對象記憶體布局 (13)

前篇：http://blog.csdn.net/pathuang68/archive/2009/04/24/4105626.aspx

 

下面來看看虛基類對對象記憶體布局的影響。虛基類的主要作用就是在所有的衍生類別中，保留且僅保留一份虛基類的suboject。

 

a. 一個虛基類的情況

#include <iostream>

using namespace std;

 

class Base

{

public:

         int base_member;

};

 

class Derived : public virtual Base {};

 

int main(void)

{

         Base b;

         Derived d;

 

         cout << sizeof(b) << endl;

         cout << sizeof(d) << endl;

 

         return 0;

}

運行結果：

 

注意使用了虛繼承，即class Derived : public virtual Base。這次Derived的對象大小為什麼為8 bytes呢？這是因為編譯器會給Derived對象安插一個虛基類表指標vbptr，下面給出Derived對象的memory layout：

 

 

虛基類表指標vbptr指向Derived類的virtual bass class table(虛基類表)，虛基類表中存放的是Derived類的虛基類表指標到虛基類執行個體指標的位移量。

 

如果Derived有兩個虛基類，那麼這兩個虛基類的執行個體指標，分別位於虛基類表的第2項和第3項，最後一項為0，意味著虛基類表的結束。虛基類表中的第1項是什嗎？目前不清楚，但是它的值大部分時候為0(請牛人指教，編譯器是VC6)。下面我們來檢驗之。

 

在main函數的return前，增加如下語句：

         cout << "Derived object d's vbptr = " << (unsigned long*)(&d) << endl;

         cout << "Address of virtual base class table = " << (unsigned long*)*(unsigned long*)(&d) << endl;

         cout << "Item 1 in virtual base class table = " << *(unsigned long*)*(unsigned long*)(&d) << endl;

         cout << "Item 2 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 1) << endl;

         cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 2) << endl;

 

         cout << "The address of virtual base class Base = " << (Base*)(&d) << endl;

編譯後運行結果：

不難發現，虛基類樣本地址 = vbptr + offset，即0x0012FF78 = 0x0012FF74 + 4。圖示如下：

 

b. 我們來看看Derived有兩個虛基類的情況：

#include <iostream>

using namespace std;

class Base1

{

public:

         int base1_member;

};

 

class Base2

{

public:

         int base2_member;

};

 

class Derived : public virtual Base1, public virtual Base2 {};

 

int main(void)

{

         Base1 b1;

         Base2 b2;

         Derived d;

 

         cout << sizeof(b1) << endl;

         cout << sizeof(b2) << endl;

         cout << sizeof(d) << endl;

 

         cout << "Derived object d's vbptr = " << (unsigned long*)(&d) << endl;

         cout << "Address of virtual base class table = " << (unsigned long*)*(unsigned long*)(&d) << endl;

         cout << "Item 1 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 0) << endl;

         cout << "Item 2 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 1) << endl;

         cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 2) << endl;

         cout << "Item 3 in virtual base class table = " << *((unsigned long*)*(unsigned long*)(&d) + 3) << endl;

 

         cout << "The address of virtual base class: Base1's instance = " << (Base1*)(&d) << endl;

         cout << "The address of virtual base class: Base2's instance = " << (Base2*)(&d) << endl;

 

         return 0;

}

編譯運行結果如下：

Derived對象的memory layout圖解如下：

 

不管Derived類有多少個虛基類，它只有一個vbptr和一個virtual base class table。

 

後篇：http://blog.csdn.net/pathuang68/archive/2009/04/24/4105851.aspx