using tr2::direct_bases get nth element of result - c++

Question

struct T1 {};
struct T2: T1 {};
typedef tr2::direct_bases<T2>::type NEW_TYPE ;
should return my something like a touple to bases types. How can I get the nth element
of this __reflection_typelist<...>. I search for something like tuple_element for the reflection list.

Answer 1

You can use this simple metafunction to turn the typelist into an std::tuple:
#include <tr2/type_traits>
#include <tuple>
template<typename T>
struct dbc_as_tuple { };
template<typename... Ts>
struct dbc_as_tuple<std::tr2::__reflection_typelist<Ts...>>
{
typedef std::tuple<Ts...> type;
};
At this point, you could work with it as you would normally work with a tuple. For instance, this is how you could retrieve elements of the type list:
struct A {};
struct B {};
struct C : A, B {};
int main()
{
using namespace std;
using direct_base_classes = dbc_as_tuple<tr2::direct_bases<C>::type>::type;
using first = tuple_element<0, direct_base_classes>::type;
using second = tuple_element<1, direct_base_classes>::type;
static_assert(is_same<first, A>::value, "Error!"); // Will not fire
static_assert(is_same<second, B>::value, "Error!"); // Will not fire
}

Answer 2

Write your own?
template <typename R, unsigned int N> struct get;
template <typename T, typename ...Args, unsigned int N>
struct get<std::tr2::__reflection_typelist<T, Args...>, N>
{
typedef typename get<std::tr2::__reflection_typelist<Args...>, N - 1>::type type;
};
template <typename T, typename ...Args>
struct get<std::tr2::__reflection_typelist<T, Args...>, 0>
{
typedef T type;
};
Or even using first/next:
template <typename R, unsigned int N>
struct get
{
typedef typename get<typename R::next::type, N - 1>::type type;
};
template <typename R>
struct get<R, 0>
{
typedef typename R::first::type type;
};
At this point, I'd say the source code is the best documentation.