C++派生类对象内存布局

最新推荐文章于 2021-02-16 16:13:24 发布

minico

最新推荐文章于 2021-02-16 16:13:24 发布

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分类专栏：编程语言 Windows平台编程文章标签： c++ c class

本文链接：https://blog.youkuaiyun.com/minico/article/details/4974327

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问题提出：

类C继承自基类A和基类B，现在有一个类C的对象，将其地址分别转换为基类A的指针和基类B的指针，那么这两个指针的值相等吗？

答案：

不相等。

解释：

由上图可见，派生类对象在内存中是把各个基类相关信息放在一起，然后存放自己的成员。类C对象地址转换为基类A和基类B的指针后它们的值如上图所示。

下面是一个验证程序：

// cpp_learning.cpp : Defines the entry point for the console application.
//

#include <iostream>
using namespace std;
class A
{
public:
    int a;
    int b;
    virtual void message()
    {
        cout << "BASE A" <<endl;
    }
};

class B
{
public:
    int c;
    int d;
    virtual void message()
    {
        cout << "BASE B" <<endl;
    }

};

class C:public A,public B
{
public:
    int e;
    virtual void message()
    {
        cout << "DERIVE C" <<endl;
    }
};

void castA(A a)
{
    cout << "A.a=" << a.a <<endl;
    cout << "A.b=" << a.b <<endl;
    a.message();

}

void castB(B b)
{
    cout << "B.c=" << b.c <<endl;
    cout << "B.d=" << b.d <<endl;
    b.message();
}

void castAref(A &a)
{
    cout << "A.a=" << a.a <<endl;
    cout << "A.b=" << a.b <<endl;
    a.message();

}

void castBref(B &b)
{
    cout << "B.c=" << b.c <<endl;
    cout << "B.d=" << b.d <<endl;
    b.message();
}

int main()
{
    C c;
    c.a = 1;
    c.b = 2;
    c.c = 3;
    c.d = 4;
    c.e = 5;
    C c1;
    A *pa = (A*)&c;
    B *pb = (B*)&c;
    A &a = c;
    B &b = c;

if ((int)pa == (int)pb)
    {
        cout << "pa == pb"<< endl;
    }
    else
    {
        cout << "pa != pb" <<endl;
    }

if ((int)&a == (int)&b)
    {
        cout << "&a == &b"<< endl;
    }
    else
    {
        cout << "&a != &b" <<endl;
    }

if ((int)&a == (int)pa)
    {
        cout << "&a == pa"<< endl;
    }
    else
    {
        cout << "&a != pa" <<endl;
    }

if ((int)&b == (int)pb)
    {
        cout << "&b == pb"<< endl;
    }
    else
    {
        cout << "&b != pb" <<endl;
    }

castA(c);
    castB(c);
    castAref(c);
    castBref(c);
    cout << "pa=" <<(int)pa << endl;
    cout << "pb=" <<(int)pb << endl;
    cout << "&c.e=" <<(int)&c.e << endl;
    cout << "sizeof A=" << sizeof(A) <<endl;
    cout << "sizeof B=" << sizeof(B) <<endl;
    cout << "sizeof C=" << sizeof(C) <<endl;

return 0;
}

执行结果：

pa != pb
&a != &b
&a == pa
&b == pb
A.a=1
A.b=2
BASE A
B.c=3
B.d=4
BASE B
A.a=1
A.b=2
DERIVE C
B.c=3
B.d=4
DERIVE C
pa=2293536
pb=2293548
&c.e=2293560
sizeof A=12
sizeof B=12
sizeof C=28