Multiplication of Template Arguments

Question

I have the following problem:

template< size_t... N_i >
class A
{
  // ...
  std::array< float, /* product of the unpacked elements of N_i */ > arr;
};

As you can see above, I try to declare an std::array named arr as a member of a class A. Here, I want arr to have the size product of the unpacked elements of N_i, e.g., in case of A<2,3,4>, "arr" should have the size 2*3*4=24.
Does anyone know how this can be implemented?

Answer 1

In C++17:

std::array < float, (... * N_i)> arr;

In C++14:

// outside the class
template<size_t... Ns>
constexpr size_t product(){
    size_t p = 1;
    for(auto n : { Ns... }) p *= n;
    return p;
}

// then
std::array< float, product<N_i...>()> arr;

In C++11:

template<size_t... N_i>
struct product { 
    static const size_t value = 1; 
};

template<size_t N_1, size_t... N_i>
struct product<N_1, N_i...> {
    static const size_t value = N_1 * product<N_i...>::value;
};

std::array< float, product<N_i...>::value> arr;

Alternatively, you can use a recursive constexpr function.