C++ tuple explode

explode.cc

#include <cstdio>
#include <tuple>
#include <utility>
#include <type_traits>

template<int ...I> struct index_tuple_type {
	template<int N> using append = index_tuple_type<I..., N>;
};
template<int N> struct make_index_impl {
	using type = typename make_index_impl<N-1>::type::template append<N-1>;
};
template<> struct make_index_impl<0> { using type = index_tuple_type<>; };
template<int N> using index_tuple = typename make_index_impl<N>::type;

template <typename I, typename ...Args>
struct func_traits;

template <typename R, int ...I, typename ...Args>
struct func_traits<R, index_tuple_type<I...>, Args...> {
	template <typename TT, typename FT>
	static inline R call(TT &&t, FT &&f) {
		return f(std::get<I>(std::forward<TT>(t))...);
	}
};

#define _explode_variant(_T)										\
template <typename FT, typename ...Args, typename R = typename std::result_of<FT(Args&&...)>::type>	\
inline R explode(_T t, FT &&f) {									\
	return func_traits<R, index_tuple<sizeof...(Args)>, Args...>					\
		::call(std::forward<_T>(t), std::forward<FT>(f));					\
}

_explode_variant(const std::tuple<Args...>&)
_explode_variant(      std::tuple<Args...>&)
_explode_variant(      std::tuple<Args...>&&)

#undef _explode_variant

//----------------------------------------------------------------------

void test1(int i, char c) {
	printf("%d %c\n", i, c);
}

struct S {
	int operator()(int, int) { return 5; }
	double operator()(int) { return 3.14; }
};

struct Object {
	Object() = default;
	Object(const Object&)  { printf("copying\n"); }
	Object(Object&&) { printf("moving\n"); }
};

void test5(Object a, Object b, Object c) {
	printf("in test5, moving\n");
}

void test6(Object a, Object b, Object c) {
	printf("in test6, copying\n");
}

int main() {
	std::tuple<int, char> t1{57, 'a'};
	explode(t1, test1);

	S s;
	// test correct return value deduction, based on implicit type conversion
	std::tuple<float> t2{5};
	printf("%f\n", explode(t2, s));

	std::tuple<int, int> t3{1, 7};
	printf("%d\n", explode(t3, s));

	// lambdas
	auto t4 = std::make_tuple("value: ", 10);
	explode(std::move(t4), [](const char *prompt, int value) { printf("%s%d\n", prompt, value); });

	// move!
	printf("starting test 5\n");
	auto t5 = std::make_tuple(Object{}, Object{}, Object{});
	printf("exploding, moving!\n");
	explode(std::move(t5), test5);

	// copy
	printf("starting test 6\n");
	auto t6 = std::make_tuple(Object{}, Object{}, Object{});
	printf("exploding, copying!\n");
	explode(t6, test6);
}