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派生類地址比基類地址少

編輯：C++入門知識

大家對虛表並不陌生，都知道每個含有虛函數的類對象都有1個虛指針，但是在現實使用中，卻總是因為這而調試半天，才發現原來是虛指針惹的禍。我這幾天在調試代碼時候也中招了，我的問題是這樣的，如下圖，CTree是最底層基類（非虛類）， CSamplerTree(虛類)派生自CTree，CMSamplerTree，CASamplerTree派生自CSamplerTree，

CTree中包括兩個成員變量，QList <CTree *> childList;樹中有多少個孩子節點；CTree *parent;當前樹節點的父親節點，程序中我大量使用CTree *pTree指針指向CSamplerTree、CMSamplerTree、CASamplerTree ，從而達到統一處理的目的，從而使代碼很簡潔，復用性高。但是誰曾想到，程序一運行就會崩潰，通過調試發現，CSamplerTree、CMSamplerTree、CASamplerTree的指針當指向CTree的指針時，地址均加了4，為什麼呢？為了加深理解，我做了一個簡單的測試代碼：

[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();
}下面是輸出的結果，從結果可以看出派生類指針指向基類指針，指針地址會加4,基類指針指向派生類時，指針地址會減4。

base
200672 200676
derived
base
200740 200744
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下面我們看看派生類對象和基類對象的內存是如何組織的，我們在上例的基礎上引入2個變量，代碼如下：

[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);
}