C++ Functions

Why use functions?

Functions are one of the most important building blocks in C++. They help you organize a program into smaller, understandable pieces. When code is split into functions, it is easier to read, easier to test, and much easier to fix when something goes wrong. This idea of writing code once and reusing it is often called DRY: Don't Repeat Yourself.

• Reuse: write the logic once and call it many times.
• Organization: give a meaningful name to a group of statements.
• Readability: a well-named function tells the reader what the code does.
• Testing: small functions are easier to check for correctness.

Creating and calling a function

C++ already provides predefined functions, such as main(), which is where every program starts running. To create your own function, you write a return type, a name, a pair of parentheses (), and a body inside curly braces {}. To run the function, you call it by writing its name followed by parentheses.

#include <iostream>
using namespace std;

// Create a function called myFunction
void myFunction() {
  cout << "I just got executed!";
}

int main() {
  myFunction(); // Call the function
  return 0;
}

// Outputs: I just got executed!

The keyword void means the function does not return any value. The name myFunction is your own choice, but by convention function names start with a lowercase letter. Notice that defining a function does not run it; the code inside only runs when you call myFunction() from main().

#include <iostream>
using namespace std;

void greet() {
  cout << "Hello!\n";
}

int main() {
  greet();
  greet();
  greet();
  return 0;
}

// Outputs:
// Hello!
// Hello!
// Hello!

Declaration and definition

A function has two parts. The declaration (the return type, name, and parameters) tells the compiler the function exists. The definition is the body with the actual code. You can split them so that main() can appear at the top of your file while the details live below it.

#include <iostream>
using namespace std;

// Declaration (prototype)
void myFunction();

int main() {
  myFunction();
  return 0;
}

// Definition
void myFunction() {
  cout << "I was defined after main()!";
}