What is Union in C: Definition, Usage, and Examples

In C, a union is a special data type that allows storing different data types in the same memory location. Only one member of the union can hold a value at a time, sharing the same memory space for all members.

How It Works

A union in C is like a shared box where you can keep one item at a time, but the box is big enough to hold the largest item you might want to store. Unlike a struct where each member has its own space, all members of a union overlap in memory.

This means if you change one member, it affects the others because they share the same memory. Think of it as a multi-tool where only one tool can be used at a time, but all tools share the same handle.

Example

This example shows a union with an integer and a float. We assign a value to the integer member, then to the float member, and print both to see how the shared memory works.

c

#include <stdio.h>

union Data {
    int i;
    float f;
};

int main() {
    union Data data;

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

    data.f = 220.5f;
    printf("data.f = %.1f\n", data.f);

    // Now printing data.i again shows unexpected value
    printf("data.i after assigning data.f = %d\n", data.i);

    return 0;
}

Output

data.i = 10
data.f = 220.5
data.i after assigning data.f = 1120403456

When to Use

Use a union when you need to store different types of data but only one at a time, saving memory. This is useful in situations like:

• Handling different data formats in the same memory space.
• Implementing variant data types or tagged unions.
• Working with hardware registers or protocols where data interpretation changes.

It helps reduce memory usage compared to using separate variables for each type.

Key Points

• A union shares the same memory for all its members.
• Only one member can hold a meaningful value at a time.
• Memory size of a union equals the size of its largest member.
• Changing one member affects the others because of shared memory.
• Useful for saving memory when storing different types alternatively.

Key Takeaways

A union stores different data types in the same memory location, but only one at a time.

The size of a union equals its largest member's size.

Changing one union member changes the shared memory, affecting other members.

Use unions to save memory when you need to store different types alternatively.

Unions are helpful in low-level programming and handling variant data.