复制构造函数初始化列表中的引用成员会导致指针悬空

问：

我有一个带有引用成员的 A 类。我编写了一个在初始化列表中初始化的复制构造函数。但结果似乎很奇怪，打印出来的值不应该是 100 吗？我的程序什么时候修改了 和 的值？numnuma.numaa.num

#include <iostream>
using namespace std;

class A{
public:
    int& num;
    A(int n):num(n){}
    A(const A& obj):num(obj.num){}

    void print(){
        cout << num << endl;
    }
};

int main(){

    A a(100);
    A aa = a;
    a.print();  //Expected to be 100, but it isn't
    aa.print(); //Also expected to be 100, but it isn't

    //The address of a.num and aa.num are the same, so both of them are referencing to the same place. But the question is why the value isn't 100 but a strange value
    cout << &(a.num) << " " << &(aa.num) <<endl;
}

输出为：

-1077613148
-1077613148
0xbfc4ed94 0xbfc4ed94