Friend Function in C++

Friend Function

As we know that private data members can only be accessed within a class and not outside it.

Definition: A friend function is capable to access the private/protected data of a class in which it is declared as a friend, though it is a nonmember function of that class

Basic properties of friend

  • A function can be declared as a friend to any number of classes
  • A friend function tough not a member function has full rights and access to the members of the class
Friend Function

Creating a friend function

#include<iostream>
using namespace std;

class demo
{
private:
int a=5;
friend void display(demo);//friend function declaration
};

void display(demo d)//friend function defination
{
cout<<"a: "<<d.a<<endl;//access of private data
}
main()
{
demo d;
display(d);//calling a friend function
}

O/P

a:4

In the above code display() function is made friend with demo class so that display() function can access the private data of the class demo

Friend class

  • A class  declared with a friend with another class is able to access private/protected  data members of that class
  • When a class is made a friend class, all the member functions of that class becomes friend functions to that class

Creating friend class

  • In this program, all member functions of class test2 will be friend functions of class test1.
  • Thus, any member function of class test2 can access the private and protected data of class test1.
  • But, member functions of class test1 cannot access the data of class test2.
#include<iostream>
using namespace std;
class test1
{
int a,b;
public:
void get()
{
cout<<"enter 2 integers";
cin>>a>>b;
}
friend class test2;//defining friend class
};

class test2
{
test1 t1;
public:
void put()
{
t1.get();
cout<<"a: "<<t1.a<<endl;
cout<<"b: "<<t1.b<<endl;
}
};
main()
{
test2 t2;
t2.put();
}

O/P

enter 2 integers 3  4
a:3
b:4

Advantages

Code reusability: instead of creating separate member functions to access private data of each class ,a single member function can access private data of many classes

Disadvantages

security: friend functions should be used only when absolutely necessary because friend mechanism alters the security of private data which is against data hiding

Real-time applications of Friend

  • Device integration is possible only by  friend
    ex: GPRS, Satellite communication
  • Friend class is also used in APIs(Application Programming Interface)
    ex: money transfer from one bank to another

Note:

Java and Unix System never support Friend Mechanism

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