#include <iostream>
using namespace std;

class A
{
private:
    int a;
public:
    A(){a = 0;}
    A(int value){a = value;}
    void display(){ cout<<a<<endl;}
};

class B
{
private:
    A a;
public:
    B(const A & aa){ a = aa;}
    A & test(){return a;}
};

int main()
{
    A a(6);
    a.display();
    B b(a);
    A & aa = b.test();  // pay attention here
                                   // 请你将这个 & 去掉，你看看运行的结果是什么
    aa.display();
    return 0;
}