virtual 关键字定义虚函数
多态是通过指向虚函数表的虚指针实现的。

虚指针和虚表

D d;
d.f2();      // 注意实际调用 B::f2(); 而B::f2() 中调用了 f1(); 是通过虚指针调用的 (VPTR[0])(); 是调用了D::f1();
d.B::f1();   // 明确指定调用 B::f1(); 没有通过虚指针
d.f1();      // 通过虚指针和虚表调用 D::f1();
d.D::f1();   //调用D::f1();

例子

#include <stdio.h>
class CParent 
{ 
public: 
    void Intro() { 
        printf("I'm a Parent, "); 
        Hobby(); 
    }   
    virtual void Hobby() { 
        printf("I like football!");
    }   
}; 

class CChild : public CParent { 
public:
    void Intro() { 
        printf("I'm a Child, "); 
        Hobby(); 
    }   
    void Hobby() { 
        printf("I like basketball!\n"); 
    }   
}; 

int main(int agc, char *arv[]) { 
        CChild *pChild = new CChild();
        CParent *pParent = (CParent*)pChild; 
        pParent->Intro(); 
        return 0;  
}

#include <stdio.h>  
#include <iostream>  
using namespace std;  
  
class A  
{  
protected:  
    int m_data;  
public:  
    A(int data = 0)  
    {  
        m_data = data;  
    }  
    int GetData()  
    {  
        return doGetData();  
    }  
    virtual int doGetData()  
    {  
        return m_data;  
    }  
};  
  
class B: public A  
{  
protected : int m_data;  
public:   
    B(int data = 1)  
    {  
        m_data  =  data;  
    }  
    int doGetData()  
    {  
        return m_data;  
    }  
  
};  
  
class C : public B  
{  
protected :  int m_data;  
public :  
    C(int data = 2)  
    {  
        m_data = data;  
    }  
};  
int main()  
{  
    C c(10);  
    cout<<c.GetData() << endl;  
    cout<<c.A::GetData()<<endl;  
    cout<<c.B::GetData()<<endl;  
    cout<<c.C::GetData()<<endl;  
  
    cout<<c.doGetData()<<endl;  
    cout<<c.A::doGetData()<<endl;  
    cout<<c.B::doGetData()<<endl;  
    cout<<c.C::doGetData()<<endl;  
      
    return 0;  
}
/*            
结果： 1   1   1   1  1  0  1  1 
*/