Introduction:
- Union is a user defined data type.
- Unions were used when memory at premium.
- A union basically allows a variable to have more than one type; but only one of these types can be used.
| Syntax: | Example: |
| union identity { Members; } ; | union Test { char c; short s; }; |
| STRUCTURE | UNION |
| Define with struct keyword. | Define with union keyword |
| The size of the structure is equal to the sum of the sizes of its members. | The size of the union is equal to the size of the largest member. |
| Each member within a structure is assigned a unique storage area. | Memory allocated is shared by individual members of the union. |
| Individual members can be accessed at a time. | Only one member can be accessed at a time. |
| Altering the value of a member will not affect other members of the structure. | Altering the value of any of the member will alter other member values. |
Program to display size of union: The sum of all sizes of variables defined in structures is the size of total structure.
| #include<stdio.h> struct st{ char a; short b; float c; }; union un{ char a; short b; float c; }; int main(){ struct st s ; union un u ; printf(“Size of structure : %d \n”, sizeof(s)); printf(“Size of union : %d \n”, sizeof(u)); return 0; } |
Accessing members of union: We use dot(.) operator to access the members of union.
Note: We can define any number of variables inside the union, but we cannot use multiple variables at a time. We will get odd results like follows.
| #include<stdio.h> union un{ int a, b; }; int main(){ union un u ; u.a = 10; printf(“b value is : %d \n”, u.b); u.b = 20; printf(“a value is : %d \n”, u.a); return 0; } |
| #include<stdio.h> union un { char a[2]; int b; }; int main() { union un u ; u.a[0] = 2 ; u.a[1] = 3 ; printf(“b value is : %u \n”, u.b); return 0; } |