Friend Function

Friend Function

Definition :

The friend function is a function that is not a member function of the class but it can access the private and protected members of the class.

The friend function is given by a keyword friend.

These are special functions which are declared anywhere in the class but have given special permission to access the private members of the class.

C++ Program

#include<iostream>

using namespace std;

class test

{

   int data;

   friend int fun(int x);//declaration of friend function

public:

   test()//constructor

   {

   data = 5;

   }

};

int fun(int x)

{

   test obj;

   //accessing private data by friend function

   return obj.data + x;

}

int main()

{

   cout << "Result is = "<< fun(4)<<endl;

}

Output

Result is = 9

 

Characteristics of Friend Function

1) The friend functions are not part of the class and they are external.

2) The friend function is not called using the object of that class. Infact, it is called like a normal function without object.

3) The friend function can be declared in public or private part without affecting its meaning.

4) The friend function can be friend of more than one class, hence they can be used for message passing between the classes.

5) The friend of one class can be friend of another class.

 

Friend class : Similar to a friend function one can declare a class as a friend to another class. This allows the friend class to access the private data members of the another class. In the following program class B is declared as friend of class A. Therefore class B can access the variable data, and variable is private member of class A.

C++ Program

#include<iostream>

using namespace std;

class A

{

private:

   int data;

   friend class B;//class B is friend of class A

public:

   A()//constructor

   {

      data = 5;

   }

};

class B

{

public:

   int sub(int x)

   {

   A obj1; //object of class A

   //the private data of class A is accessed in class B

   //data contains 5 and x contains 2

   return obj1.data – x;

   }

};

void main()

{

   B obj2;

   clrscr();

   cout << "Result is = "<< obj2.sub(2):

}

Output

Result is =  3

 

For certain specific application one can declare either a friend function or a friend class. But if all the functions or classes are declared as friend then it will lose the purpose of data encapsulation and data hiding.

Example: 1

Write a C++ program to assign 'n' projects to 'm' programmers based on skill set of programmers using friend function. Use static variable to count number of assignments.

Solution :

#include <iostream>

using namespace std;

class Programmers;

class Projects

{

   static int n;

public:

   void setassignments (int i){n=i;}

   friend void display (Projects,Programmers);

};

class Programmers

{

   static int m;

public:

   void setassignments (int i) {m=i;}

   friend void display(Projects,Programmers);

};

void display(Projects N,Programmers M)

{

   cout<<"\n number of assignments in projects are: ";

   cout<<N.n;

   cout<<"\n number assignments allocated to programmers are: ";

   cout<<M.m;

}

int Projects::n;

int Programmers::m;

void main()

{

   Projects prj;

   prj.setassignments(10);

   Programmers prm;

   prm.setassignments(5);

   display(prj,prm);

}

 

Example: 2

Write a C++ program to solve eight queens problem with friend function

Solution :

/**************************************************************************

This program is to implement n‒queens problem using backtracking and friend function

**************************************************************************/

#include<iostream>

#include<cmath>

using namespace std;

class Backtrack

{

private:

   int x[20];

public:

   Backtrack(){}

   void print board (int);

   friend void Queen(int,int,Backtrack);  // Declaring the Queen function as Friend function

   int place(int,int);

};

/* This function is for printing the solution to n‒queens problem */

void Backtrack:: print board(int n)

{

   cout<<"\n\nSolution....\n";

   for(int i=1;i<=n;i++)

   {

      cout<<<<"                "<<i;

   }

   for(i=1;i<=n;i++)

   {

      cout<<"\n\n"<<i;

      for(int j=1;j<=n;j++)// for nXn board

      {

      if(x[i]= =j)

         cout<<"               Q";//Queen at i,j position

      else

         cout<<"               ‒";// empty slot

}

}

   cout<<"\n Press any key to continue...";

