关于多重继承和虚基类的问题

Question

以下是源代码。
为什么输出的sphere的球心是（0,0,3）而不是（1,2,3）？

 #include <iostream>
using namespace std;

class Point2D
{
protected:
    double x;
    double y;
public:
    Point2D() { x = 0; y = 0; }
    Point2D(double a, double b)
    {
        x = a;
        y = b;
    }
    virtual void show()
    {
        cout << "The 2D point is:(" << x << ',' << y << ')'<<endl;
    }
};

class Circle:virtual public Point2D
{
protected:
    double radius;
public:
    Circle() { radius = 0; }
    Circle(double a, double b, double c) :Point2D(a, b)
    {
        radius = c;
    }
    virtual void show() 
    { 
        cout << "The center of circle is:(" << x << ',' << y << ')' << endl;
        cout << "The radius of circle is:" << radius << endl;
    }
};

class Point3D :virtual public Point2D
{
protected:
    double z;
public:
    Point3D(double a, double b, double c) :Point2D(a, b)
    {
        z = c;
    }
    virtual void show()
    {
        cout << "The 3D point is:(" << x << ',' << y << ',' << z << ')' << endl;
    }
};

class Sphere :public Circle, public Point3D
{
public:
    Sphere(double a, double b, double c, double r) :Point3D(a, b, c)
    {
        radius = r;
    }
    virtual void show()
    {
        cout << "The center of sphere is:(" << x << ',' << y << ','<<z<<')' << endl;
        cout << "The radius of sphere is:" << radius << endl;
    }
};

int main()
{
    Point2D p1(1, 1);
    Circle c1(1, 2, 2);
    Point3D p2(1.5, 2.5, 3.5);
    Sphere s1(1, 2, 3, 4);
    //p1 = c1;
    Point2D* pointer = &p1;
    pointer->show();
    pointer = &c1;
    pointer->show();
    pointer=&p2;
    pointer->show();
    pointer = &s1;
    pointer->show();
    system("pause");
    return 0;
}

Answer 1

这里Point2D是Circle和Point3D的父类，会早于它们初始化，由于你没有显式的指定构造函数参数，因此这里Point2D直接默认构造成为（0，0），此时Point2D的构造已经完成，因此Point3D的构造函数中
Point3D(double a, double b, double c) : Point2D(a,b)
这部分会被跳过，于是x和y便保留了默认构造时候的（0,0）。
你可以显式的初始化所有基类

class Sphere :public Circle, public Point3D
{
public:
    Sphere(double a, double b, double c, double r)
        : Point2D(a, b)
        , Circle(a, b, r) 
        , Point3D(a, b, c)
    {
        radius = r;
    }
    virtual void show()
    {
        cout << "The center of sphere is:(" << x << ',' << y << ',' << z << ')' << endl;
        cout << "The radius of sphere is:" << radius << endl;
    }
};

也可以在Circle和Point3D的构造函数中，基类的初始化隐去，与radius或者z一起显式的赋值（只不过这个时候构造已经完成了，你只是在修改值而不是初始化），效果应该是一样的。