Unions in C

In the previous lessons, you learned about structures and nested structures.

Now we study a concept that looks similar to structures, but behaves very differently in memory.

This concept is called a union.

Why Do We Need Unions?

Sometimes, a program needs to store:

• Only one value at a time
• But of different possible types

Using a structure wastes memory in such cases.

Unions help optimize memory usage.

What Is a Union?

A union is a user-defined data type like a structure.

But the key difference is:

All members of a union share the same memory location.

Only one member can hold a valid value at a time.

Defining a Union

union Data {
    int i;
    float f;
    char c;
};

This defines a union with three members.

Creating a Union Variable

union Data d1;

Memory is allocated based on the largest member.

Accessing Union Members

Access members using the dot operator, just like structures:

d1.i = 10;
d1.f = 3.14;

But remember:

Storing a new value overwrites the previous one.

Complete Example

#include <stdio.h>

union Data {
    int i;
    float f;
    char c;
};

int main() {
    union Data d;

    d.i = 10;
    printf("Integer: %d\n", d.i);

    d.f = 3.5;
    printf("Float: %.2f\n", d.f);

    d.c = 'A';
    printf("Char: %c\n", d.c);

    return 0;
}

Each assignment replaces the previous value.

Union vs Structure (Conceptual)

Key differences:

• Structure → each member has separate memory
• Union → all members share memory

Structure size = sum of member sizes (plus padding) Union size = size of largest member

Memory Understanding

Example:

• int → 4 bytes
• float → 4 bytes
• char → 1 byte

Union size will be 4 bytes.

Structure size would be larger.

Real-World Analogy

Think of a TV remote display:

• It can show channel number
• Or volume level
• Or battery status

But only one thing is shown at a time.

That is how a union works.

Where Unions Are Used

• Embedded systems
• Low-level hardware programming
• Protocol parsing
• Memory-efficient applications

Mini Practice

• Create a union with int and char
• Assign values one by one
• Print after each assignment

Quick Quiz

Q1. What is the main purpose of a union?

To save memory by sharing storage.

Q2. Can a union store multiple values at once?

No.

Q3. Which determines the size of a union?

The largest member.

Q4. Do unions and structures use the same syntax?

Yes.

Q5. Are unions useful in real applications?

Yes, especially where memory is limited.