typelist.cpp

#include <iostream>
#include <type_traits>

template <typename...>
struct typelist;

template <uint32_t n, typename Head, typename...Tail>
struct nthElement;

template <uint32_t n, typename Head, typename...Tail>
struct nthElement<n, typelist<Head, Tail...>> 
{
    static_assert(n <= sizeof...(Tail), "Index out of bounds!");
    using Type = typename nthElement<n - 1, typelist<Tail...>>::Type;
};

template <typename Head, typename...Tail>
struct nthElement<0, typelist<Head, Tail...>>
{
    using Type = Head;
};

template <uint32_t n, typename List>
using nthElementOf = typename nthElement<n, List>::Type;

template <typename List1, typename List2>
struct appendTypes;

template <typename...Types1, typename...Types2>
struct appendTypes<typelist<Types1...>, typelist<Types2...>>
{
    using Type = typelist<Types2..., Types1...>;
};

template <typename NewTypes, typename List>
using appendTypesTo = typename appendTypes<NewTypes, List>::Type;

template <typename List1, typename List2>
struct prependTypes;

template <typename...Types1, typename...Types2>
struct prependTypes<typelist<Types1...>, typelist<Types2...>>
{
    using Type = typelist<Types1..., Types2...>;
};

template <typename NewTypes, typename List>
using prependTypesTo = typename prependTypes<NewTypes, List>::Type;


int main(void)
{
    using List = typelist<int, bool, double>;
    static_assert(std::is_same_v<nthElementOf<0, List>, int>);
    static_assert(std::is_same_v<nthElementOf<1, List>, bool>);
    static_assert(std::is_same_v<nthElementOf<2, List>, double>);
    static_assert(std::is_same_v<prependTypesTo<typelist<double>, typelist<int, bool>>, typelist<double, int, bool>>);
    static_assert(std::is_same_v<appendTypesTo<typelist<double>, typelist<int, bool>>, typelist<int, bool, double>>);
}