派生类地址比基类地址少

大家对虚表并不陌生，都知道每个含有虚函数的类对象都有1个虚指针，但是在现实使用中，却总是因为这而调试半天，才发现原来是虚指针惹的祸。我这几天在调试代码时候也中招了，我的问题是这样的，如下图，CTree是最底层基类（非虚类）， CSamplerTree(虚类)派生自CTree，CMSamplerTree，CASamplerTree派生自CSamplerTree，

CTree中包括两个成员变量，QList <CTree *> childList;树中有多少个孩子节点；CTree *parent;当前树节点的父亲节点，程序中我大量使用CTree *pTree指针指向CSamplerTree、CMSamplerTree、CASamplerTree ，从而达到统一处理的目的，从而使代码很简洁，复用性高。但是谁曾想到，程序一运行就会崩溃，通过调试发现，CSamplerTree、CMSamplerTree、CASamplerTree的指针当指向CTree的指针时，地址均加了4，为什么呢？为了加深理解，我做了一个简单的测试代码：

[cpp]
#include <stdio.h>class CBase {
public:
    CBase() {}
    void func()
    {
        printf("base\n");
    }
};
class CDerived : public CBase {
public:
    CDerived() {}
    virtual void func1()
    {
        printf("derived\n");
    }
};
void main()
{
     CBase *pBase = new CDerived();
     pBase->func();
     CDerived *pDerived = (CDerived *)pBase;
     printf("%d %d\n", pDerived, pBase);
     pDerived->func();
     CBase *pBase1 = new CBase();
     pBase1->func();
     CDerived *pDerived1 = (CDerived *)pBase1;
     printf("%d %d\n", pDerived1, pBase1);
     pDerived1->func();
}

#include <stdio.h>class CBase {
public:
    CBase() {}
    void func()
    {
        printf("base\n");
    }
};
class CDerived : public CBase {
public:
CDerived() {}
virtual void func1()
{
  printf("derived\n");
}
};
void main()
{
     CBase *pBase = new CDerived();
     pBase->func();
     CDerived *pDerived = (CDerived *)pBase;
     printf("%d %d\n", pDerived, pBase);
     pDerived->func();
     CBase *pBase1 = new CBase();
     pBase1->func();
     CDerived *pDerived1 = (CDerived *)pBase1;
     printf("%d %d\n", pDerived1, pBase1);
     pDerived1->func();
}下面是输出的结果，从结果可以看出派生类指针指向基类指针，指针地址会加4,基类指针指向派生类时，指针地址会减4。

base
200672 200676
derived
base
200740 200744
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下面我们看看派生类对象和基类对象的内存是如何组织的，我们在上例的基础上引入2个变量，代码如下：

[cpp]
#include <stdio.h>class CBase {
public:
    CBase() {}
    void func()
    {
        printf("base\n");
    }
    int a;
};
class CDerived : public CBase {
public:
    CDerived() {}
    virtual void func1()
    {
        printf("derived\n");
    }
       int b;
};
void main()
{
     CBase *pBase = new CDerived();
     CDerived *pDerived = (CDerived *)pBase;
     printf("%d %d\n", pDerived, pBase);
     printf("%d %d %d\n", &pDerived->a, &pDerived->b, &pBase->a);
}

#include <stdio.h>class CBase {
public:
    CBase() {}
    void func()
    {
        printf("base\n");
    }
    int a;
};
class CDerived : public CBase {
public:
CDerived() {}
virtual void func1()
{
  printf("derived\n");
}
       int b;
};
void main()
{
     CBase *pBase = new CDerived();
     CDerived *pDerived = (CDerived *)pBase;
     printf("%d %d\n", pDerived, pBase);
     printf("%d %d %d\n", &pDerived->a, &pDerived->b, &pBase->a);
}

200672 200676
200676 200680 200676
Press any key to continue
从输出结果我们可看出，CDerived对象的起始地址存放的是虚表指针vptr，接下来的是基类的成员变量，接下来再是自身的成员变量。

补充：软件开发 , C++ ,