C++继承时的对象内存位置(一)有成员变量遮蔽时的内存分布

#include <cstdio>
#include<iostream>
using namespace std;
//基类A
class A{
public:
    A(int a, int b);
public:
    void display();

public:
    int m_a;
    int m_b;
};
A::A(int a, int b): m_a(a), m_b(b){}
void A::display(){
    printf("m_a=%d, m_b=%d\n", m_a, m_b);
}
//派生类B
class B: public A{
public:
    B(int a, int b, int c);
    void display();

public:
    int m_c;
};
B::B(int a, int b, int c): A(a, b), m_c(c){ }
void B::display(){
    printf("m_a=%d, m_b=%d, m_c=%d\n", m_a, m_b, m_c);
}

//声明并定义派生类C
class C: public B{
public:
    C(char a, int b, int c, int d);
public:
    void display();
public:
    int m_b;  //遮蔽A类的成员变量

    int m_c;  //遮蔽B类的成员变量
    int m_d;  //新增成员变量
};
C::C(char a, int b, int c, int d): B(a, b, c), m_b(b), m_c(c), m_d(d){ }
void C::display(){
    printf("A::m_a=%d, A::m_b=%d, B::m_c=%d\n", m_a, A::m_b, B::m_c);
    printf("C::m_b=%d, C::m_c=%d, C::m_d=%d\n", m_b, m_c, m_d);
}

int main(){
    A obj_a(99, 10);

    B obj_b(84, 23, 95);
    cout<<"&obj_a.m_a="<<&obj_a.m_a<<"  &obj_a.m_b="<<&obj_a.m_b<<"  &obj_a="<<&obj_a<<endl;

    cout<<"&obj_b.m_a="<<&obj_b.m_a<<"  &obj_b.m_b="<<&obj_b.m_b<<"  &obj_b.m_c="<<&obj_b.m_c<<"  &obj_b="<<&obj_b<<endl;
    obj_a.display();
//创建C类对象obj_c
    C obj_c(84, 23, 95, 60);
    obj_c.display();

    cout<<"&obj_c.m_a="<<&obj_c.m_a<< "  &obj_c.A::m_b="<<&obj_c.A::m_b<< "  &obj_c.B::m_c="<<&obj_c.B::m_c<<"  &obj_c.m_b="<<&obj_c.m_b<<"  &obj_c.m_c="<<&obj_c.m_c<<"  &obj_c.m_d="<<&obj_c.m_d<<"  &obj_c="<<&obj_c<<endl;
    obj_c.display();




    obj_b.display();
    return 0;
}

&obj_a.m_a=0x7fffc1edf9dc  &obj_a.m_b=0x7fffc1edf9e0  &obj_a=0x7fffc1edf9dc
&obj_b.m_a=0x7fffc1edf9e4  &obj_b.m_b=0x7fffc1edf9e8  &obj_b.m_c=0x7fffc1edf9ec  &obj_b=0x7fffc1edf9e4
m_a=99, m_b=10
A::m_a=84, A::m_b=23, B::m_c=95
C::m_b=23, C::m_c=95, C::m_d=60
&obj_c.m_a=0x7fffc1edf9f0  &obj_c.A::m_b=0x7fffc1edf9f4  &obj_c.B::m_c=0x7fffc1edf9f8  &obj_c.m_b=0x7fffc1edf9fc  &obj_c.m_c=0x7fffc1edfa00  &obj_c.m_d=0x7fffc1edfa04  &obj_c=0x7fffc1edf9f0
A::m_a=84, A::m_b=23, B::m_c=95
C::m_b=23, C::m_c=95, C::m_d=60
m_a=84, m_b=23, m_c=95

假设 obj_c 的起始地址为 0X1300，那么它的内存分布如下图所示

当基类 A、B 的成员变量被遮蔽时，仍然会留在派生类对象 obj_c 的内存中，C 类新增的成员变量始终排在基类 A、B 的后面。

总结：在派生类的对象模型中，会包含所有基类的成员变量。这种设计方案的优点是访问效率高，能够在派生类对象中直接访问基类变量，无需经过好几层间接计算。