C++ 虚函数，纯虚函数的一些问题

#include <iostream>
using namespace std;

#define cendl cout << endl;

class AA{//这是一个纯虚函数
public :
    virtual void fun() = 0;
};

class A{
    int data_a;
public:
    A(){
        data_a = 1;
        cout << "A " ;
    }
    A(int a){
        data_a = a;
        cout << "A(a) " ;
    }
    void show(){
        cout << data_a<<‘ ‘ ;
    }
    virtual void fun(){
        cout << "this is A ";
    }
    ~A(){
        cout << "~A " ;
    }
};
class B : public A{
    int data_b;
public:
    B(){
        data_b = 2;
        cout << "B " ;
    }
    B(int a, int b) :A(a){
        data_b = b;
        cout << "B(a) " ;
    }
    void show(){
        A::show();
        cout << data_b << ‘ ‘;
    }
    virtual void fun(){
        cout << "this is B ";
    }
    ~B(){
        cout << "~B " ;
    }
};

class C :public A{
int data_c;
public:
C(){ data_c = 3; cout << "C " ; }
C(int a, int b) :A(a){
data_c = b;
cout << "C(a) " ;
}
void show(){
A::show();
cout << data_c << ‘ ‘;
}
virtual void fun(){
cout << "this is C ";
}
~C(){
cout << "~C " ;
}
};

int main()

{

A *a1 = new A(2);
cendl;
A *a2 = new B(2,3);
cendl;
A *a3 = new C(2, 3);
cendl;
a1->fun();
cendl;
a2->fun();
cendl;
a3->fun();
cendl;
delete a1;
cendl;
delete a2;
cendl;
delete a3;
system("pause");
return 0;

}