Structures and Unions

User-Defined Types: Structures and Unions in C++

The Lunchbox Analogy 🍱

Imagine you have a lunchbox. Inside, you can organize different foods in separate compartments: a sandwich here, an apple there, and some cookies in another spot. Each compartment holds something different, but they all travel together as ONE lunchbox.

That’s exactly what a Structure is in C++!

Now imagine a different lunchbox—a magical one. It only has ONE compartment, but that compartment can transform to hold EITHER a sandwich OR an apple OR cookies—but only ONE thing at a time.

That’s a Union!

Let’s dive into this delicious journey of organizing data!

Part 1: Structures (struct)

What is a Structure?

A structure is like a custom container you design yourself. You decide what goes inside and give each compartment a name.

Real Life Example:

• A Student ID Card has: Name, Age, Roll Number, Grade
• All these pieces of info travel together as ONE card
• That’s a structure!

Creating Your First Structure

struct Student {
    string name;
    int age;
    int rollNumber;
    char grade;
};

Think of this as designing your lunchbox blueprint:

• name = compartment for the sandwich
• age = compartment for the apple
• rollNumber = compartment for cookies
• grade = compartment for juice

Using a Structure

Student alice;         // Create a lunchbox
alice.name = "Alice";  // Put sandwich in
alice.age = 10;        // Put apple in
alice.rollNumber = 42; // Put cookies in
alice.grade = 'A';     // Put juice in

The dot (.) is like opening a specific compartment!

Why Use Structures?

Without structures, you’d carry everything separately:

// Messy way - like carrying food
// in separate bags
string name1 = "Alice";
int age1 = 10;
string name2 = "Bob";
int age2 = 11;
// Gets confusing fast!

With structures, everything is organized:

// Clean way - one lunchbox per person
Student alice;
Student bob;
// Much better!

Structure Initialization

Method 1: One by one

Student s1;
s1.name = "Emma";
s1.age = 9;

Method 2: All at once (like packing lunch the night before)

Student s2 = {"Liam", 10, 101, 'B'};

Method 3: Named initialization (C++20)

Student s3 = {
    .name = "Noah",
    .age = 11,
    .rollNumber = 102,
    .grade = 'A'
};

Structures with Functions

Structures can travel to functions just like regular variables!

void printStudent(Student s) {
    cout << "Name: " << s.name;
    cout << " Age: " << s.age;
}

// Using it:
Student myStudent = {"Zoe", 8, 55, 'A'};
printStudent(myStudent);

Nested Structures

A lunchbox inside a lunchbox? Yes!

struct Date {
    int day;
    int month;
    int year;
};

struct Student {
    string name;
    Date birthday;  // Structure inside!
};

Student kid;
kid.name = "Max";
kid.birthday.day = 15;
kid.birthday.month = 6;
kid.birthday.year = 2014;

Array of Structures

Many lunchboxes in a row!

Student classroom[30];  // 30 students!

classroom[0].name = "First Student";
classroom[1].name = "Second Student";

Part 2: Unions

What is a Union?

Remember the magical transforming compartment? A union is exactly that—ONE storage space that can hold different types of data, but only ONE at a time.

Real Life Example:

• A parking spot can hold a car OR a motorcycle OR a bicycle
• But only ONE vehicle at a time!
• That’s a union!

Creating a Union

union Container {
    int number;
    float decimal;
    char letter;
};

The Key Difference from Structures

Structure (struct):          Union:
┌─────────┬─────────┐       ┌─────────┐
│ number  │ decimal │       │ number  │
├─────────┼─────────┤   OR  │ decimal │
│ letter  │ (empty) │       │ letter  │
└─────────┴─────────┘       └─────────┘
All compartments exist!     ONE space, shared!

Memory Magic

Structure: Each member gets its own space

struct S {
    int a;    // 4 bytes
    float b;  // 4 bytes
    char c;   // 1 byte
};
// Total: ~9+ bytes

Union: All members SHARE the same space

union U {
    int a;    // 4 bytes
    float b;  // 4 bytes
    char c;   // 1 byte
};
// Total: 4 bytes (size of largest)

Using a Union

union Value {
    int whole;
    float decimal;
};

Value v;
v.whole = 42;       // Store an integer
cout << v.whole;    // Works! Shows 42

v.decimal = 3.14;   // Now store a float
// v.whole is now GARBAGE!
// Only v.decimal is valid

Warning: Only the LAST thing you put in is valid!

When to Use Unions?

1. Saving Memory - When you have limited space
2. Either/Or Data - When something can be one type OR another
3. Low-level Programming - Working close to hardware

Practical Union Example

union IPAddress {
    unsigned int full;
    unsigned char parts[4];
};

IPAddress ip;
ip.full = 3232235777;  // One number

// Access individual parts!
cout << (int)ip.parts[0]; // 192
cout << (int)ip.parts[1]; // 168
cout << (int)ip.parts[2]; // 1
cout << (int)ip.parts[3]; // 1
// That's 192.168.1.1!

Structures vs Unions: Quick Comparison

graph TD
    A["Need to store data?"] --> B{Multiple values<br>at SAME time?}
    B -->|Yes| C["Use STRUCT"]
    B -->|No, only one| D["Use UNION"]
    C --> E["Each member&lt;br&gt;has own space"]
    D --> F["All members&lt;br&gt;share space"]

Fun Challenge: Build a Game Character!

struct Position {
    int x;
    int y;
};

struct Character {
    string name;
    int health;
    Position location;
};

Character hero;
hero.name = "Knight";
hero.health = 100;
hero.location.x = 0;
hero.location.y = 0;

You just created a game character with a name, health bar, and position on the map—all packed neatly in one structure!

Key Takeaways

1. Structures = Multiple compartments, all available at once
2. Unions = One compartment that transforms, only one thing at a time
3. Use dot notation (.) to access members
4. Structures are for grouping related data
5. Unions are for saving memory when you need either/or storage

You Did It! 🎉

You now understand how to create your own custom data types in C++! Structures help you organize related information together, while unions help you save memory when you only need one value at a time.

Think of it this way:

• struct = A lunchbox with compartments
• union = A magical single compartment that shape-shifts

Now go build something amazing with your new knowledge! 🚀