How to use Folding Expression to initialize an array?

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()];