upcast 做了什么操作

#include <stdio.h>

using namespace std;

class A{
private:
        int i;
public:
        A(int ii):i(ii){}
        void print(){ printf("A::print() %d\n",i); }
};

class B:public A{
private:
        int j;
public:
        B(int ii,int jj):A(ii),j(jj){}
        void print(){ printf("B::print() %d ",j);A::print(); }
};

int main()
{
        A a(1);
        B b(2,3);

        a.print();
        b.print();

        a = b;
        a.print();
        return 0;
}