C++ Functions

What is a Function?

A function is a reusable block of code that performs a specific task. Functions reduce repetition and make code modular.

// Declaration (prototype)
returnType functionName(parameters);

// Definition
returnType functionName(parameters) {
    // body
    return value;
}

#include <iostream>
using namespace std;

void greet() {
    cout << "Hello from a function!" << endl;
}

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

Output:

Hello from a function!
Hello from a function!

Parameters and Arguments

void printInfo(string name, int age) {
    cout << name << " is " << age << " years old." << endl;
}

int main() {
    printInfo("Alice", 30);
    printInfo("Bob",   25);
    return 0;
}

Default Parameters

void greet(string name = "Guest") {
    cout << "Hello, " << name << "!" << endl;
}

greet();          // Hello, Guest!
greet("Alice");   // Hello, Alice!

Return Values

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

double circleArea(double r) {
    return 3.14159 * r * r;
}

int main() {
    cout << add(5, 3)            << endl;   // 8
    cout << circleArea(5.0)      << endl;   // 78.54
    return 0;
}

Function Overloading

Multiple functions can share the same name if they have different parameter lists:

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

cout << multiply(3, 4);       // 12    (int version)
cout << multiply(2.5, 3.0);   // 7.5  (double version)

#include <iostream>
using namespace std;

bool isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int main() {
    for (int i = 2; i <= 50; i++) {
        if (isPrime(i)) cout << i << " ";
    }
    cout << endl;
    return 0;
}

Summary

• Functions are reusable code blocks with a name, parameters, and return type.
• Use void for functions that return nothing.
• Default parameters allow optional arguments.
• Function overloading enables same name, different signatures.