add two variadic template list of integers

Question

Given this type

template<std::size_t N, int ...content>
struct list {
    inline int reduce() {
        int result = 0;
        constexpr int arr[N] = { content... };
        for(std::size_t k = 0; k < N; ++k) {
            result += arr[k];
        }

        return result;
    }
};

I'd like to implement a function add, which returns a new list containing the element-by-element addition of the two input lists. In other words (pseudo-code):

add([a0, a1, a2], [b0, b1]) -> [a0 + b0, a1 + b2, a2]

Problem:

• I don't even know how to declare the return type of such a function
• and i don't know if it's possible

Answer 1

Here is how I would do it:

#include <iostream>
#include <utility>

template<std::size_t N, int ...content>
struct list {
    inline int reduce() {
        int result = 0;
        constexpr int arr[N] = { content... };
        for(std::size_t k = 0; k < N; ++k) {
            result += arr[k];
        }

        return result;
    }
};


template <std::size_t I, int ...A>
constexpr int list_at(list<sizeof...(A),A...>)
{
    if constexpr (I < sizeof...(A))
    {
        constexpr int arr[] {A...};
        return arr[I];
    }
    else
    {
        return 0;
    }
}

template <int ...A, int ...B, std::size_t ...I>
constexpr auto list_sum_low(list<sizeof...(A),A...>,
                            list<sizeof...(B),B...>,
                            std::index_sequence<I...>)
{
    return list<sizeof...(I), (list_at<I>(list<sizeof...(A),A...>{}) +
                               list_at<I>(list<sizeof...(B),B...>{}))...>{};
}

template <int ...A, int ...B>
constexpr auto list_sum(list<sizeof...(A),A...>, list<sizeof...(B),B...>)
{
    constexpr int a = sizeof...(A), b = sizeof...(B);
    return list_sum_low(list<a,A...>{}, list<b,B...>{},
                        std::make_index_sequence<(a > b ? a : b)>{});
}


template <int ...A>
void print_list(list<sizeof...(A),A...>)
{
    (void(std::cout << ' ' << A) , ...);
}


int main()
{
    constexpr auto x = list_sum(list<4, 1,2,3,4>{}, list<2, 10,20>{});
    print_list(x);
}

Also, note that there is no need have size_t N template parameter for class list. Parameter packs know their own size.