Friend Classes

Friend Class can access private or protected members and functions of other class that specifies it as a friend.

Syntax for specifying friend class:

Here's an example:

#include <iostream>
#include <string>

using namespace std;

class Phone
{
private:
    // Private members that can be accessed
    // inside friend class body:
    string model;
    double price;

public:
    Phone(string modelValue, double priceValue)
    {
        model = modelValue;
        price = priceValue;
    }

Phone() {};

// Specifying friend class:
    friend class Store;
};

class Store
{
private:
    // Declaring array of phones:
    Phone phones[5];

public:
    void listPhones()
    {
        for (int i = 0; i < 5; i++)
        {
            Phone phone = phones[i];

// The private members model and price can be
            // accessed in the friend class body (Store).
            cout << "Model: " << phone.model << endl;
            cout << "Price: " << phone.price << endl;
            cout << endl;
        }
    }

void setPhoneAtIndex(Phone phone, int index)
    {
        phones[index] = phone;
    }

Store() {};
};

void main()
{
    Store store;

Phone phone_1("Samsung Galaxy S 20", 1300);
    Phone phone_2("Samsung Galaxy Fold", 2400);
    Phone phone_3("Iphone 11", 2600);
    Phone phone_4("Oneplus 7", 1200);
    Phone phone_5("Xiaomi Redmi Note 7", 1000);

store.setPhoneAtIndex(phone_1, 0);
    store.setPhoneAtIndex(phone_2, 1);
    store.setPhoneAtIndex(phone_3, 2);
    store.setPhoneAtIndex(phone_4, 3);
    store.setPhoneAtIndex(phone_5, 4);

store.listPhones();
}

Output:

Explanation:

Atline 24 we declare a friend class. Inside the class Store any private / protected member or function of the class Phone can be accessed.
At line 31 an array of phones is declared. It's filled with elements at lines 66 - 70.
At lines 42 and 43 private members of the Phone class are accessed. This is possible only due to the declaration of a friend class at line 24.