Last Updated : 04 Sep, 2023
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The Union is a user-defined data type in C language that can contain elements of the different data types just like structure. But unlike structures, all the members in the C union are stored in the same memory location. Due to this, only one member can store data at the given instance.
Syntax of Union in C
The syntax of the union in C can be divided into three steps which are as follows:
C Union Declaration
In this part, we only declare the template of the union, i.e., we only declare the members’ names and data types along with the name of the union. No memory is allocated to the union in the declaration.
union union_name { datatype member1; datatype member2; ...};Keep in mind that we have to always end the union declaration with a semi-colon.
Different Ways to Define a Union Variable
We need to define a variable of the union type to start using union members. There are two methods using which we can define a union variable.
- With Union Declaration
- After Union Declaration
1. Defining Union Variable with Declaration
union union_name { datatype member1; datatype member2; ...} var1, var2, ...;2. Defining Union Variable after Declaration
union union_name var1, var2, var3...;
where union_name is the name of an already declared union.
Access Union Members
We can access the members of a union by using the ( . ) dot operator just like structures.
var1.member1;
where var1 is the union variable and member1 is the member of the union.
The above method of accessing the members of the union also works for the nested unions.
var1.member1.memberA;
Here,
- var1 is a union variable.
- member1 is a member of the union.
- memberA is a member of member1.
Initialization of Union in C
The initialization of a union is the initialization of its members by simply assigning the value to it.
var1.member1 = some_value;
One important thing to note here is that only one member can contain some value at a given instance of time.
Example of Union
C
// C Program to demonstrate how to use union
#include <stdio.h>
// union template or declaration
union un {
int member1;
char member2;
float member3;
};
// driver code
int main()
{
// defining a union variable
union un var1;
// initializing the union member
var1.member1 = 15;
printf("The value stored in member1 = %d",
var1.member1);
return 0;
}
Output
The value stored in member1 = 15
Size of Union
The size of the union will always be equal to the size of the largest member of the array. All the less-sized elements can store the data in the same space without any overflow.
Memory Allocation in C Union
Example 1: C program to find the size of the union
C
// C Program to find the size of the union
#include <stdio.h>
// declaring multiple unions
union test1 {
int x;
int y;
} Test1;
union test2 {
int x;
char y;
} Test2;
union test3 {
int arr[10];
char y;
} Test3;
// driver code
int main()
{
// finding size using sizeof() operator
int size1 = sizeof(Test1);
int size2 = sizeof(Test2);
int size3 = sizeof(Test3);
printf("Sizeof test1: %d\n", size1);
printf("Sizeof test2: %d\n", size2);
printf("Sizeof test3: %d", size3);
return 0;
}
Output
Sizeof test1: 4Sizeof test2: 4Sizeof test3: 40
Difference between C Structure and C Union
The following table lists the key difference between the structure and union in C:
| Structure | Union |
|---|---|
| The size of the structure is equal to or greater than the total size of all of its members. | The size of the union is the size of its largest member. |
| The structure can contain data in multiple members at the same time. | Only one member can contain data at the same time. |
| It is declared using the struct keyword. | It is declared using the union keyword. |
FAQs on C Unions
1. What is the size of the given union?
union un {int a;int arr[20];}Ans: The size of the given union is 20 x 4 bytes = 80 bytes. Even if the array is a collection of similar data elements, it is considered to be a single entity by the C compiler.
2. Can we store data in multiple union members at the same time?
No. We can only store data in a single member at the same time. For example in the following C program, both x and y share the same location. If we change x, we can see the changes being reflected in y.
C
// C program to check if we can store data in multiple union
// members
#include <stdio.h>
// Declaration of union is same as structures
union test {
int x, y;
};
int main()
{
// A union variable t
union test t;
t.x = 2; // t.y also gets value 2
printf("After making x = 2:\n x = %d, y = %d\n\n", t.x,
t.y);
t.y = 10; // t.x is also updated to 10
printf("After making y = 10:\n x = %d, y = %d\n\n", t.x,
t.y);
return 0;
}
Output
After making x = 2: x = 2, y = 2After making y = 10: x = 10, y = 10
3. What are the applications of unions?
Unions can be useful in many situations where we want to use the same memory for two or more members. For example, suppose we want to implement a binary tree data structure where each leaf node has a double data value, while each internal node has pointers to two children, but no data. If we declare this as:
C
struct NODE {
struct NODE* left;
struct NODE* right;
double data;
};
then every node requires 16 bytes, with half the bytes wasted for each type of node. On the other hand, if we declare a node as the following, then we can save space.
C
struct NODE {
bool is_leaf;
union {
struct {
struct NODE* left;
struct NODE* right;
} internal;
double data;
} info;
};
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