Failing to accumulate compile time constants - c++

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