为什么使用虚函数

看下面的例子，使用虚函数的用意。

class animal { public:    animal(){};    void run()    { 　　　　　cout<< “animal run”;    } }; class dog :public animal { public:    dog(){};    void run()    {         cout<< ”dog run“;    } }; class cat :public animal { public:    cat(){};    void run()    { 　　　　 cout<< ”cat run“;    } }; void main() {    animal* ani;    ani =new dog();    ani->run();    ani =new cat();    ani->run(); }   输出结果都是：animal run，没有调用各派生类的run函数，虚函数解决这一问题。
class animal
{
public:
   animal(){};
   void run()
   {
　　　　　cout<< “animal run”;
   }
};

class dog :public animal
{
public:
   dog(){};
   void run()
   {
        cout<< ”dog run“;
   }
};

class cat :public animal
{
public:
   cat(){};
   void run()
   {
　　　　 cout<< ”cat run“;
   }
};

void main()
{
   animal* ani;

   ani =new dog();
   ani->run();

   ani =new cat();
   ani->run();
}

输出结果都是：animal run，没有调用各派生类的run函数，虚函数解决这一问题。

class animal
{
public:
   animal(){};
   virtualvoid run()
   {

      cout << "animal run";
   }
};

class dog:public animal
{
public:
   dog(){};
   void run()
   {
　　　　cout << "dog run";
   }
};

class cat:public animal
{
public:
   cat(){};
   void run()
   {
　　　　cout << "cat run";
   }
};

void main()
{
   animal* ani;    ani =new dog();    ani->run();    ani =new cat();    ani->run();

   ani =new dog();
   ani->run();

   ani =new cat();
   ani->run();
}

输出结果为：dog run   cat run，若animal有很多派生类，则只声明一个animal指针即可，虚函数的作用明显体现。