Mastering C++: From Fundamentals to Advanced Programming

Chapter 4: Functions and Scope in C++

🔹 1. Introduction

As your C++ programs grow, you’ll need a way to reuse logic, organize code, and reduce repetition — that's where functions come in.

Functions help divide complex problems into manageable blocks, making code:

• Modular
• Reusable
• Maintainable

In this chapter, we’ll also explore scope — understanding where variables can be accessed and how they behave.

🔹 2. What is a Function?

A function is a reusable block of code that performs a specific task.

✅ General Syntax:

returnType functionName(parameters) {

    // code

    return value;

}

🔹 3. Function Example

#include <iostream>

using namespace std;

int add(int a, int b) {

    return a + b;

}

int main() {

    cout << add(5, 3);  // Output: 8

    return 0;

}

🧠 Key Concepts:

• int add(int a, int b) is a function definition
• int is the return type
• add(5, 3) is the function call

🔹 4. Function Declaration & Definition

• Declaration (Prototype):

int add(int, int);  // Just declares it

• Definition:

int add(int x, int y) {

    return x + y;

}

✅ You can declare functions at the top and define them later in the file — useful for organizing large projects.

🔹 5. Return Type & Parameters

🔹 6. Default Arguments

void greet(string name = "Guest") {

    cout << "Hello, " << name;

}

int main() {

    greet();        // Hello, Guest

    greet("Alice"); // Hello, Alice

}

✅ Parameters can have default values, which are overridden when arguments are passed.

🔹 7. Function Overloading

C++ allows functions with same name but different parameter types or counts.

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

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

cout << add(2, 3);        // Calls int version

cout << add(2.5f, 3.5f);  // Calls float version

✅ Compile-time polymorphism in action.

🔹 8. Recursive Functions

A function that calls itself is called recursive.

int factorial(int n) {

    if (n <= 1) return 1;

    return n * factorial(n - 1);

}

Use recursion carefully to avoid stack overflows!

🔹 9. Scope in C++

Scope defines where a variable can be accessed or modified.

✅ Local Example:

void greet() {

    int age = 30;  // Local

}

✅ Global Example:

int x = 5;

void show() {

    cout << x;  // Accessible

}

✅ Static Variable:

void counter() {

    static int count = 0;

    count++;

    cout << count << " ";

}

✅ Output on 3 calls: 1 2 3

🔹 10. Inline Functions (Compiler Suggestion)

inline int square(int x) {

    return x * x;

}

✅ The compiler may replace the function call with the actual code to optimize performance (used for small functions).

🔹 Summary Table