1 Functions in C++

1.1 Purpose

Functions encapsulate specific operations, reducing code duplication and improving modularity.

1.2 Defining and Calling Functions

Syntax for Definition:

return_type function_name(parameter list) { functon body return expression; }

Example: Sum of Two Integers

int add(int a, int b) {
    // a and b are formal parameters
    return a + b;
}

Calling a Function:

function_name(argument list);

int main() {
    int x = 4, y = 5;
    int result = add(x, y);  // x and y are actual arguments
    cout << result;
    return 0;
}

1.3 Pass by Value

• During a function call, the values of actual arguments are copied into formal parameters.
• Modifying formal parameters does not affect actual arguments.

#include<iostream>
using namespace std;

// Function to swap two numbers using pass by value
void swap(int n1, int n2) {
    cout << "Before swap:\n";
    cout << "n1 = " << n1 << ", n2 = " << n2 << endl;  // Prints 4, 5
    int temp = n1;
    n1 = n2;
    n2 = temp;
    cout << "After swap:\n";
    cout << "n1 = " << n1 << ", n2 = " << n2 << endl;  // Prints 5, 4
}

int main() {
    int a = 4, b = 5;
    swap(a, b);
    cout << "a = " << a << ", b = " << b << endl;  // Prints 4, 5
    return 0;
}

1.4 Common Function Patterns

• With parameters, no return value
• With parameters, with return value
• No parameters, with return value
• No parameters, no return value

1.5 Function Declaration (Prototype)

• Informs the compiler about the function's name and how to call it.
• The actual definition can appear later in the code.
• Declarations can appear multiple times; definitions only once.

#include<iostream>
using namespace std;

// Declaration: tells compiler that max exists
int max(int num1, int num2);

int main() {
    int a = 4, b = 5;
    int m = max(a, b);
    cout << "m = " << m << endl;
    return 0;
}

// Definition appears after main
int max(int num1, int num2) {
    return (num1 > num2) ? num1 : num2;
}

1.6 Splitting Functions Across Files

Purpose: Improves code organization.

Steps:

1. Create a header file (.h).
2. Create a source file (.cpp).
3. Write function declarations in the header.
4. Write function definisions in the source file.

Example: Sum of Two Integers

add.h

#pragma once
int add(int num1, int num2);

add.cpp

#include "add.h"

int add(int num1, int num2) {
    return num1 + num2;
}

main.cpp

#include <iostream>
#include "add.h"
using namespace std;

int main() {
    int x = 4, y = 5;
    int sum = add(x, y);
    cout << "Sum = " << sum << endl;
    return 0;
}