c++ - Compare two sets of types for equality

Question

How can one check if two parameter packs are the same, ignoring their internal order?

So far I only have the frame (using std::tuple), but no functionality.

#include <tuple>
#include <type_traits>

template <typename, typename>
struct type_set_eq : std::false_type
{
};

template <typename ... Types1, typename ... Types2>
struct type_set_eq<std::tuple<Types1...>, std::tuple<Types2...>>
    : std::true_type
{
    // Should only be true_type if the sets of types are equal
};

int main() {
    using t1 = std::tuple<int, double>;
    using t2 = std::tuple<double, int>;
    using t3 = std::tuple<int, double, char>;

    static_assert(type_set_eq<t1, t1>::value, "err");
    static_assert(type_set_eq<t1, t2>::value, "err");
    static_assert(!type_set_eq<t1, t3>::value, "err");
}

Every type is not allowed to occur more than once in a set.

Answer 1

Boost.Hana solution:

constexpr auto t1 = hana::tuple_t<int, double>;
constexpr auto t2 = hana::tuple_t<double, int>;
constexpr auto t3 = hana::tuple_t<int, double, char>;

auto same = [](auto a, auto b)
{
    auto to_occurrences_map = [](auto t)
    {
        return hana::fold(t, hana::make_map(), [](auto m, auto x)
        { 
            if constexpr(!hana::contains(decltype(m){}, x)) 
            { 
                return hana::insert(m, hana::make_pair(x, 1)); 
            }
            else { return ++(m[x]); }
        });
    };

    return to_occurrences_map(a) == to_occurrences_map(b);
};

static_assert(same(t1, t1));
static_assert(same(t1, t2));
static_assert(!same(t1, t3));

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