { 
        typedef int2type<Elem::value + Acc::value> type;
    };
};

/* foldl implementation */
template <class L, class Fun, class Accin> 
struct foldl;

/* primary template : foldl(List, Fun, Accin */
template <class L, class Fun, class Accin>
struct foldl { 
    /* call Fun(H, AccIn) -> NewAcc */
    typedef typename Fun::template apply<typename L::head, Accin>::type NewAcc;
    /* foldl(Tail, Fun, NewAcc) */
    typedef typename foldl<typename L::tail, Fun, NewAcc>::type type;
};

/* specialization to stop fold */
template <class Fun, class Accin>
struct foldl<null_typelist, Fun, Accin> { 
    typedef typename Accin::type type;
};

int main(int argc, char **argv) { 
    typedef make_typelist<one, two, three, four>::type mylist;
    typedef foldl<mylist, adder, zero>::type folder;
    printf("%d\n", folder::value);
    return 0;
}