构造函数和析构函数问题

#include <iostream>using namespace std;class A{public:    A() { cout<<"A构造了"<<endl; } void Reset();    void SetData(int newx);    void ShowData(); ~A() { cout<<"A析构了"<<endl; }private:    int x;};class B{public: B() {  cout<<"B构造了"<<endl; }    void Reset();    void SetData(A newy1,A newy2);    void ShowData(); ~B() { cout<<"B析构了"<<endl; }private:    A y1,y2;};void A :: Reset(){    x=0;}void A :: SetData(int newx){    x=newx;}void A ::ShowData(){    cout<<"A类：x="<<x<<endl;}void B ::Reset(){    y1.Reset(); y2.Reset();}void B ::SetData(A newy1,A newy2){    y1=newy1; y2=newy2;}void B ::ShowData(){    cout<<"B类嵌套";    y1.ShowData(); cout<<"B类嵌套"; y2.ShowData();}void main(){    A a;    B b; a.SetData(100); b.SetData(a,a);    a.ShowData();    b.ShowData();    a.Reset();    b.Reset();    a.ShowData();    b.ShowData();} 结果中为什么会出现那么多个析构？谁能解释下每句构造和析构是怎么来的吗？