Friends of C++

Friend function

Remember that a member which is not public (Private and protected )  can not be accessed from outside the class. 

Three are some situations where you may need to access these. Keyword friend  is used for this. A friend functions and classes can access all members of a class. 

This concept is controversial. People say friend concept violates data encapsulation 

A friend function is a non-member function but still can access all the members of a class including private members. Such a function is declared within class body with the prefix "friend"

class Number
{
 int num;
public:
 Number(int m){/*code*/}
 friend void printNum(Number ob);/*friend function*/
};

void printNum(Number obj)
{
 cout<<obj.num<<"endl";
}

printNum() is not member of class Number. But it can still access all members including private members, because it is a friend.

Friend class

An object of a friend class can access all the members of a class.

#include<iostream>
using std::cout;
class A;
class Number
{
 int num;
public:
 Number(int a):num(a){};
 friend class A;
};
class A
{
public:
 void printNumber(Number ob){cout<<ob.num;}
 int getNumber(Number ob){return ob.num;}
};
int main()
{
 A ob1;
 Number ob2(10);
 ob1.printNumber(ob2); 
}

Output
10

ob1 - is an object of A class. So it should not be able to access private members of Number class. But as A class is declared as a friend of Number class. So printNumber() and getNumber() functions of A class can access private data num of Number class.

Declaration of class A above class Number is called forward declaration. You need to use forward declaration for friend class.