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...>
};
CodePudding user response：
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 --------