I have two tuples -- TypeSets modeled as tuples, and thus guaranteed to contain each type at maximum once in their parameter packs, to be exact -- (say A = std::tuple<T1, T2> and B = std::tuple<T2, T3>), and I wish to obtain a typedef that corresponds to a tuple of types in the intersection of A and B (in this case, tuple_intersect<A,B>::type = std::tuple<T2>). How do I go about this?
You can use the indices trick along with has_type (from here):
#include <tuple>
#include <type_traits>
// ##############################################
// from https://stackoverflow.com/a/25958302/678093
template <typename T, typename Tuple>
struct has_type;
template <typename T>
struct has_type<T, std::tuple<>> : std::false_type {};
template <typename T, typename U, typename... Ts>
struct has_type<T, std::tuple<U, Ts...>> : has_type<T, std::tuple<Ts...>> {};
template <typename T, typename... Ts>
struct has_type<T, std::tuple<T, Ts...>> : std::true_type {};
// ##############################################
template <typename S1, typename S2>
struct intersect
{
template <std::size_t... Indices>
static constexpr auto make_intersection(std::index_sequence<Indices...> ) {
return std::tuple_cat(
std::conditional_t<
has_type<
std::tuple_element_t<Indices, S1>,
S2
>::value,
std::tuple<std::tuple_element_t<Indices, S1>>,
std::tuple<>
>{}...);
}
using type = decltype(make_intersection(std::make_index_sequence<std::tuple_size<S1>::value>{}));
};
struct T1{};
struct T2{};
struct T3{};
using A = std::tuple<T1, T2>;
using B = std::tuple<T2, T3>;
int main()
{
static_assert(std::is_same<std::tuple<T2>, intersect<A, B>::type>::value, "");
}