C++ Friend Function and Friend Classes

In C++, there are situations when we want to allow a non-member function or a non-member class to access the private and protected members of a class. This can be achieved by using the concept of friend functions and friend classes.

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Friend Function

A friend function of a class is a function that is not a member of the class but has access to the private and protected members of the class. A friend function can be declared inside the class using the keyword "friend". The friend function can be a normal function or a member function of another class.

The syntax for declaring a friend function is as follows:

class className
{
   private:
      // private members

   protected:
      // protected members

   public:
      // public members
      friend returnType functionName(arguments);
};

A friend function can be defined outside the class or inside the class. If the friend function is defined inside the class, it must be preceded by the keyword "friend".

The friend function can be called just like a normal function, and it can access the private and protected members of the class.

Here is a sample C++ program to demonstrate the use of a friend function in C++.

#include <iostream>

using namespace std;

class Rectangle {
private:
    int width;
    int height;

public:
    Rectangle(int w = 0, int h = 0) : width(w), height(h) {}

    // Friend function
    friend int area(Rectangle &);
};

// Friend function definition
int area(Rectangle &rect) {
    return rect.width * rect.height;
}

int main() {
    Rectangle rect(5, 7);

    cout << "Area of rectangle: " << area(rect) << endl;

    return 0;
}

Output

Area of rectangle: 35

In this example, the area function is a friend function of the Rectangle class, and it has access to the private and protected members of the class.

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Friend Class

A friend class is a class that has access to the private and protected members of another class. A friend class can be declared inside a class using the keyword "friend". The syntax for declaring a friend class is as follows:

class className1 {
   private:
      // private members

   protected:
      // protected members

   public:
      // public members
      friend class className2;
};

In this example, the className2 is a friend class of the className1. The friend class can access the private and protected members of the className1.

Here is a sample C++ program to demonstrate the use of friend classes in C++.

#include <iostream>
using namespace std;

class Rectangle;

class Square {
    private:
        int side;
    public:
        Square(int s) : side(s) {}
        friend class Rectangle;
};

class Rectangle {
    private:
        int length;
        int breadth;
    public:
        Rectangle(int l, int b) : length(l), breadth(b) {}
        Rectangle(Square &s) {
            length = s.side;
            breadth = s.side;
        }
        int area() {
            return length * breadth;
        }
};

int main() {
    Square square(10);
    Rectangle rectangle(square);
    cout << "Area of rectangle: " << rectangle.area() << endl;
    return 0;
}

Output

Area of rectangle: 100

In this example, we have created two classes Square and Rectangle. Square class has a private variable side, and a friend class Rectangle. The Rectangle class has a private variable length and breadth. The Rectangle class has a constructor which takes an object of Square class as an argument and initializes the length and breadth with the side of the square.

The main function creates an object of Square class and passes it to the constructor of Rectangle class. The area function of the Rectangle class is used to calculate the area of the rectangle. The friend class relationship between Square and Rectangle allows the Rectangle class to access the private variables of the Square class.