If i declare datatype of pure virtual function and used that pure function by assigning different values to it .i get con

Question

The code is given below:

#include <iostream>
using namespace std;

class Enemy {
private : int a;
public:
virtual void attack(int a) = 0;
};

class Ninja: public Enemy {
public:
void attack(int a) {
cout << "Ninja!"<<endl;
return attack(a);

}
};

class Monster: public Enemy {
public:
void attack(int a) {
cout << "Monster!"<<endl;
return attack(a);

}
};

int main()
{

Ninja n;
Monster m;
Enemy *e1 = &n;
Enemy *e2 = &m;
e1->attack(5);
e2->attack(6);

return 0;
}