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.

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

// C++ program to demonstrate the working of friend function

#include <iostream>
using namespace std;

class Distance {
    private:
        int meter;
        
        // friend function
        friend int addFive(Distance);

    public:
        Distance() : meter(0) {}
        
};

// friend function definition
int addFive(Distance d) {

    //accessing private members from the friend function
    d.meter += 5;
    return d.meter;
}

int main() {
    Distance D;
    cout << "Distance: " << addFive(D);
    return 0;
}

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

// Add members of two different classes using friend functions

#include <iostream>
using namespace std;

// forward declaration
class ClassB;

class ClassA {
    
    public:
        // constructor to initialize numA to 12
        ClassA() : numA(12) {}
        
    private:
        int numA;
        
         // friend function declaration
         friend int add(ClassA, ClassB);
};

class ClassB {

    public:
        // constructor to initialize numB to 1
        ClassB() : numB(1) {}
    
    private:
        int numB;
 
        // friend function declaration
        friend int add(ClassA, ClassB);
};

// access members of both classes
int add(ClassA objectA, ClassB objectB) {
    return (objectA.numA + objectB.numB);
}

int main() {
    ClassA objectA;
    ClassB objectB;
    cout << "Sum: " << add(objectA, objectB);
    return 0;
}

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 declarationclass 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 ClassAfriend 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

// C++ program to demonstrate the working of friend class

#include <iostream>
using namespace std;

// forward declaration
class ClassB;

class ClassA {
    private:
        int numA;

        // friend class declaration
        friend class ClassB;

    public:
        // constructor to initialize numA to 12
        ClassA() : numA(12) {}
};

class ClassB {
    private:
        int numB;

    public:
        // constructor to initialize numB to 1
        ClassB() : numB(1) {}
    
    // member function to add numA
    // from ClassA and numB from ClassB
    int add() {
        ClassA objectA;
        return objectA.numA + numB;
    }
};

int main() {
    ClassB objectB;
    cout << "Sum: " << objectB.add();
    return 0;
}

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.