Why do we use pointers with Polymorphism?

Question

I'm watching tutorials from thenewboston on youtube ,and I have few questions about Polymorphism. Here is his code:

#include <iostream>
using namespace std;
class Enemy{


   protected:
   int attackPower;
public:
    void setAttackPower(int a){
    attackPower=a;
    }
};

class Ninja:public Enemy{
public:
    void attack(){
    cout<<"I am a ninja,ninja chop! -"<<attackPower<<endl;}
};

class Monster:public Enemy{
public:
void attack() {
cout<<"monnster must eat you!!! -"<<attackPower<<endl;
}
};

int main()
{
    Ninja n;
    Monster m;
    Enemy *enemy1=&n;
    Enemy *enemy2=&m;
    enemy1->setAttackPower(29);
    enemy2->setAttackPower(99);
    n.attack();
    m.attack();

}

My question is : Can I write the code in main() like this(or shouldn't I and WHY??):

Ninja n;
Monster m;
//Enemy *enemy1=&n;
//Enemy *enemy2=&m;
//enemy1->setAttackPower(29);
//enemy2->setAttackPower(99);
n.setAttackPower(99);
m.setAttackPower(29);
n.attack();
m.attack();

Answer 1

Can I write the code in main() like this [...]

Absolutely, you can! The reason for this is that your new example does not use polymorphic behavior. Unlike the original example, which hid the knowledge of run-time type of Enemy objects from compile-time code, your rewritten code keeps types available.

Here is what's not going to work without a pointer or a reference:

void setPowerAndAttack(Enemy enemy, int power) {
//                     ^^^^^^^^^^^
//  This is not going to work without pointer/reference
    enemy.setAttackPower(power);
    attack();
}
...
Ninja n;
Monster m;
setPowerAndAttack(n, 99);
setPowerAndAttack(m, 29);

Even though the code would compile, Enemy in setPowerAndAttack is not going to exhibit polymorphic behavior due to object slicing.

You need to make enemy a pointer or a reference to keep polymorphic behavior:

void setPowerAndAttack(Enemy& enemy, int power)
//                          ^

Very Important: You need to make attack function virtual in the Enemy class in order to have any polymorphic behavior at all. This is not clear from watching the video:

class Enemy {
protected:
   int attackPower;
public:
    void setAttackPower(int a) {
        attackPower=a;
    }
    virtual void attack();      // <<== Add this line
    virtual ~Enemy() = default; // <<== Add a virtual destructor
};