Unions and Memory Efficiency

A union is a special data type that allows storing different data types in the same memory location.

In this lesson, you will learn:

• How to define and use unions.
• The difference between structures and unions.
• How unions optimize memory usage.

What is a Union?

A union is similar to a structure, but all members share the same memory space.

Example:

union Data { int i; float f; char str[20]; };

Here, i, f, and str share the same memory location.

Declaring and Using a Union

Unions are declared and used like structures:

union Data d1;

Assigning values:

d1.i = 10;

Only one member can store a value at a time.

Example: Using a Union

This program demonstrates how a union stores values in a shared memory location.

#include <stdio.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data d1;
    d1.i = 10;
    printf("Integer: %d\n", d1.i);
    d1.f = 20.5;
    printf("Float: %f\n", d1.f);
    return 0;
}

Memory Allocation in Unions

Unlike structures, a union's size is determined by its largest member.

Example:

union Data { int i; // 4 bytes float f; // 4 bytes char str[20]; // 20 bytes };

The size of this union is 20 bytes, not 28.

Difference Between Struct and Union

Structure:

• Each member has a separate memory location.
• Size is the sum of all members.

Union:

• All members share the same memory location.
• Size is determined by the largest member.

Example: Comparing Struct and Union

This program shows the memory size of a struct and a union.

#include <stdio.h>

struct StructExample {
    int i;
    float f;
    char str[20];
};

union UnionExample {
    int i;
    float f;
    char str[20];
};

int main() {
    printf("Size of struct: %lu bytes\n", sizeof(struct StructExample));
    printf("Size of union: %lu bytes\n", sizeof(union UnionExample));
    return 0;
}

When to Use Unions?

Unions are useful when:

• Memory efficiency is a priority.
• A variable needs to store different types at different times.
• Working with hardware registers.

Summary

In this lesson, you learned:

• How unions work and how they save memory.
• The difference between structures and unions.
• How to use unions in practical scenarios.

Next, we will explore enumerations in C!