What are functions?

Function is named code block for one task.

Write once. Call it whenever needed.

Coffee machine analogy. Press button, get coffee.

Why do functions exist?

• Reduce code duplication
• Improve readability
• Simplify debugging
• Make programs easier to maintain
• Encourage code reuse

Real-world use cases

• Banking applications
• Calculator programs
• Game development
• Student management systems
• File processing
• Scientific calculations
• Web applications
• Data processing software

Core concepts

• Function declaration (prototype)
• Function definition
• Function call
• Return value (optional)

Function declaration (prototype)

Tells compiler about function before use.

return_type functionName(parameter_list);

int add(int, int);

Function definition

Holds the actual implementation.

return_type functionName(parameters)
{
    // Function body
}

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

Function call

int result = add(10, 20);

Function parameters

Parameters receive values passed in.

void greet(string name)

Passed value is called an argument.

Return type

return sends a value back.

double calculateArea(double radius)
{
    return 3.14159 * radius * radius;
}

Use void if function returns nothing.

Types of functions

Type                           | Description
------------------------------ | -----------------------------------
No parameters, no return value | Runs a task, no input or output
Parameters, no return value    | Takes input, returns nothing
No parameters, returns value   | Returns result, no input
Parameters and return value    | Takes input, returns a result

Function scope

Local vars exist only inside their function.

Recursion

Recursive function calls itself for smaller subproblems.

Must have a base case to stop.

Example 1: Function without parameters

#include <iostream>
using namespace std;

void greet()
{
    cout << "Welcome to C++!";
}

int main()
{
    greet();

    return 0;
}

Example 2: Function with parameters

#include <iostream>
using namespace std;

void displaySum(int a, int b)
{
    cout << a + b;
}

int main()
{
    displaySum(15, 20);

    return 0;
}

Example 3: Function returning a value

#include <iostream>
using namespace std;

int square(int number)
{
    return number * number;
}

int main()
{
    int result = square(8);
    cout << result;

    return 0;
}

Example 4: Finding the largest number

#include <iostream>
using namespace std;

int largest(int a, int b)
{
    if (a > b)
    {
        return a;
    }

    return b;
}

int main()
{
    cout << largest(45, 72);

    return 0;
}

Example 5: Recursive factorial

#include <iostream>
using namespace std;

int factorial(int number)
{
    if (number <= 1)
    {
        return 1;
    }

    return number * factorial(number - 1);
}

int main()
{
    cout << factorial(5);

    return 0;
}

Common mistakes and pitfalls

• Calling before declaring. Fix: declare first
• Missing return in non-void. Fix: return a value
• Wrong argument count. Fix: match parameters
• Local vars used outside scope. Fix: keep them inside
• Infinite recursion. Fix: add a base case

// Wrong
int square(int number)
{
    number * number;
}

// Correct
int square(int number)
{
    return number * number;
}

Best practices

• Each function does one task
• Use descriptive function names
• Avoid overly long functions
• Pass only needed parameters
• Return values over global variables
• Reuse functions, avoid duplication

When not to use this

• Functions doing too many tasks
• Tiny functions adding complexity
• Excessive nested function calls
• Recursion where loops fit better

Summary

• Functions split programs into reusable parts
• Can take params, return values, both, neither
• Declaration informs compiler beforehand
• Definition holds the real code
• Call runs the function
• Local vars stay in their function
• Recursion needs a base case
• Good functions aid readability and reuse

Frequently asked questions

What is a function in C++?

Named code block doing a specific task.

What is the difference between a parameter and an argument?

Parameter is declared. Argument is passed value.

What is the purpose of the return statement?

Ends function, may send back a value.

What is recursion?

Function calling itself on smaller subproblems.

When should I use void functions?

Use when function acts but returns nothing.