C++ Friend Function

Sometimes a class keeps its data private, but we still want a particular function outside the class to access that private data. C++ gives us a way to do this using a friend function. A friend function is a function that is not a member of the class, but the class gives it permission to access its private and protected members.

It may sound a little strange at first, so let’s understand C++ Friend Function with an example.

Why Do We Need a Friend Function in C++?

Normally, a private variable cannot be accessed from outside the class.

For example:

class Student {
private:
    int marks;
};

We cannot do this:

student.marks = 90;

But sometimes we may want a particular outside function to access marks. That’s where a friend function can help.

How to Create a Friend Function in C++?

We use the friend keyword inside the class.

class Student {
private:
    int marks;

public:
    friend void showMarks(Student student);
};

The function is declared inside the class using friend, but the function itself is written outside the class.

Simple C++ Friend Function Example

#include <iostream>
using namespace std;

class Student {
private:
    int marks = 90;

public:
    friend void showMarks(Student student);
};

void showMarks(Student student) {
    cout << "Marks: " << student.marks;
}

int main() {
    Student student;

    showMarks(student);

    return 0;
}

Output:

Marks: 90

Look at this line:

friend void showMarks(Student student);

It tells C++:

showMarks() is allowed to access the private members of Student.

That’s why this works:

cout << student.marks;

even though marks is private.

C++ Friend Class

A friend class is a class that is allowed to access the private and protected members of another class. We use the friend keyword to give another class this permission.

Simple C++ Friend Class Example

#include <iostream>
using namespace std;

class Student {
private:
    int marks = 90;

    friend class Teacher;
};

class Teacher {
public:
    void showMarks(Student student) {
        cout << "Marks: " << student.marks;
    }
};

int main() {
    Student student;
    Teacher teacher;

    teacher.showMarks(student);

    return 0;
}

Output:

Marks: 90

Look at this line:

friend class Teacher;

It tells C++ that the Teacher class is allowed to access the private and protected members of Student.

That’s why Teacher can access:

student.marks

even though marks is private.

Friend Function vs Friend Class

The difference is simple:

• Friend function → gives access to one specific function.
• Friend class → gives access to all member functions of another class.

Mini Quiz

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C++ PROGRAMMING - QUIZ 17

1 / 5

Q1. What is a friend function in C++?

A function that can access private members with permission

A function that must be inside the class

A function that creates objects

A function that cannot access class data

2 / 5

Q2. Which keyword is used to declare a friend function?

private

friend

public

class

3 / 5

Q3. Is a friend function a member of the class?

Yes, always

No, it is not a class member

Only when it is private

Only when it is public

4 / 5

Q4. How is a friend function normally called?

student.showMarks()

friend.showMarks()

showMarks(student)

Student.showMarks()

5 / 5

Q5. What can a friend function access?

Only public members

Only protected members

Private and protected members

Only constructors

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#include <iostream>
using namespace std;

class Student {
private:
    int marks;

public:
    Student(int m) {
        marks = m;
    }

    friend void checkResult(Student student);
};

void checkResult(Student student) {

    cout << "Marks: " << student.marks << "\n";

    if (student.marks >= 40) {
        cout << "Student Passed";
    }
    else {
        cout << "Student Failed";
    }
}

int main() {

    int marks;

    cout << "Enter marks: ";
    cin >> marks;

    Student student(marks);

    checkResult(student);

    return 0;
}

What’s Next?

In the next chapter, we will learn about C++ Encapsulation and see how data can be protected inside a class while allowing controlled access through public functions.

Frequently Asked Questions (FAQs)

Written by Shubhranshu Shekhar, who has trained 20000+ students in coding.

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