template<size_t L, size_t M>
struct matrix {

private:
    double cells[L][M];

    template<size_t L1, size_t M1, size_t N1>
    friend matrix<L1, N1> operator*(matrix<L1, M1> const&, matrix<M1, N1> const&);
};

template<size_t L, size_t M, size_t N>
matrix<L, N> operator*(matrix<L, M> const& a, matrix<M, N> const& b) {
    matrix<L, N> tmp{};
    for(size_t i = 0; i < L; ++i) {
        for(size_t j = 0; j < N; ++j) {
            for(size_t r = 0; r < M; ++r) {
                tmp.cells[i][j] += a.cells[i][r]*b.cells[r][j];
            }
        }
    }
    return tmp;
}

int main() {
    matrix<2, 3> a{};
    matrix<3, 5> b{};
    auto c = a*b;
}

Return value
If the function succeeds, the return value is a handle to the module.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.