C++ Functions

Defining a Function

#include <iostream>
using namespace std;

// returnType functionName(parameters) { body }
int add(int a, int b) {
    return a + b;
}

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

int main() {
    cout << add(3, 4) << endl;          // 7
    cout << circleArea(5.0) << endl;    // 78.539...
    return 0;
}

Pass by Value vs Reference

void doubleByValue(int x) { x *= 2; }  // copy, original unchanged
void doubleByRef(int& x)  { x *= 2; }  // reference, original changes

int n = 5;
doubleByValue(n);  // n still 5
doubleByRef(n);    // n is now 10

Default Arguments

void greet(string name, string greeting = "Hello") {
    cout << greeting << ", " << name << "!" << endl;
}
greet("Alice");           // Hello, Alice!
greet("Bob", "Hi");       // Hi, Bob!

Function Overloading

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

multiply(2, 3);       // int version → 6
multiply(2.5, 3.0);   // double version → 7.5

Recursion

int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}
// factorial(5) → 120

Inline Functions

inline int square(int x) { return x * x; }
// Compiler may expand this at the call site for performance
cout << square(4);  // 16