virtual base classes

virtual base classes用来实现菱形继承解决多个重复subobject的问题

//: C09:VirtualBase.cpp
// Shows a shared subobject via a virtual base.
#include <iostream>
using namespace std;

class Top
{
protected:
    int x;
public:
    Top(int n)
    {
        x = n;
    }
    virtual ~Top() {}
    friend ostream&
    operator<<(ostream& os, const Top& t)
    {
        return os << t.x;
    }
};

class Left : virtual public Top
{
protected:
    int y;
public:
    Left(int m, int n) : Top(m)
    {
        y = n;
    }
};

class Right : virtual public Top
{
protected:
    int z;
public:
    Right(int m, int n) : Top(m)
    {
        z = n;
    }
};

class Bottom : public Left, public Right
{
    int w;
public:
    Bottom(int i, int j, int k, int m)
        : Top(i), Left(0, j), Right(0, k)
    {
        w = m;
    }

    friend ostream&
    operator<<(ostream& os, const Bottom& b)
    {
        return os << b.x << ',' << b.y << ',' << b.z
               << ',' << b.w;
    }
};

int main()
{
    Bottom b(1, 2, 3, 4);
    cout << sizeof b << endl;
    cout << b << endl;
    cout << static_cast<void*>(&b) << endl;
    Top* p = static_cast<Top*>(&b);
    cout << *p << endl;
    cout << static_cast<void*>(p) << endl;
    cout << dynamic_cast<void*>(p) << endl;

    return 0;
} ///:~

输出结果：

28
1,2,3,4
0043FABC
1
0043FAD0
0043FABC

对象大小与实现有关，28 = 4 * 4 + 2 * 4 + 1 * 4（大概是4个int，2个ptr to virtual base，1 个vptr）

注意Bottom里初始化Top的方式，Left、Right给出的对应参数会被编译器巧妙地忽略