C++ Object Class Friend Function

A friend function in C++ is a function that is granted access to the private and protected members of a class. It is declared inside the class but defined outside the class. The purpose of a friend function is to allow external functions to access private and protected data members of a class.

Syntax

The syntax of a friend function is as follows:

class MyClass {
   private:
      int x;
   public:
      friend void friendFunction(MyClass&);
};

void friendFunction(MyClass& obj) {
   obj.x = 10;
}

In the above example, the function friendFunction() is a friend function declared inside the class MyClass. It takes an object of MyClass as an argument and modifies the private member x.

Example

#include<iostream>
using namespace std;

class MyClass {
   private:
      int x;
   public:
      MyClass() {
         x = 0;
      }
      friend void friendFunction(MyClass&);
};

void friendFunction(MyClass& obj) {
   obj.x = 10;
}

int main() {
   MyClass obj;
   cout << "Before friend function: " << obj.x << endl;
   friendFunction(obj);
   cout << "After friend function: " << obj.x << endl;
   return 0;
}

Output

Before friend function: 0
After friend function: 10

Explanation

In the above example, we have defined a class MyClass with a private member x and a friend function friendFunction(). The function friendFunction() takes an object of MyClass as an argument and modifies the private member x.

In the main() function, we create an object obj of MyClass. Before calling the friend function, the value of x is 0. After calling the friend function, the value of x is changed to 10.

Use

A friend function can be used in the following scenarios:

• To access private or protected members of a class from outside the class
• To make two or more classes communicate with each other without exposing their implementation details
• To simplify the implementation of operators such as << and >>

Important Points

• A friend function can be declared inside or outside the class
• A friend function cannot access members of a class directly. It has to be passed an object of the class as an argument
• Friends are not members of a class but can be part of the namespace in which the class is defined
• Friendship is not transitive. If class A is a friend of class B and class B is a friend of class C, then class A is not automatically a friend of class C.

Summary

In summary, a friend function in C++ is a function that is granted access to the private and protected members of a class. It is declared inside the class but defined outside the class. A friend function can be used to access private or protected members of a class from outside the class, make two or more classes communicate with each other without exposing their implementation details, and simplify the implementation of operators.