flattening (C++11)

gistfile1.cpp

template <typename... T> struct type {};

struct A{}; struct B{}; struct C{}; struct D{}; struct E{}; struct F{}; 
struct G{}; struct H{}; struct I{}; struct J{}; struct K{}; struct L{};
struct M{}; struct N{}; struct O{}; struct P{};

typedef type<A, B, type<C, D>, type<E, F, type<G, H>, I, J>, 
                        type<type<type<K, type<L>>, M>, N>, O, P> init;

typedef type<A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P> expected;

template <typename... T> struct cons;

template <typename H, typename... T> 
struct cons<H, type<T...>> {
    typedef ::type<H, T...> type;
};

template <typename... T> struct flatten;

template <> struct flatten<type<>> { typedef ::type<> type; };

template <typename... T, typename... U>
struct flatten<type<type<T...>, U...>> { 
    typedef typename flatten< ::type<T..., U...>>::type type; 
};

template <typename H, typename... T>
struct flatten<type<H, T...>> {
    typedef typename cons<H, typename flatten< ::type<T...>>::type>::type type;
};

#include <typeinfo>
#include <iostream>

int main() {
    std::cout << typeid(init).name() << std::endl;
    std::cout << typeid(flatten<init>::type).name() << std::endl;
    std::cout << typeid(expected).name() << std::endl;
}