如何解决“声明朋友时必须使用的类”错误？

class two;
class one
{
    int a;
    public:
        one()
        {
            a = 8;
        }
    friend two;
};

class two
{
    public:
        two() { }
        two(one i)
        {
            cout << i.a;
        }
};

int main()
{
    one o;
    two t(o);
    getch();
}
class two;
class one
{
    int a;
    public: