Getting the sum of a variadic size_t... argument in a variadic template in C++

Question

I am trying to create an n dimensional array template class (as a wrapper for std::array or c++ arrays) in c++ that allocates a single array block for the whole n dimensional array (To avoid the overhead of using n arrays with n indexes).

In doing this I want my template to be in the following format where sizes represent the size of each dimension.

template <class Type, size_t...  sizes>
class array_dimensional{
private:
    std::array<Type, /*here is the problem, how do 
       I get the product of all the sizes?*/> allocated_array;
...

My problem is that I am not sure how to get the product of all the sizes.

Is it possible to do this, and if so how?

Answer 1

In C++14, a constexpr function may be easier to read:

template<size_t... S>
constexpr size_t multiply() {
    size_t result = 1;
    for(auto s : { S... }) result *= s;
    return result;
}

In C++17, just use a fold expression: (... * sizes).