Functions in C++ | Void & Return Types Explained With Examples

What You’ll Learn

By the end of this post, you will:

Understand what functions are and why they’re used
Learn how to define and call functions in C++
Use parameters and return values effectively
Differentiate between pass-by-value and pass-by-reference

What is a Function?

A function is a reusable block of code that performs a specific task. It allows you to avoid repeating code and helps keep programs clean and organized.

Every C++ program starts with a function called main() — but you can create your own functions too.

Why Use Functions?

Imagine you’re calculating the square of a number in five different places. Instead of writing the same code five times, you can write it once as a function and call it whenever needed.

Function Basics

General Syntax:

return_type function_name(parameter_list) {
    // code block
    return value;
}

return_type – Type of value the function returns (int, void, etc.)
function_name – Name used to call the function
parameter_list – Inputs (if any)
return – Sends a value back to the caller

Example 1: A Simple Function

#include <iostream>
using namespace std;

void greet() {
    cout << "Welcome to Painless Programming!" << endl;
}

int main() {
    greet();  // function call
    greet();
    return 0;
}

Output:

Welcome to Painless Programming!
Welcome to Painless Programming!

Here, greet() is a function with no parameters and no return value (void).

Example 2: Function with Parameters

void sayHello(string name) {
    cout << "Hello, " << name << "!" << endl;
}

int main() {
    sayHello("Ali");
    sayHello("Sara");
    return 0;
}

Example 3: Function with Return Value

int add(int a, int b) {
    return a + b;
}

int main() {
    int result = add(5, 3);
    cout << "Sum: " << result << endl;
    return 0;
}

Function Declaration and Definition

You can declare a function before main() and define it later.

int multiply(int, int); // declaration (prototype)

int main() {
    cout << multiply(4, 5) << endl;
    return 0;
}

int multiply(int a, int b) { // definition
    return a * b;
}

This is useful when your function is defined after main().

Pass-by-Value vs. Pass-by-Reference

Pass-by-Value

Default in C++. The function gets a copy of the argument. Changes don’t affect the original.

void change(int x) {
    x = 100;
}

int main() {
    int a = 5;
    change(a);
    cout << a << endl; // Still 5
    return 0;
}

Pass-by-Reference

Pass the actual variable using reference (&). Changes affect the original.

void change(int &x) {
    x = 100;
}

int main() {
    int a = 5;
    change(a);
    cout << a << endl; // Now 100
    return 0;
}

Multiple Parameters and Return Types

You can pass as many parameters as needed, and return one value.

double average(int x, int y) {
    return (x + y) / 2.0;
}

C++ does not allow returning multiple values directly from a function, but you can simulate it using references or structures (covered in later posts).

Quiz: Functions

Question 1/7

Mini Exercises

Write a function isEven(int x) that returns true if a number is even.
Create a function factorial(int n) that returns the factorial of n.
Write a function swap(int &a, int &b) that swaps the values of two variables.
Define a function that prints the multiplication table of a given number.

Common Mistakes to Avoid

Forgetting to return a value in non-void functions
Using pass-by-value when you actually need to modify the original
Not declaring functions before main() if defining them after

Summary

In this post, you learned:

How to define and use your own functions in C++
How to use parameters and return values
The difference between pass-by-value and pass-by-reference
How functions can simplify and organize your code

What’s Next?

In the next post, we’ll dive into Memory in C++, including stack vs heap memory, pointers, references, and dynamic memory allocation.