C++ Functions: Pass By Reference

Welcome to The Coding College! In this tutorial, we will explore the concept of pass by reference in C++ and how it enables functions to modify the original variables passed to them.

What Is Pass By Reference?

In pass by reference, instead of passing a copy of the variable, a function receives a reference (alias) to the original variable. This allows the function to directly modify the value of the original variable.

Key Features:

• No additional memory is used for copying data.
• Enables functions to modify the caller’s variables.
• Efficient for large objects or arrays.

Syntax for Pass By Reference

Use the & symbol to declare a reference parameter in the function.

return_type function_name(parameter_type ¶meter_name) {
    // Function body
}

• &: Indicates the parameter is passed by reference.
• The function operates directly on the original variable.

Example: Pass By Reference

#include <iostream>
using namespace std;

// Function to swap two numbers
void swap(int &a, int &b) {
    int temp = a;
    a = b;
    b = temp;
}

int main() {
    int x = 5, y = 10;

    cout << "Before swapping: x = " << x << ", y = " << y << endl;

    swap(x, y); // Passing by reference

    cout << "After swapping: x = " << x << ", y = " << y << endl;

    return 0;
}

Output:

Before swapping: x = 5, y = 10  
After swapping: x = 10, y = 5  

Benefits of Pass By Reference

1. Efficiency: No memory overhead for copying large objects.
2. Direct Modification: Allows functions to modify the original variables.
3. Consistency: Avoids the need to return multiple values.

Practical Use Cases

1. Updating Values

#include <iostream>
using namespace std;

void updateValue(int &value) {
    value += 10; // Modifies the original variable
}

int main() {
    int number = 5;

    cout << "Before update: " << number << endl;

    updateValue(number); // Pass by reference

    cout << "After update: " << number << endl;

    return 0;
}

Output:

Before update: 5  
After update: 15  

2. Multiple Outputs Using References

#include <iostream>
using namespace std;

void calculate(int a, int b, int &sum, int &product) {
    sum = a + b;
    product = a * b;
}

int main() {
    int x = 5, y = 10, sum, product;

    calculate(x, y, sum, product); // Pass by reference for sum and product

    cout << "Sum: " << sum << ", Product: " << product << endl;

    return 0;
}

Output:

Sum: 15, Product: 50  

Difference Between Pass By Reference and Pass By Value

Const References

To prevent a function from modifying a referenced variable, use the const keyword.

Example: Const Reference

#include <iostream>
using namespace std;

void display(const int &value) {
    cout << "Value: " << value << endl;
    // value += 10; // Error: Cannot modify a const reference
}

int main() {
    int number = 20;

    display(number); // Pass by const reference

    return 0;
}

Benefits of Const References:

• Prevents accidental modification.
• Enables the function to accept temporary objects.

When to Use Pass By Reference

1. When the function needs to modify the original variable.
2. When working with large objects to avoid the overhead of copying.
3. When returning multiple values through reference parameters.

Explore More on The Coding College

Pass by reference is an essential feature for writing efficient and flexible C++ programs. To learn more about advanced topics like move semantics, rvalue references, and templates, visit The Coding College.