对象内存布局 (12)

下面来看看虚基类对对象内存布局的影响。虚基类的主要作用就是在所有的派生类中，保留且仅保留一份虚基类的suboject。

a. 一个虚基类的情况

#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;
}

运行结果：



注意使用了虚继承，即class Derived : public virtual Base。这次Derived的对象大小为什么为8 bytes呢？这是因为编译器会给Derived对象安插一个虚基类表指针vbptr，下面给出Derived对象的memory layout：



虚基类表指针vbptr指向Derived类的virtual bass class table(虚基类表)，虚基类表中存放的是Derived类的虚基类表指针到虚基类实例指针的偏移量。

在main函数的return前，增加如下语句：

#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;
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;
return 0;
}

编译后运行结果：

编译后运行结果：



不难发现，虚基类示例地址 = vbptr + offset，即0x0012FF78 = 0x0012FF74 + 4。图示如下：





b. 我们来看看Derived有两个虚基类的情况：

#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;

}

编译运行结果如下：



Derived对象的memory layout图解如下：



不管Derived类有多少个虚基类，它只有一个vbptr和一个virtual base class table。