How to use Folding Expression to initialize an array? - c++

Question

I come across with a weird problem in which MSVC doesn't let me to use fold expression to initialize an array in the following:
#include <iostream>
template <typename T, std::size_t ...dims>
class Matrix {
public:
void print()
{
std::cout << (... + dims) << '\n';
}
T matrix[(... + dims)]; // <-- error C2059: syntax error: '...'
};
int main()
{
Matrix<int, 3, 3, 3> m;
m.print();
Matrix<int, 3, 2, 1> n;
n.print();
return 0;
}
Here is the errors:
(10): error C2059: syntax error: '...' (11): note: see
reference to class template instantiation 'Matrix' being
compiled (10): error C2238: unexpected token(s) preceding ';'
I tried GCC and everything just worked perfectly fine!
Is there any workaround to use fold expression directly to initialize an array with MSVC?
Thank you so much guys!

Answer 1

This looks like a bug in the MS compiler. As with any bug of such kind, it's hard to tell what exactly goes wrong unless you know MS compiler internals.
A workaround is the introduction of an intermediate member variable:
template<typename T, std::size_t... dims>
class Matrix {
// ...
static constexpr std::size_t my_size = (... + dims);
T matrix[my_size];
};
or a static member function:
static constexpr std::size_t my_size() { return (... + dims); }
T matrix[my_size()];