多态和多态对象模型

1 单继承的多态，以以下代码为例

class A{public:    virtual void f1()    {        cout<<"A::f1"<<endl;    }    virtual void f2()    {        cout<<"A::f2"<<endl;    }    void f3()    {        cout<<"A::f3"<<endl;    }};class B :public A{public:    virtual void f1()    {        cout<<"B::f1"<<endl;    }    virtual void f2()    {        cout<<"B::f2"<<endl;    }    void f3()    {        cout<<"A::f3"<<endl;    }};int main(){    A* a;    B b;    a=&b;    a->f1();    a->f2();    a->f3();    return 0;}

2.继承的多态以以下代码为例

#include<iostream>using namespace std;class A{public:    virtual void f1()    {        cout<<"A::f1"<<endl;    }public:    int _a;};class B{public:    virtual void f2()    {        cout<<"B::f2"<<endl;    }public:    int _b;};class C:public B,public A{public:    virtual void f3()    {        cout<<"C::f3"<<endl;    }public:    int _c;};typedef void(*VFUNC)();void PrintVTable(void* vtable){    printf("vtable:0x%p\n", vtable);    VFUNC* array = (VFUNC*)vtable;    for (size_t i = 0; array[i] != 0; ++i)    {        printf("vtable[%d]:0x%p->", i,array[i]);        array[i]();    }    cout<<"================================"<<endl;}int main(){    A a;    B b;    C c;    PrintVTable(*((int**)&c));    return 0;}

这个多继承说明了基类有虚表时，派生类会继承基类的虚数表，并且是先继承谁，就会把自己的虚函数存储在谁的虚表中，我们通过调用监视窗口和打印虚表来看清这一问题。

所以说C有两个虚表。
3.菱形继承

class A{public:    virtual void f1()    {        cout<<"A::f1"<<endl;    }public:    int _a;};class B:public A{public:    virtual void f1()    {        cout<<"B::f1"<<endl;    }    virtual void f2()    {        cout<<"B::f2"<<endl;    }public:    int _b;};class C:public A{public:    virtual void f3()    {        cout<<"C::f3"<<endl;    }public:    int _c;};class D:public B,public C{public:    virtual void f1()    {        cout<<"D::f1"<<endl;    }    virtual void f4()    {        cout<<"D::f4"<<endl;    }public:    int _d;};typedef void (*V_FUNC)();  void PrintVTable(int vtable)  {      int* VfArray = (int*)vtable;      printf("vtable:0x%p\n",VfArray);      for(int i = 0;VfArray[i] != 0;++i)      {          printf("vfun[%d]:0x%p->",i,VfArray[i]);          V_FUNC f = (V_FUNC)VfArray[i];          f();      }      cout<<"*********************************"<<endl;  }  int main(){    A a;    B b;    C c;    D d;    d.B::_a=1;    d.C::_a=2;    d._b=3;    d._d=4;    PrintVTable(*((int*)&d));      PrintVTable(*(int*)((char*)&d+12));      return 0;}

菱形继承也符合单继承和多继承的规律
总结：
当基类有虚函数即有虚表时，派生类就会继承其虚表。一个只有虚表的类就是4个字节大小，就是一个指针的大小。