C++ Functions - Pass By Reference

Pass by Reference is a method of passing arguments to a function where the function receives a reference (alias) to the original variable instead of a copy. This allows the function to directly modify the original value stored in memory.

• No copy of the variable is created.
• Changes made inside the function affect the original variable.
• Improves performance when working with large objects because copying is avoided.

#include <iostream>
using namespace std;

void updateValue(int &num){
    
    num = num + 10;
}

int main(){
    
    int number = 20;

    cout << "Before Function Call: " << number << endl;

    updateValue(number);

    cout << "After Function Call: " << number << endl;

    return 0;
}

Before Function Call: 20
After Function Call: 30

Explanation

• number contains the value 20.
• The function receives number as a reference using &.
• Any modification made to num directly affects number.
• After the function call, the original value becomes 30.

Syntax

returnType functionName(dataType &parameter){

// Function body

}

#include <iostream>
using namespace std;

void func(int& x) {
  x--;
}

int main() {
  int a = 5;
  cout << a << endl;
  func(a);
  cout << a;
}

5
4

Explanation: The function func() receives an integer by reference. Since x refers to the original variable a, the statement x-- directly decreases a by 1. As a result, the value of a changes from 5 to 4, demonstrating that pass by reference allows a function to modify the original variable.

Swap function using Pass-By-Reference

The swap function swaps the values of the two variables it receives as arguments. Below is the C++ program to swap the values of two variables using pass-by-reference.

#include <bits/stdc++.h>
using namespace std;

// Swap function
void swap(int &a, int &b) {
    int temp;
    temp = b;
    b = a;
    a = temp;
}

int main() {
    int x = 3, y = 7;

    // Before swapping
    cout << "Before Swapping: " 
       << endl;
    cout << "x: " << x << " y: " 
       << y << endl;

    // Call the function
    swap(x, y);

    // After swapping
    cout << "After Swapping: " 
       << endl;
    cout << "x: " << x << " y: " 
       << y;
    return 0;
}

Before Swapping: 
x: 3 y: 7
After Swapping: 
x: 7 y: 3

Explanation: In this program, the swap function swaps the values of two variables by passing them as references. When the function is called, it modifies the values of a and b directly. When we print the value of x and y after calling swap function, x value is 7 and y value is 3.

Pass by Reference vs Pass by Value

The following table compares the key differences between Pass by Reference and Pass by value in C++: