So i have a list of pairs where the first member is a constant integer and the second is a type, is there anyway to unpack it into an array of the first member and a tuple of the second members?
struct MA {}
struct MB {}
struct MC {}
template <int I, class T> struct MyPair{};
How can I make a template meta function such that it has these two members:
MyStruct<1, MA, 2, MB, 3, MC> {
std::array<int, 3> arr = {1, 2, 3};
using Tuple = std::tuple<MA, MB, MC>;
};
Just define two helper metafunctions to get I and T:
template<class> struct GetFirst;
template<int I, class T> struct GetFirst<MyPair<I, T>> {
static constexpr int value = I;
};
template<class> struct GetSecond;
template<int I, class T> struct GetSecond<MyPair<I, T>> {
using type = T;
};
template<class... MyPairs>
struct MyStruct {
std::array<int, sizeof...(MyPairs)> arr{GetFirst<MyPairs>::value...};
using Tuple = std::tuple<typename GetSecond<MyPairs>::type...>;
};
//////////////////////////////////////////////////
using S = MyStruct<MyPair<1, MA>, MyPair<2, MB>, MyPair<3, MC>>;
static_assert(std::is_same_v<S::Tuple, std::tuple<MA, MB, MC>>);
assert((S{}.arr == std::array{1, 2, 3}));
You can't mix type and non-type parameters in a variadic template, so it's not possible to have
MyStruct<1, MA, 2, MB, 3, MC, ...>
without wrapping (int, Type) into a type.
As JeJo mentioned in a comment below, both metafunctions can be joined into a single one:
template<class> struct MyPairTraits;
template<int I, class T> struct MyPairTraits<MyPair<I, T>> {
static constexpr int i = I;
using Type = T;
};