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 */
  
    // Now class  LinkedList can 
    // access private members of Node 
    friend class LinkedList; 
}; 

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 class A is a friend of B, then B doesn’t become a 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 showB() 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; } 
  
    // global friend function 
    friend void showA(A&); 
}; 
  
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.

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