C++纯虚函数

代码：

#include <iostream>class Pet {public:    virtual void speak() = 0;    virtual void eat() = 0;};class Dog : public Pet {public:    void speak() {        std::cout<<"speak"<<std::endl;    }    void eat() {        std::cout<<"eat"<<std::endl;    }};class littleDog : public Dog {public:    void speak() {        std::cout<<"small speak"<<std::endl;    }    void eat() {        std::cout<<"little eat"<<std::endl;    }};int main() {    Dog dog;    littleDog ld;    Dog *p = &dog;    p->speak();    p->eat();    p = &ld;    p->speak();    p->eat();    return 0;}

结果：

为什么？