When should you use 'friend' in C++?

A friend function of a class is defined outside that class' scope but it has the right to access all private and protected members of the class. Even though the prototypes for friend functions appear in the class definition, friends are not member functions.

A friend can be a function, function template, or member function, or a class or class template, in which case the entire class and all of its members are friends.

To declare a function as a friend of a class, precede the function prototype in the class definition with keyword friend as follows −

class Box {
   double width;
   public:
   double length;
   friend void printWidth( Box box );
   void setWidth( double wid );
};

To declare all member functions of class ClassTwo as friends of class ClassOne, place the following declaration in the definition of class ClassOne

Example

friend class ClassTwo;  
For example,

#include <iostream>
using namespace std;
class Box {
   double width;
   public:
   friend void printWidth( Box box );
   void setWidth( double wid );
};

// Member function definition
void Box::setWidth( double wid ) {
   width = wid;
}

// Note: printWidth() is not a member function of any class.
void printWidth( Box box ) {
   /* Because printWidth() is a friend of Box, it can
   directly access any member of this class */
   cout << "Width of box : " << box.width <<endl;
}

// Main function for the program
int main() {
   Box box;

   // set box width without member function
   box.setWidth(10.0);

   // Use friend function to print the wdith.
   printWidth( box );

   return 0;
}

Output

This will give the output −

Width of box: 10

Even though the function was not a member of the class, it could directly access the member variables of that class. This can be very useful in certain situations.