C++ friend Function and friend Classes

Data hiding is a fundamental concept of object-oriented programming. It restricts the access of private members from outside of the class.

Similarly, protected members can only be accessed by derived classes and are inaccessible from outside. For example,

class MyClass {
    private:
        int member1;
}

int main() {
    MyClass obj;

    // Error! Cannot access private members from here.
    obj.member1 = 5;
}

However, there is a feature in C++ called friend functions that break this rule and allow us to access member functions from outside the class.

Similarly, there is a friend class as well, which we will learn later in this tutorial.

friend Function in C++

A friend function can access the private and protected data of a class. We declare a friend function using the friend keyword inside the body of the class.

class className {
    ... .. ...
    friend returnType functionName(arguments);
    ... .. ...
}

Example 1: Working of friend Function

Output

Distance: 5

Here, addFive() is a friend function that can access both private and public data members.

Though this example gives us an idea about the concept of a friend function, it doesn't show any meaningful use.

A more meaningful use would be operating on objects of two different classes. That's when the friend function can be very helpful.

Example 2: Add Members of Two Different Classes

Output

Sum: 13

In this program, ClassA and ClassB have declared add() as a friend function. Thus, this function can access private data of both classes.

One thing to notice here is the friend function inside ClassA is using the ClassB. However, we haven't defined ClassB at this point.

// inside classA 
friend int add(ClassA, ClassB);

For this to work, we need a forward declaration of ClassB in our program.

// forward declaration
class ClassB;

friend Class in C++

We can also use a friend Class in C++ using the friend keyword. For example,

class ClassB;

class ClassA {
   // ClassB is a friend class of ClassA
   friend class ClassB;
   ... .. ...
}

class ClassB {
   ... .. ...
}

When a class is declared a friend class, all the member functions of the friend class become friend functions.

Since ClassB is a friend class, we can access all members of ClassA from inside ClassB.

However, we cannot access members of ClassB from inside ClassA. It is because friend relation in C++ is only granted, not taken.

Example 3: C++ friend Class

Output

Sum: 13

Here, ClassB is a friend class of ClassA. So, ClassB has access to the members of classA.

In ClassB, we have created a function add() that returns the sum of numA and numB.

Since ClassB is a friend class, we can create objects of ClassA inside of ClassB.