C Programming - Beginner to Brilliance / Structures and Unions

Differences between Structures & Unions

📌 Definition Recap

Concept

Description

🧱 Structure (struct)

A collection of variables (members) of different types, stored in separate memory locations, but grouped under a single name.

🧬 Union (union)

Similar to a structure, but all members share the same memory location, and only one member can hold a value at a time.

🧠 Memory Allocation – The Core Difference

🔍 In a struct:

Each member has its own memory location.

struct Sample {
    int a;      // 4 bytes
    float b;    // 4 bytes
    char c;     // 1 byte
};

Total size = sum of all members + padding (usually around 12 bytes)

🔍 In a union:

All members share the same memory location — the size of the largest member decides the total size.

union Sample {
    int a;      // 4 bytes
    float b;    // 4 bytes
    char c;     // 1 byte
};

Total size = 4 bytes (largest of the three)

📊 Comparison Table

🔢 Feature

🧱 Structure

🧬 Union

🔧 Memory

Sum of all members' sizes (+ padding)

Size of the largest member

🧠 Storage

Each member has a separate location

All members share the same memory

📌 Active members

All can store and hold values simultaneously

Only one member holds value at a time

🔁 Value Retention

Value of all members is retained

Assigning a value to one member will overwrite others

📥 Initialization

All members can be initialized

Only the first member can be initialized at declaration

🧾 Use Cases

Complex data records (e.g., Employee)

Space-saving when only one field is needed at a time

🧰 Efficiency

Less memory efficient

More memory efficient

🧪 Syntax

struct Student { int id; };

union Data { int num; };

🎯 When to Use What?

Situation

Use

You need to store multiple pieces of information together

✅ Use Structure

You need to store only one of many types to save space

✅ Use Union

📦 Real-Life Analogy

Concept

Analogy

Structure

A lunchbox with compartments: rice, curry, salad — all stored separately but together

Union

A single glass where you can pour either water or juice, but not both at the same time