求输出

#include <iostream>

using namespace std;

class A
{
public:
    A()  {    cout<<"A"<<endl;    }
    ~A() {    cout<<"~A"<<endl;   }
};

class B:public A
{
public:
    B(A &a):_a(a)
    {
        cout<<"B"<<endl;
    }
    ~B()
    {
        cout<<"~B"<<endl;
    }
private:
    A _a;
};

int main(void)
{
    A a;
    B b(a);
    return 0;
}