Failing to accumulate compile time constants

Question

I am trying to create a variadic int argument vectors and perform + operators on them.

For example:

vec<1,2,3> v1;
vec<4,5>   v2;
auto res = v1+v2;
res.print(); // should print 5 7 3 vec<5,7,3> is the result.

And now I want to create struct vecSum, which will sum up the vectors given to it;

namespace ex {

    template<int... N>
    struct vec
    {
        static const int size = sizeof...(N);
        void print() {
            ((std::cout << N << " "), ...);
        }
    };

    template<int...N, int...M, int ...S>
    auto add(vec<N...>, vec<M...>, std::index_sequence<S...>) {
        constexpr int arr1[sizeof...(S)]{ N... };
        constexpr int arr2[sizeof...(S)]{ M... };
        return vec<(arr1[S] + arr2[S])...>{};
    }

    template<int...N, int...M>
    auto operator+(vec<N...> v1, vec<M...> v2) {
        constexpr size_t size = std::max(sizeof...(N), sizeof...(M));
        return add(v1, v2, std::make_index_sequence<size>{});
    }

    template<typename... Args>
    auto all(Args... args) { return (... + args); }

    template<typename... Ts>
    struct vecSum
    {
        static constexpr auto res = all(Ts);
    };
}

This test works fine:

ex::vec<1, 2, 3> v1;
ex::vec<4, 5> v2;
ex::vec<2> v3;
auto r = ex::all(v1,v2,v3);
r.print();

This prints 7 7 3;

What I want to achieve is:

vecSum<vec<1,3,4>, vec<3>, vec<3,4,5>> z;
z::res.print(); and I expect 7 7 9

Instead I have this error:

error C2275: 'Ts': illegal use of this type as an expression
error C2119: 'res': the type for 'auto' cannot be deduced from an empty initializer

Can someone please hint what's wrong here? I know I am trying to pass types parameter pack Ts as expression, is there a workaround or fix for this?

Answer 1

I see some little problems in your code...

---

1. expand Ts

In vecSum you define res as follows

static constexpr auto res = all(Ts);

You have to expand Ts as follows

// ...............................VVVVV
static constexpr auto res = all(Ts{}...);

---

2. std::index_sequence contains std::size_t, not int, values

Your add() is defined as follows

template<int...N, int...M, int ...S>
auto add(vec<N...>, vec<M...>, std::index_sequence<S...>) 

But the S... values are std::size_t

// ........................VVVVVVVVVVV
template<int...N, int...M, std::size_t ...S>
auto add(vec<N...>, vec<M...>, std::index_sequence<S...>) 

or template deduction doesn't works.

---

3. constexpr functions

If vecSum has to be constexpr, you have to define all necessary functions (add(), all(), operator+()) as constexpr.

---

4. print() const

The method vec::print() is better const, given that you want print a constexpr vecSum::res value.

---

The following is a full compiling corrected example

#include <utility>
#include <iostream>
#include <type_traits>

namespace ex {

    template<int... N>
    struct vec
    {
        static const int size = sizeof...(N);
        void print() const {
            ((std::cout << N << " "), ...);
        }
    };

    template<int...N, int...M, std::size_t ...S>
    constexpr auto add(vec<N...>, vec<M...>, std::index_sequence<S...>) {
        constexpr int arr1[sizeof...(S)]{ N... };
        constexpr int arr2[sizeof...(S)]{ M... };
        return vec<(arr1[S] + arr2[S])...>{};
    }

    template<int...N, int...M>
    constexpr auto operator+(vec<N...> v1, vec<M...> v2) {
        constexpr size_t size = std::max(sizeof...(N), sizeof...(M));
        return add(v1, v2, std::make_index_sequence<size>{});
    }

    template<typename... Args>
    constexpr auto all(Args... args) { return (... + args); }

    template<typename... Ts>
    struct vecSum
    {
        static constexpr auto res = all(Ts{}...);
    };
}

int main ()
 { 

   ex::vecSum<ex::vec<1,3,4>, ex::vec<3>, ex::vec<3,4,5>> z;
   z.res.print();
 }