父类的数据发生改变后 子类的数据要怎样做 才可以跟着一起继承新的数据

父类对象的数据发生改变后
子类的数据要怎样做 才可以跟着一起继承新的数据
这是举例

#include <iostream>
using namespace std;
class A
{
private:
    int A_num;

public:
    A(int n)
    {
        A_num = n;
    }
    void setnum(int n)
    {
        A_num = n;
    }
    int getnum()
    {
        return A_num;
    }
};
class B : public A
{
private:
    int B_num;

public:
    B(int n) : A(n)
    {
        B_num = A::getnum();
    }
    void show()
    {
        cout << B_num << endl;
    }
};
int main()
{
    B b(1);
    b.show();
    b.setnum(2);
    b.show();
}

可以看到在执行b.setnum(2)后 b中的B-num没有变成2
该怎样改才能输出2

运行结果：

代码修改如下：

#include <iostream>
using namespace std;
//点类Point
class Point
{
private:
    double x;
    double y;

public:
    Point(double xv = 0, double yv = 0); /*构造函数*/
    Point(const Point &p);               /*拷贝构造*/
    ~Point();                            /*析构函数*/
    void setX(double xv);                /*设置X坐标*/
    void setY(double yv);                /*设置Y坐标*/
    double getX() const;                 /*获取X坐标*/
    double getY() const;                 /*获取Y坐标*/
    virtual void show() const;           /*显示*/
};
Point::Point(const double xv, const double yv)
{ /*构造函数*/
    x = xv;
    y = yv;
    cout << "Point Constructor run" << endl;
}
Point::Point(const Point &p)
{ /*拷贝构造*/
    x = p.x;
    y = p.y;
    cout << "Point CopyConstructor run" << endl;
}
Point::~Point()
{ /*析构函数*/
    cout << "Point Destructor run" << endl;
}
void Point::setX(double xv)
{ /*设置X坐标*/
    x = xv;
}
void Point::setY(double yv)
{ /*设置Y坐标*/
    y = yv;
}
double Point::getX() const
{ /*获取X坐标*/
    return x;
}
double Point::getY() const
{ /*获取Y坐标*/
    return y;
}
void Point::show() const
{ /*显示*/
    cout << "Point(X=" << x << ",Y=" << y << ")";
}
//平面图形类Plane
class Plane
{
public:
    virtual double length() const = 0; /*周长*/
    virtual double area() const = 0;   /*面积*/
};
//立体图形类Solid
class Solid
{
public:
    virtual double volume() const = 0; /*体积*/
    virtual double s_Area() const = 0; /*表面积*/
};

class Circle : public Plane,public Point
{
private:
    double radius;

protected:
    static double PI;

public:
    Circle(double x = 0, double y = 0, double r = 0);
    Circle(const Circle &);
    ~Circle();
    void setR(double);
    double getR() const;
    void show() const;
    double area() const;
    double length() const;
};
double Circle::PI = 3.14159;
Circle::Circle(double x, double y, double r) 
{
    //X = Point::getX();
    //Y = Point::getY();
    setX(x);
    setY(y);
    radius=r;
    cout << "Circle Constructor run" << endl;
}
Circle::Circle(const Circle &c) 
{
    setX(c.getX());
    setY(c.getY());
    radius = c.radius;
    PI = c.PI;
    cout << "Circle CopyConstructor run" << endl;
}
Circle::~Circle()
{
    cout << "Circle Destructor run" << endl;
}
void Circle::setR(double r)
{
    radius = r;
}
double Circle::getR() const
{
    return radius;
}
void Circle::show() const
{
    cout << "Circle(";
    cout <<"("<<getX()<<","<<getY()<<")";
    cout << ",Radius=" << radius << ")";
}
double Circle::area() const
{
    return PI * radius * radius;
}
double Circle::length() const
{
    return 2 * PI * radius;
}
class Sphere : public Solid,public Circle
{
protected:
    static double PI;
/*
private:
    int X;
    int Y;
    double radius;*/

public:
    Sphere(double x = 0, double y = 0, double r = 0);
    Sphere(Sphere &);
    ~Sphere();
    double volume() const;
    double s_Area() const;
    void show() const;
};
double Sphere::PI = 3.14159;
Sphere::Sphere(double x, double y, double r) 
{
    //X = Point::getX();
    //Y = Point::getY();
    //radius = getR();
    setX(x);
    setY(y);
    setR(r);
    cout << "Sphere Constructor run" << endl;
}
Sphere::Sphere(Sphere &s)
{
    setX(s.getX());
    setY(s.getY());
    setR(s.getR());
    PI = s.PI;
    cout << "Sphere CopyConstructor run" << endl;
}
Sphere::~Sphere()
{
    cout << "Sphere Destructor run" << endl;
}
double Sphere::volume() const
{
    return 4 * PI * getR() * getR() * getR() / 3;
}
double Sphere::s_Area() const
{
    return 4 * PI * getR() * getR();
}
void Sphere::show() const
{
    cout << "Sphere(Circle(Point(X=" << getX() << ",Y=" << getX() << "),Radius=" << getR() << "))";
}
void show(Point *p)
{ /*点基类的显示函数*/
    p->show();
}
void length(Plane *p)
{ /*平面图形的周长函数*/
    cout << "Length=" << p->length() << endl;
}
void area(Plane &p)
{ /*平面图形的面积函数*/
    cout << "Area=" << p.area() << endl;
}

void volumn(Solid *s)
{ /*立体图形的体积函数*/
    cout << "Volumn=" << s->volume() << endl;
}
void s_Area(Solid &s)
{ /*立体图形的表面积函数*/
    cout << "S_Area=" << s.s_Area() << endl;
}
//主函数
int main(void)
{
    double r;
    cin >> r;
    Sphere s1(1, 2, 3), s2(s1);
    show(&s1);
    cout << endl;
    area(s1);
    length(&s1);
    s_Area(s1);
    volumn(&s1);
    s2.setR(r);
    show(&s2);
    cout << endl;
    area(s2);
    length(&s2);
    s_Area(s2);
    volumn(&s2);
    return 0;
}

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