派生类指针赋值给基类指针

#include <iostream>
using namespace std;
//基类A
class A{
public:
    A(int a);
public:
    void display();
protected:
    int m_a;
};
A::A(int a): m_a(a){ }
void A::display(){
    cout<<"Class A: m_a="<<m_a<<endl;

    cout<<"A:: &m_a=" <<&m_a<<endl;
}
//中间派生类B
class B: public A{
public:
    B(int a, int b);
public:
    void display();
protected:
    int m_b;
};
B::B(int a, int b): A(a), m_b(b){ }
void B::display(){
    cout<<"Class B: m_a="<<m_a<<", m_b="<<m_b<<endl;
    cout<<"B:: &m_a=" <<&m_a<<"  &m_b="<<&m_b<<endl;
}
//基类C
class C{
public:
    C(int c);
public:
    void display();
protected:
    int m_c;
};
C::C(int c): m_c(c){ }
void C::display(){
    cout<<"Class C: m_c="<<m_c<<endl;
    cout<<"C:: &m_c=" <<&m_c<<endl;
}
//最终派生类D
class D: public B, public C{
public:
    D(int a, int b, int c, int d);
public:
    void display();
private:
    int m_d;
};
D::D(int a, int b, int c, int d): B(a, b), C(c), m_d(d){ }
void D::display(){
    cout<<"Class D: m_a="<<m_a<<", m_b="<<m_b<<", m_c="<<m_c<<", m_d="<<m_d<<"\n"<<endl;
    cout<<"D:: &m_a=" <<&m_a<<"  &m_b="<<&m_b<<"  &m_c="<<&m_c<<"  &m_d="<<&m_d<<endl;
}
int main(){

    cout<<"--------初始化四个class----------"<<endl;
    A *pa = new A(1);
    cout<<"pa="<<pa<<endl;
    pa ->display();
    cout<<"-----------------------"<<endl;
    B *pb = new B(2, 20);
    cout<<"pb="<<pb<<endl;
    pb ->display();
    cout<<"-----------------------"<<endl;
    C *pc = new C(3);
    cout<<"pc="<<pc<<endl;
    pc ->display();
    cout<<"-----------------------"<<endl;

    D *pd = new D(4, 40, 400, 4000);
    cout<<"pd="<<pd<<endl;
    pd ->display();
    cout<<"-----------------------"<<endl;


    cout<<"-----------赋值之后------------"<<endl;

    pa = pd;
    pa -> display();
    cout<<"-----------------------"<<endl;
    pb = pd;
    pb -> display();
    cout<<"-----------------------"<<endl;
    pc = pd;
    pc -> display();
    cout<<"-----------------------"<<endl;
    cout<<"pa="<<pa<<endl;
    cout<<"pb="<<pb<<endl;
    cout<<"pc="<<pc<<endl;
    cout<<"pd="<<pd<<endl;
    return 0;
}

/home/ledi/.CLion2016.2/system/cmake/generated/neww-a0ddff1d/a0ddff1d/Debug/neww
--------初始化四个class----------
pa=0x559deed7e280
Class A: m_a=1
A:: &m_a=0x559deed7e280
-----------------------
pb=0x559deed7e2a0
Class B: m_a=2, m_b=20
B:: &m_a=0x559deed7e2a0  &m_b=0x559deed7e2a4
-----------------------
pc=0x559deed7e2c0
Class C: m_c=3
C:: &m_c=0x559deed7e2c0
-----------------------
pd=0x559deed7e2e0
Class D: m_a=4, m_b=40, m_c=400, m_d=4000

D:: &m_a=0x559deed7e2e0  &m_b=0x559deed7e2e4  &m_c=0x559deed7e2e8  &m_d=0x559deed7e2ec
-----------------------
-----------赋值之后------------
Class A: m_a=4
A:: &m_a=0x559deed7e2e0
-----------------------
Class B: m_a=4, m_b=40
B:: &m_a=0x559deed7e2e0  &m_b=0x559deed7e2e4
-----------------------
Class C: m_c=400
C:: &m_c=0x559deed7e2e8
-----------------------
pa=0x559deed7e2e0
pb=0x559deed7e2e0
pc=0x559deed7e2e8
pd=0x559deed7e2e0

Process finished with exit code 0

在进行指针赋值之前，我们可以看出

在A 中 pa=&m_a

在B 中pb=&m_a (b继承了Ａ)

在C 中pc=&m_c

在Ｄ中　pd＝&m_a　（D继承了Ｂ,C）

A	B	C	D
a	a		a
	b		b
		c	c
			d

pa = pd;
pb = pd;
pc = pd;

之前有说过　将派生类对象赋值给基类对象时，会舍弃派生类新增的成员

将派生类的指针赋值给基类的指针的时候，比如　pa = pd;新增加的对象（ｂｃｄ）在Ａ中没有对应位置被丢弃，

pa 指向D 的第一个元素a 的地址　＆m_a

A			D
a	--------------->		a
			b
			c
			d

pa 指向D 的第一个元素a 的地址　＆m_a

B			D
a	--------------->		a
b			b
			c
			d

pa 指向D 的第三个元素a 的地址　＆m_c

C			D
			a
			b
c	--------------->		c
			d

参考１

参考２