C Unions

A union is a user-defined type similar to structs in C except for one key difference.

Structures allocate enough space to store all their members, whereas unions can only hold one member value at a time.

How to define a union?

We use the union keyword to define unions. Here's an example:

union car < char name[50]; int price; >;

The above code defines a derived type union car .

Create union variables

When a union is defined, it creates a user-defined type. However, no memory is allocated. To allocate memory for a given union type and work with it, we need to create variables.

Here's how we create union variables.

union car < char name[50]; int price; >; int main()

Another way of creating union variables is:

union car < char name[50]; int price; >car1, car2, *car3;

In both cases, union variables car1 , car2 , and a union pointer car3 of union car type are created.

Access members of a union

We use the . operator to access members of a union. And to access pointer variables, we use the -> operator.

In the above example,

To access price for car1 , car1.price is used.
To access price using car3 , either (*car3).price or car3->price can be used.

Difference between unions and structures

Let's take an example to demonstrate the difference between unions and structures:

#include union unionJob < //defining a union char name[32]; float salary; int workerNo; >uJob; struct structJob < char name[32]; float salary; int workerNo; >sJob; int main()

Output

size of union = 32 size of structure = 40

Why this difference in the size of union and structure variables?

Here, the size of sJob is 40 bytes because

the size of name[32] is 32 bytes
the size of salary is 4 bytes
the size of workerNo is 4 bytes

However, the size of uJob is 32 bytes. It's because the size of a union variable will always be the size of its largest element. In the above example, the size of its largest element, ( name[32] ), is 32 bytes.

With a union, all members share the same memory.

Example: Accessing Union Members

#include union Job < float salary; int workerNo; >j; int main() < j.salary = 12.3; // when j.workerNo is assigned a value, // j.salary will no longer hold 12.3 j.workerNo = 100; printf("Salary = %.1f\n", j.salary); printf("Number of workers = %d", j.workerNo); return 0; >