求助 解释下 运行结果

#include<iostream.h>
class A
{
public:
    A(){cout<<"类A的构造函数"<<endl;}
    ~A(){cout<<"类A的析构函数"<<endl;}
};
class B:public A
{
public:
    B(int x=0):b(x){cout<<"类B的构造函数"<<endl;}
    ~B(){cout<<"类B的析构函数"<<endl;}
    int get(){return b;}
private:
    int b;
};
class C:public A
{
public:
    C(int y=0):c(y){cout<<"类C的构造函数"<<endl;}
    ~C(){cout<<"类C的析构函数"<<endl;}
    int get(){return c;}
private:
    int c;
};
class D:public B,public C
{
public:
    D(int x=0,int y=0):B(x),C(y){cout<<"类D的构造函数"<<endl;}
    ~D(){cout<<"类D的析构函数"<<endl;}
    D add(B &p,C &q)
    {
        D D1;
        D1.d=p.get()+q.get();
        return D1;
    }
    void put(){cout<<d<<endl;}
private:
    int d;
};
void main()
{
    B B1(1);
    C C1(2);
    D D2(3,4);
    D2=D2.add(B1,C1);
    D2.put();
}