Friend class and function in C++

Friend Class A friend class can access private and protected members of other class in which it is declared as friend. It is sometimes useful to allow a particular class to access private members of other class. For example a LinkedList class may be allowed to access private members of Node.

class Node 
{ 
private: 
  int key; 
  Node *next; 
  /* Other members of Node Class */
  
  friend class LinkedList; // Now class  LinkedList can  
                           // access private members of Node 
}; 

Friend Function Like friend class, a friend function can be given special grant to access private and protected members. A friend function can be:
a) A method of another class
b) A global function

class Node 
{ 
private: 
  int key; 
  Node *next; 
  
  /* Other members of Node Class */
  friend int LinkedList::search(); // Only search() of linkedList  
                                  // can access internal members 
}; 

Following are some important points about friend functions and classes:
1) Friends should be used only for limited purpose. too many functions or external classes are declared as friends of a class with protected or private data, it lessens the value of encapsulation of separate classes in object-oriented programming.

2) Friendship is not mutual. If a class A is friend of B, then B doesn’t become friend of A automatically.

3) Friendship is not inherited (See this for more details)

4) The concept of friends is not there in Java.

A simple and complete C++ program to demonstrate friend Class

#include <iostream> 
class A { 
private: 
    int a; 
public: 
    A() { a=0; } 
    friend class B;     // Friend Class 
}; 
  
class B { 
private: 
    int b; 
public: 
    void showA(A& x) { 
        // Since B is friend of A, it can access 
        // private members of A 
        std::cout << "A::a=" << x.a; 
    } 
}; 
  
int main() { 
   A a; 
   B b; 
   b.showA(a); 
   return 0; 
} 

Output:

A::a=0

A simple and complete C++ program to demonstrate friend function of another class

#include <iostream> 
  
class B; 
  
class A 
{ 
public: 
    void showB(B& ); 
}; 
  
class B 
{ 
private: 
    int b; 
public: 
    B()  {  b = 0; } 
    friend void A::showB(B& x); // Friend function 
}; 
  
void A::showB(B &x) 
{ 
    // Since show() is friend of B, it can 
    // access private members of B 
    std::cout << "B::b = " << x.b; 
} 
  
int main() 
{ 
    A a; 
    B x; 
    a.showB(x); 
    return 0; 
}

Output:

B::b = 0

A simple and complete C++ program to demonstrate global friend

#include <iostream> 
  
class A 
{ 
    int a; 
public: 
    A() {a = 0;} 
    friend void showA(A&); // global friend function 
}; 
  
void showA(A& x) { 
    // Since showA() is a friend, it can access 
    // private members of A 
    std::cout << "A::a=" << x.a; 
} 
  
int main() 
{ 
    A a; 
    showA(a); 
    return 0; 
}

Output:

A::a = 0

References:
http://en.wikipedia.org/wiki/Friend_class
http://en.wikipedia.org/wiki/Friend_function
http://www.cprogramming.com/tutorial/friends.html
http://www.parashift.com/c++-faq/friends-and-encap.html

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Friend class and function in C++

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