C++11 课堂上的指针

我不知道为什么我的代码不起作用。问题如下图所示

class A
{
private:
    B* b;
public:
    friend class B;
};

class B
{
private:
    A *a;
public:
    friend class A;
    void h()
    {
        (a->b)->h();
    }
};

class B;

class A
{
private:
    B* b;
public:
    friend class B;
};

class B
{
private:
    A *a;
public:
    friend class A;
    void h()
    {
        (a->b)->h();
    }
};