对象内存布局 (8)

在内存对象布局 (5)的代码中，在Derived类中将三个基类中的虚函数分别覆盖一个，即分别覆盖Base1中声明的vfBase1_1()，Base2中声明的vfBase2_1()以及Base3中声明的vfBase3_1()。保持其他代码不变，修改后的Derived代码如下：

#include <iostream>
using namespace std;

class Base1
{
public:
    int m_base1;
    inline virtual void vfBase1_1()
    {
        cout << "This is in Base1::vfBase1_1()" << endl;
    }
    inline virtual void vfBase1_2()
    {
        cout << "This is in Base1::vfBase1_2()" << endl;
    }
};
class Base2
{
public:
    int m_base2;
    inline virtual void vfBase2_1()
    {
        cout << "This is in Base2::vfBase2_1()" << endl;
    }
    inline virtual void vfBase2_2()
    {
        cout << "This is in Base2::vfBase2_2()" << endl;
    }
};
class Base3
{
public:
    int m_Base3;
    inline virtual void vfBase3_1()
    {
        cout << "This is in Base3::vfBase3_1()" << endl;
    }
    inline virtual void vfBase3_2()
    {
        cout << "This is in Base3::vfBase3_2()" << endl;
    }
};
class Derived : public Base1, public Base2, public Base3
{
public:
    int m_derived;
    inline virtual void fd()
    {
        cout << "This is in Derived::fd()" << endl;
    }
    inline void vfBase1_1()
    {
        cout << "This is in Derived::vfBase1_1()" << endl;
    }
    inline void vfBase2_1()
    {

        cout << "This is in Derived::vfBase2_1()" << endl;

    }

    inline void vfBase3_1()

    {

        cout << "This is in Derived::vfBase3_1()" << endl;

    }
};
typedef void (*VFun)(void);
template<typename T>
VFun virtualFunctionPointer(T* b, int i)
{
    return (VFun)(*((int*)(*(int*)b) + i));
}
int main(void)
{
    Derived d;
    cout << "The size of Derived object = 	" << sizeof(Derived) << endl;
    cout << endl;
    cout << "1st virtual function table: " << endl;
    int i = 0;
    while(virtualFunctionPointer(&d, i)&&i<3)
    {
        VFun pVF = virtualFunctionPointer(&d, i++);
        pVF();
    }
    cout << endl;
    cout << "2nd virtual function table: " << endl;
    i = 0;
    //以32字长的机器，找到下一个继承base class的vptr
    int* tmp = ((int*)&d)+sizeof(Base1)/4;
    //虚函数表中的虚函数后面不为NULL？如果不加i的限制会出现段错误，不能结束循环
    while(virtualFunctionPointer(tmp, i)&&i<2)

    {
        VFun pVF = virtualFunctionPointer(tmp, i++);
        pVF();
    }
    cout << endl;
    cout << "3rd virtual function table: " << endl;
    i = 0;
    tmp = ((int*)&d) + sizeof(Base2)/4;
    while(virtualFunctionPointer(tmp, i)&&i<2)
    {
        VFun pVF = virtualFunctionPointer(tmp, i++);
        pVF();
    }
    return 0;
}

运行结果：

Derived对象的memory layout图解如下：

 

Derived中覆盖的虚函数，分别出现在三个不同的虚函数表中，而且分别代替个基类的原虚函数的位置，即：

第一个虚函数表中，Derived::vfBase1_1()代替了Base::vfBase1_1()的位置，Base::vfBase1_1()不再在虚函数表中出现；

第二个虚函数表中，Derived::vfBase2_1()代替了Base::vfBase2_1()的位置，Base::vfBase2_1()不再在虚函数表中出现；

第三个虚函数表中，Derived::vfBase3_1()代替了Base::vfBase3_1()的位置，Base::vfBase3_1()不再在虚函数表中出现；

在Derived中自己定义的虚函数，总是处在第一个虚函数表的最后一项的位置。