C++ 菱形继承

#include <iostream>

using namespace std;
class A
{
public:
    A(){cout << "A" << endl;}
    ~A(){cout << "~A" << endl;}
    int a;
};

class B:virtual public A
{
public:
    B(){cout << "B" << endl;}
    ~B(){cout << "~B" << endl;}
private:
    int b;
};

class C:virtual public A
{
public:
    C(){cout << "C" << endl;}
    ~C(){cout << "~C" << endl;}
    private:
    int c;
};

class D:public B,public C
{
public:
    D(){cout << "D" << endl;}
    ~D(){cout << "~D" << endl;}
private:
    int d;
};


int main()
{
    D d;
    d.B::a = 100;
    cout << d.a << endl;
    cout << d.B::a << endl;
    cout << d.C::a << endl;

    d.a = 10;
    cout << d.a << endl;
    cout << d.B::a << endl;
    cout << d.C::a << endl;

    return 0;
}