Understanding Friend Functions in C++ Made Simple!

Written by: Manish Methani

October 17, 2021 at 2:04pm IST. 5 min 9 sec

Table of Contents:

Introduction to Friend Functions

Do you need to access private members of an object from outside its class? Well, introducing the friend function! It's a special function capable of accessing private or protected members of an object. This function is 'friends' with a class and can be called with an object as an argument.

Let's break it down with some examples to understand this concept better:

Program without Using Friend Function (Before Scenario)

Consider the following code:

#include <iostream>
using namespace std;

class Demo {
    int firstNumber, secondNumber; // private data members
public:
    Demo(int a, int b) {
        firstNumber = a;
        secondNumber = b;
    }
    
    void add(Demo);
};

// Function attempting to access private members (Error Scenario)
void add(Demo demoInstance) {
    int c = demoInstance.firstNumber + demoInstance.secondNumber;
    cout << "Addition is " << c;
}

int main() {
    Demo a(12, 13);
    add(a);
    return 0;
}

Running this code would result in a compilation error stating that 'firstNumber' and 'secondNumber' are private members of 'Demo'. This shows that accessing private members directly from outside the class isn't allowed.

Program Using Friend Function (After Scenario)

Now, let's modify the code to use a friend function:

#include 
using namespace std;

class Demo {
    int firstNumber, secondNumber; // private data members
public:
    Demo(int a, int b) {
        firstNumber = a;
        secondNumber = b;
    }
    
    friend void add(Demo); // Declare 'add' function as a friend
};

// Friend function that accesses private members
void add(Demo demoInstance) {
    int c = demoInstance.firstNumber + demoInstance.secondNumber;
    cout << "Addition is " << c;
}

int main() {
    Demo a(12, 13);
    add(a);
    return 0;
}

Here, we've made the add() function a friend of the Demo class. This allows add() to access the private members firstNumber and secondNumber. When running the program, it should display:

Addition is 25

This showcases how using a friend function allows accessing private members from outside the class, enabling operations that would otherwise result in compilation errors.

Understanding friend functions in C++ enables better control over data accessibility within classes and facilitates smooth handling of private members when required.