Unions

In the previous topic, we learned about structures, which allow us to store different types of data together.

A union also allows us to group different data types, but there is one important difference: all members of a union share the same memory location.

What is a Union?

A union is a user-defined data type in C where all members share the same memory location.

It is defined using the union keyword:

union Data
{
    int number;
    float price;
    char grade;
};

We can create a union variable:

union Data data;

How Does a Union Work?

Consider:

union Data
{
    int number;
    float price;
};

If we store a value in number:

data.number = 10;

we can access number.

But if we then store:

data.price = 20.5;

the same memory is used for price, so the previously stored number value is no longer reliably available.

Think of it like a single box that can hold different types of things, but only one is meant to be stored at a time.

Example

#include <stdio.h>

union Data
{
    int number;
    float price;
};

int main()
{
    union Data data;

    data.number = 10;
    printf("Number: %d\n", data.number);

    data.price = 20.5;
    printf("Price: %.1f\n", data.price);

    return 0;
}

Output:

Number: 10
Price: 20.5

The important point is that number and price use the same memory area.

Union vs Structure

This is the most important difference to remember.

For example:

struct Data
{
    int number;
    float price;
};

Both number and price have their own storage.

But:

union Data
{
    int number;
    float price;
};

They share the same storage.

Why Use Unions?

Unions are useful when:

• Only one member needs to be used at a time.

• Memory efficiency is important.

• Working with low-level or embedded systems.

• Representing data that can have different possible types.

In Simple Words

A structure stores all its members separately, while a union makes all its members share the same memory location.

The easiest way to remember:

Structure → Separate memory

Union → Shared memory