Friend Function

A friend function is a non-member function that is granted exceptional permission to access the private and protected data members of a class. While data hiding normally prevents outside code from tampering with a class's internal state, declaring a function as a friend breaks this rule safely under the class's explicit control.

How to implement a friend function

1. Declare the Function Inside the Class using the keyword friend. The parameter of the function is an object declaration of the class
2. Define the Function Outside the Class

Notes:

1. The friend function is used to access or assign private variable values
2. Using assign value to private variable(s), the friend function parameter must be the address of the object (&objectName) in order for the values to change

Let's modify our paddle class in our encapsulation lession to create a friend function that accesses the private variables (xPos and yPos) and displays them.

#include <iostream>
using namespace std;
class Paddle{
    private:
    int xPos;
    int yPos;
    int interval = 5;
    public:
    Paddle(int x, int y){
        xPos = x;
        yPos = y;
    }
    void moveUp(){
        yPos = yPos + interval;
    }
    void moveDown(){
        yPos = yPos - interval;
    }
    void setXPosition(int x){
        xPos = x;
    }
    void setYPosition(int y){
        yPos = y;
    }
    //Declare the friend function
    friend void setPaddlePositions(Paddle &paddle,int x, int y);
    friend void printBallPositions(Paddle paddle);
};
//Implement the friend function
void setPaddlePositions(Paddle &paddle,int x, int y){
    paddle.xPos = x;
    paddle.yPos = y;
}
void printBallPositions(Paddle paddle){
    cout << "Paddle is at (" << paddle.xPos << "," << paddle.yPos << ")" << endl;
}
int main() {
    Paddle paddle(5,50);
    //Show the x and y coordinates of the paddle using the friend function
    printBallPositions(paddle);
    //The up arrow key pressed 5 times
    paddle.moveUp();
    paddle.moveUp();
    paddle.moveUp();
    paddle.moveUp();
    paddle.moveUp();
    printBallPositions(paddle);
    //The down arrow pressed 2 times
    paddle.moveDown();
    paddle.moveDown();
    printBallPositions(paddle);
    //Set private values using set methods
    paddle.setXPosition(50);
    paddle.setYPosition(20);
    printBallPositions(paddle);
    //Assign values to the private variable using the friend function
    setPaddlePositions(paddle,20, 80);
    printBallPositions(paddle);
     
    return 0;
}

Explanations

1. friend void setPaddlePositions(Paddle &paddle,int x, int y) declares the friend function for changing the values of the private variables
2. There is & attached to the paddle object to indicate the address of where the object is stored in order to change the values.
3. friend void printBallPositions(Paddle paddle) is used to access and print the private variables
4. setPaddlePositions(paddle,20, 80) calls the friend function to change the private variable values
5. printBallPositions(paddle) is calls the friend function to access and print the private variable values