思想：
在C++的类继承中，
建立对象时，首先调用基类的构造函数，然后在调用下一个派生类的构造函数，依次类推；
析构对象时，其顺序正好与构造相反；
例子：
#include <iostream>
using namespace std; class Shape{
public:
void Draw() {cout<<"Base::Draw()"<<endl;}
void Erase() {cout<<"Base::Erase()"<<endl;}
Shape() {Draw();}
~Shape() {Erase();}
};
//-------------------------------------------------
class Polygon:public Shape{
public:
Polygon() {Draw();}
void Draw() {cout<<"Polygon::Draw()"<<endl;}
void Erase() {cout<<"Polygon Erase()"<<endl;}
~Polygon() {Erase();}
};
//--------------------------------------------------
class Rectangle:public Polygon{
public:
Rectangle() {Draw();}
void Draw() {cout<<"Rectangle::Draw()"<<endl;}
void Erase() {cout<<"Rectangle Erase()"<<endl;}
~Rectangle() {Erase();}
};
//--------------------------------------------------
class Square:public Rectangle{
public:
Square() {Draw();}
void Draw() {cout<<"Square::Draw()"<<endl;}
void Erase() {cout<<"Square Erase()"<<endl;}
~Square() {Erase();}
};
//--------------------------------------------------
int main(){
Polygon c;
Rectangle s;
Square t;
cout<<"------------------------------------------"<<endl;
return 0;
}