问题描述

关于多重继承和虚基类的问题

以下是源代码。
为什么输出的sphere的球心是（003）而不是（123）？

 #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;}

解决方案

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(ab)
这部分会被跳过，于是x和y便保留了默认构造时候的（00）。
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。

解决方案二：
#include <iostream>
using namespace std;
class A
{
public:
int a;
};

class A1: public A
{

};
class A2: public A1
{

};
class B1: public A
{

};
class B2: public B1......
答案就在这里：有关多重继承虚基类的问题