对象内存布局 (6)

如果在对象内存布局 (5)的代码中，将Base1中的两个虚函数声明删除，同时将main函数中的下面代码注释掉(因为现在只有两张虚函数表了)：

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;
    }
};
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 = \t" << 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)+sizeof(Base2))/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：

通过修改代码的位置以及运行结果可以知道，Derived对象的memory layout图解如下：

由上面分析可知：

其一，Base1 subobject排列在Base2 subobject之后，而在Base3 subobject之前，尽管它在Derived类的继承列表中排列在首位，因为它已经没有虚函数表了，但仍然排列在Derived类的成员变量m_derived之前。

其二，在Derived类中定义的虚函数Derived::vfDerived()附加在一个虚函数表的最后，这时第一张虚函数表是类Base2的。