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 a special grant to access private and protected members. A friend function can be:

a) A member 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