Friend Functions

Friend functions in C++ provide a way to grant a non-member function access to the private members of a class. This allows the function to manipulate the data members of the class without violating encapsulation. A friend function is declared using the keyword 'friend' and can access all private and protected members of the class for which it is a friend.

Here is an example program to illustrate the usage of friend functions in C++

#include <iostream>
using namespace std;

class Rectangle {
  int width, height;
public:
  Rectangle() {}
  Rectangle(int w, int h) : width(w), height(h) {}
  friend int area(Rectangle rect);
};

int area(Rectangle rect) {
  return rect.width * rect.height;
}

int main () {
  Rectangle rect(3, 4);
  cout << "Area of rectangle is: " << area(rect) << endl;
  return 0;
}

In the above program, we define a class Rectangle with private members width and height. We then declare a non-member function 'area' as a friend of the Rectangle class. This allows the 'area' function to access the private members of the Rectangle class.

The 'area' function takes a Rectangle object as its argument and calculates the area of the rectangle using the formula width * height. In the main function, we create a Rectangle object with width 3 and height 4 and call the 'area' function to calculate its area.

Output

Area of rectangle is: 12

Summary

friend functions in C++ provide a way to grant a non-member function access to private members of a class without breaking encapsulation. By declaring a function as a friend, it gains access to all private and protected members of the class. This feature is useful when designing complex programs that require close interaction between classes and functions.