Question about Polymorphism.

[corey__cakes]

I've recently learned about polymorphism in C++. I get the jest of virtual members, abstract classes, and pointers to a base class. What I don't understand is the purpose of have a pointer to a base class. The resources I've read on polymorphism have all shown that you have an object from a derived class and a pointer of a base class pointing to that object, then calling some function from the pointer. What do I gain from doing that? I thought the point of inheritance was so I could call those same functions from an object of the derived class.

//basic includes and stuff
#include<iostream>
using namespace std;

//Base class
class Base {
	int x, y;	//variables and stuff in the Base class
public:
	void SetXandY(int a, int b) {	//generic function
		x = a;
		y = b;
	}
	int GetX() { return x; }	//accessor functions
	int GetY() { return y; }
};

//Derived Class
class Derived : public Base	{	//Derived class inherits from Base class
	int w, h;	//other members you'd add in derived classes
public:
	void SetWandH(int a, int b) {	//generic derived class function
		w = a;
		h = b;
	}
	int GetW() { return w; }	//accessor functions
	int GetH() { return h; }
};

//main function
int main() {
	
	Derived obj; //declares an object from the derived class
	Base* objpointer = &obj;
	/* We can do this because the object of the Derived class
	is inheriting from the Base Class. */
	//now we can do this
	objpointer->SetXandY(5, 20);	//this will make the x and y values for obj 5 and 20 respectively

	//shows us the data
	cout << obj.GetX() << endl;
	cout << obj.GetY() << endl;

	//holds the screen
	system("PAUSE");

	//end of main
	return 0;
}

This works and I know why it works. I understand all of it. But why do I need a pointer at all? I can call obj.SetXandY() without making a pointer and it still preforms the same. So there isn't any need for objpointer.

[blueshackstudios]

You are correct in that it is useless in your situation, but that is a bad example.

Mainly polymorphism is useful when you need a base class pointer for general use.

Consider the following:

Code: Select all

class Enemy
{
     virtual void Update()
     {
          
     }
};
class Goblin : public Enemy
{
     void Update() // Overrides virtual function in base class
     {
          // DO GOBLIN SPECIFIC STUFF / THINGS
     }
};
int main()
{
     Enemy* enemy;
     enemy = new Goblin();
     enemy->Update();

     return 0;
}

In the above situation you can inherit from enemy to make goblins, ogres, bandits, etc, and then only call enemy->Update() after setting the enemy to whatever type you need. This is especially useful for having a list like List<Enemy*> and adding all different sorts of enemies to it, and then looping the list and calling ALL of their Update()'s.