C++继承时的对象内存位置(一)

#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);
}
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();




    obj_b.display();
    return 0;
}

&obj_a.m_a=0x7ffc9098d234  &obj_a.m_b=0x7ffc9098d238  &obj_a=0x7ffc9098d234
&obj_b.m_a=0x7ffc9098d23c  &obj_b.m_b=0x7ffc9098d240  &obj_b.m_c=0x7ffc9098d244  &obj_b=0x7ffc9098d23c
m_a=99, m_b=10
m_a=84, m_b=23, m_c=95

obj_a 是基类对象，obj_b 是派生类对象。假设 obj_a 的起始地址为 0X1000，那么它的内存分布如下图所示：
假设 obj_b 的起始地址为 0X1100，那么它的内存分布如下图所示：
可以发现，基类的成员变量排在前面，派生类的排在后面。

为了让大家理解更加透彻，我们不妨再由 B 类派生出一个 C 类：

#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_d;
};
C::C(char a, int b, int c, int d): B(a, b, c), m_d(d){ }
void C::display(){
    printf("m_a=%d, m_b=%d, m_c=%d, m_d=%d\n", m_a, 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();

    C obj_c(84, 23, 95, 60);
    cout<<"&obj_c.m_a="<<&obj_c.m_a<<"  &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;
}

假设 obj_c 的起始地址为 0X1200，那么它的内存分布如下图所示：

成员变量按照派生的层级依次排列，新增成员变量始终在最后。