c++一道英文题，消除两个类构造的顺序依赖性

Define two classes, each with a static member, so that the construction of each static member involves a referece to the other. Where might such constructors appear in real code? How can these classes be modifiered to eliminate the order dependence in the constructors?

以上是原题，这话的中文意思懂，但不理解是个什么情况，网上也搜不着，
拜托大佬们指点下了。

#include <iostream>

using namespace std;

class B;
class A
{
public:
    A();
    void print(){printf("this A print");}
private:
    static B b;
};
class B
{
public:
    B();
    void print(){printf("this B print");}
private:
    static A a;
};

//对调上下顺序，修改AB构造函数先后顺序
A B::a = A();
B A::b = B();

A::A()
{
    printf("this A constructed: ");
    b.print();
    printf("\n");
}

B::B()
{
    printf("this B constructed: ");
    a.print();
    printf("\n");
}

int main()
{
    return 0;
}

不知是否你要的结果

class B;

class A
{
private:
    static B b;
public:
    A();
};
class B
{
private:
    static A a;
public:
    B()
    {
        a = A();
    }
};

A::A() { b = B(); }

int main()
{
    A a;
    return 0;
}