Call derived method which does not exist in base class from base class

Question

In this example, I create base object sphere(2), and assign its address to a derived class pointer with type conversion. Then I can call the fun() function that does not exist in base object sphere(2). And I think it is quite strange, since this is no definition for fun() in Sphere at all. But I can make type conversion and call it. Can someone explain it? Thanks in advance.

PS: The output is "Haha I am a ball with radius 2"

//---------sphere.h--------------
#ifndef SPHERE_H
#define SPHERE_H

class Sphere{
    private:
        double _radius;
    public:
        Sphere(double radius){
            _radius = radius;
        }
        double getRadius(){
            return _radius;
        }
};

#endif
//-----------ball.h--------------
#ifndef BALL_H
#define BALL_H

#include <iostream>
#include "Sphere.h"

using namespace std;

class Ball : public Sphere
{
    private:
        string _ballName;
    public:
        Ball(double radius, string ballName): Sphere(radius){
            _ballName = ballName;
        }

        string getName(){
            return _ballName;
        }

        void fun(){
            cout << "Haha I am a ball with radius " << getRadius() << endl;
        }
        void displayInfo(){
            cout << "Name of ball: " << getName()
                        << " radius of ball: " <<  getRadius() << endl;
        }
};

#endif
//-------main.cpp----------------
#include "Ball.h"
#include "Sphere.h"

int main(){
    Ball *ballPtr;
    Sphere sphere(2);

    ballPtr = (Ball *)&sphere;
    ballPtr -> fun();

    return 0;
}

Answer 1

That was mere "luck". You are invoking a function on an object while pretending it is of another type (a Ball is a Sphere, but not all Spheres are Balls, and that one certainly isn't). This is Undefined Behaviour, and can do anything, including toasting your cat. Watch out.