c ++类及其布局和转换

问题描述

我得到了这段代码来测试我的cpp理解，我很困惑：

I was given this code to test my cpp understanding, and I am quite confused:

#include "stdafx.h"
#include <iostream>
#include <cstddef>

using namespace std;

class A
{
public:
    A() : m_x(0) { }

public:
    static ptrdiff_t member_offset(const A &a)
    {
        const char *p = reinterpret_cast<const char*>(&a);
        const char *q = reinterpret_cast<const char*>(&a.m_x);

        return q - p;
    }

private:
    int m_x;
};

class B
    : public A
{
public:
    B() : m_x('a') { }

public:
    static int m_n;

public:
    static ptrdiff_t member_offset(const B &b)
    {
        const char *p = reinterpret_cast<const char*>(&b);
        const char *q = reinterpret_cast<const char*>(&b.m_x);

        return q - p;
    }

private:
    char m_x;
};

int B::m_n = 1;

class C
{
public:
    C() : m_x(0) { }
    virtual ~C() { }

public:
    static ptrdiff_t member_offset(const C &c)
    {
        const char *p = reinterpret_cast<const char*>(&c);
        const char *q = reinterpret_cast<const char*>(&c.m_x);

        return q - p;
    }

private:
    int m_x;
};

int _tmain(int argc, _TCHAR* argv[])
{
    A a;
    B b;
    C c;
    std::cout << ((A::member_offset(a) == 0) ? 0 : 1);
    std::cout << ((B::member_offset(b) == 0) ? 0 : 2);
    std::cout << ((A::member_offset(b) == 0) ? 0 : 3);
    std::cout << ((C::member_offset(c) == 0) ? 0 : 4);
    std::cout << std::endl;

    return 0;
}

为什么答案为0204？

Why is the answer 0204?

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