matrix.cpp

#include <iostream>
#include <array>

template<typename T, size_t N, size_t M>
class matrix {
public:
    template<size_t ... n>
    constexpr explicit matrix(const T (&...list)[n]) : _data{}
    {
        auto pos = &_data[0];
        ((pos = std::copy(list, list + n, pos)), ...);
    }

    constexpr matrix() : _data{} {}

    constexpr const T& operator()(size_t i, size_t j) const
    {
        return _data[i * M + j];
    }

    constexpr T& operator()(size_t i, size_t j)
    {
        return _data[i * M + j];
    }

    [[nodiscard]] constexpr auto rows() const
    {
        return N;
    }

    [[nodiscard]] constexpr auto cols() const
    {
        return M;
    }

private:
    std::array<T, N * M> _data;
};

template<typename T, size_t... n>
matrix(const T (&...list)[n]) -> matrix<T, sizeof...(n), (n + ...) / sizeof...(n)>;

int main() {
    constexpr matrix mat2 {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9},
            {10, 11, 12}
    };

    return 0;
}