What is a union

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()
{
  union car car1, car2, *car3;
  return 0;
}

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. To access pointer variables, we use also 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 <stdio.h>
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()
{
   printf("size of union = %d bytes", sizeof(uJob));
   printf("\nsize of structure = %d bytes", sizeof(sJob));
   return 0;
}

Output

size of union = 32
size of structure = 40