}

/* This function is for checking for the conflicts.  If there is no conflict for the desired position  it returns 1 otherwise it returns 0 */

int Backtrack::place (int row,int column)

int i;

for(i=1;i<=row‒1;i++)

{ //checking for column and diagonal conflicts

   if(x[i] = = column)

   return 0;

   else

   if(abs(x[i] ‒ column) = = abs(i ‒ row))

      return 0;

}

//no conflicts hence Queen can be placed

return 1;

}

/* By this function we try the next free slot  and check for proper positioning of queen */

void Queen(int row,int n,Backtrack obj)

{

   int column;

   for(column=1;column<=n;column++)

   if(obj.place(row,column))

   {

   obj.x[row] = column;

   if(row= = n)

   {

      obj.print_board(n); //printing the board configuration

   }

   else //try next queen with next position

      Queen(row+1,n,obj);

}

}

}

void main()

{

   int n,

   Backtrack obj;

   cout<<"\n\t Program for n‒Queens Using Backtracking";

   cout<<"\nEnter Number of Queens";

   cin>>n;

   Queen(1,n,obj);//trace using backtrack

}

Note : For the sake of simplicity giving the solution of 4 queeens problem. The above program also works for 8 queens

Output

Solution....

               1           2             3             4

1             ‒            Q            ‒              ‒

2             ‒             ‒            ‒               Q

3            Q            ‒             ‒               ‒

4             ‒             ‒            Q              ‒

Press any key to continue...

Solution....

              1           2             3             4

1             ‒            ‒            Q              ‒

2             Q             ‒            ‒               ‒

3            ‒            ‒             ‒                Q

4             ‒             Q            ‒              ‒

Press any key to continue...

 

Example: 3

Write a member function and friend function to subtract two complex numbers in C++.

Solution :

#include<iostream>

#include<cmath>

using namespace std;

class complex

{

public:

   float real,img:

   complex(){real=0;img=0;}

   void getData()

   {

      cout<<"\n Enter real value: ";

      cin>>real;

      cout<<"\n Enter imaginary value:" ;

      cin>>img;

   }

   friend complex complex::sub(complex obj1,complex obj2)

   {

      complex temp;

      temp.real=obj1.real‒obj2.real;

      temp.img=obj1.img‒obj2.img;

      return temp;

   }

   void display()

   {

      if(img<0)

         cout<<real<<"‒i"<<abs(img);

      else

         cout<<real<<"+i"<<img;

   }

};

void main()

{

   complex a,b;

   a.getData();

   b.getData();

   complex c=sub(a,b);

   cout<<"\n First Number: ";

   a.display();

   cout<<"\n Second Number: ";

   b.display();

   cout<<"\n The subtraction of two complex numbers is...";

   c.display();

}

Output

Enter real value: 10

Enter imaginary value: 4

Enter real value: 3

Enter imaginary value: 2

First Number: 10+i4

Second Number: 3+i2

The subtraction of two complex numbers is... 7+i2

 

Example: 4

Write a program for swapping two numbers using friend function.

Solution :

#include<iostream>

using namespace std;

class Test

{

private

   int x, y,

public:

   void setdata(int a, int b)

   {

      x= a;

      y = b;

   }

void display()

{

   cout << "x="<<x<< "y="<<y;

}

   friend void swap(Test &t);

};

void swap(Test &t)

{

   int temp;

   temp = t.x;

   t.x = t.y;

   t.y = temp;

}

void main()

{

   Test obj;

   int x1, x2;

   cout << "\nInput two Numbers:";

   cin >> x1 >> x2;

   obj.setdata(x1, x2);

   cout << "\nBefore Swapping\n";

   obj.display();

   swap(obj);

   cout << "\nAfter Swapping\n";

   obj.display();

}

Output

Input two Numbers:10 20

Before Swapping

x=10 y=20

After Swapping

x=20 y=10

 

Review Questions

1. Define friend class and specify its importance. Explain with suitable example.

2. List the characteristics of friend function.