class Base{public:    Base(){};    virtual ~Base(){};};
//类A是一个抽象类class A : public Base{public:    A(){}    virtual ~A(){}    virtual void fun() = 0; //纯虚函数};class B : public A{public:    B(){}    virtual ~B(){}    virtual void fun()    {        std::cout<<"B"<<std::endl;    }};int main(){    //A a; //抽象类A不能实例化对象，编译器会报错    A *a = new B;    a->fun();    B b;    b.fun();        return 0;}