尝试 C++20 或者把矩乘改成：

for (int i = 1; i <= n; i++)
	for (int k = 1; k <= n; k++)
		if (a.a[i][k])
			for (int j = 1; j <= n; j++)
				c.a[i][j] += a.a[i][k] * b.a[k][j];