C++ Sum parameter pack into a template argument

Question

How can I sum all the std::size_t parameters passed through the template into one std::size_t value that will define the size of the array.

template<typename T, std::size_t... N>
class Example {
    std::array<T, N + ...> data; // std::array<T, sum of all N...>
};

Answer 1

You almost have it. You need to use a fold expression for this which is the same syntax, just surrounded with (). That gives you

template<typename T, std::size_t... N>
struct Example {
    std::array<T, (N + ...)> data; // std::array<T, sum of all N...>
};

And in this example you'll get an error that tells you the array member has the size of the sum of the parameters.

template<typename T, std::size_t... N>
struct Example {
    std::array<T, (N + ...)> data; // std::array<T, sum of all N...>
};

template <typename T>
struct get_type;

int main() 
{
    Example<int, 1, 2, 3> ex;
    get_type<decltype(ex.data)>{};
}

Error:

main.cpp: In function 'int main()':
main.cpp:27:33: error: invalid use of incomplete type 'struct get_type<std::array<int, 6> >'
   27 |     get_type<decltype(ex.data)>{};
      |                                 ^
main.cpp:22:8: note: declaration of 'struct get_type<std::array<int, 6> >'
   22 | struct get_type;                                             ^
      |        ^~~~~~~~                                              |
                              // here you see 1 + 2 + 3 == 6 --------+

live example