C++ Program to swap two members using Friend Function

Pre-requisite: Friend Function
CASE 1: 
Given two numbers a & b, swap these two numbers using the friend function of C++. 

Examples:

Input : a = 5, b = 9
Output : a = 9, b = 5

Input : a = 4, b = 6
Output : a= 6, b = 4

Approach: 
Create a class Swap, declare three variables in it, i.e., a, b, and temp and create a constructor for inputs. Declare a friend function in it. Define the friend function outside the class scope by taking arguments as call by reference to pass the copy of Swap Object. Perform the swap operation with Swap variables.

C++

// C++ Program to swap two numbers using friend function
#include <iostream>

using namespace std;

class Swap {

    // Declare the variables of Swap Class
    int temp, a, b;

public:

    // Define the parameterized constructor, for inputs
    Swap(int a, int b)
    {
        this->a = a;
        this->b = b;
    }

    // Declare the friend function to swap, take arguments
    // as call by reference
    friend void swap(Swap&);
};

// Define the swap function outside class scope
void swap(Swap& s1)
{
    // Call by reference is used to passed object copy to
    // the function
    cout << "\nBefore Swapping: " << s1.a << " " << s1.b;

    // Swap operations with Swap Class variables
    s1.temp = s1.a;
    s1.a = s1.b;
    s1.b = s1.temp;
    cout << "\nAfter Swapping: " << s1.a << " " << s1.b;
}

// Driver Code
int main()
{
    // Declare and Initialize the Swap object
    Swap s(4, 6);
    swap(s);
    return 0;
}

Output:

Before Swapping: 4 6
After Swapping: 6 4

CASE 2: 
Given two objects s1 & s2 of a class, swap the data members of these two objects using friend function of C++. 

Examples:

Input : a = 6, b = 10
Output : a = 10, b = 6

Input : a = 4, b = 6
Output : a= 6, b = 4

Approach: 
Create a class Swap, declare one variable in it, i.e., num and create a constructor for inputs. Declare a friend function in it. Define the friend function outside the class scope by taking arguments as call by reference to pass the copy of Swap Object. Perform the swap operation.

C++

//C++ Program to swap data members of two objects of a class using friend function.
#include <iostream>

using namespace std;

class Swap {

    // Declare the variable of Swap Class
    int num;

public:

    // Define the parameterized constructor, for input.
    Swap(int num)
    {
        this->num = num;
    }

    // Declare the friend function to swap, take arguments
    // as call by reference
    friend void swap(Swap&, Swap&);
};

// Define the swap function outside class scope
void swap(Swap& s1, Swap& s2)
{
    // declare a temporary variable.
    int temp;
    // Call by reference is used to passed object copy to
    // the function
    cout << "\nBefore Swapping: " << s1.num << " " << s2.num;

    // Swap operations with objects of class Swap
    temp = s1.num;
    s1.num = s2.num;
    s2.num = temp;
    cout << "\nAfter Swapping: " << s1.num << " " << s2.num;
}

// Driver Code
int main()
{
    // Declare and Initialize the objects of Swap class
    Swap s1(6), s2(10);
    swap(s1,s2);
    return 0;
}

Output:

Before Swapping: 6 10
After Swapping: 10 6

Time Complexity: O(1)
Auxiliary Space: O(1)