Union
#A union is quite similar to the structures in C. It also store different data types in the same memory location. It is also a user defined data type same like structure.
union and structure are almost same
| Structure | Union |
|---|---|
struct student { int roll; char name[10]; float marks; }u; | union student { int roll; char name[10]; float marks; }u; |
Defining a union
Union can be defined in same manner as structures, for defining union use union keyword where as for defining structure use struct keyword.
Syntax
union tagname { datatype member1; datatype member2; ....... ....... };
Example of Union
union emp { int ID; char name[10]; double salary; }u;
Accessing members of an union
The member of unions can be accessed in similar manner as Structure with union reference. Suppose, we you want to access name variable in above example, it can be accessed as u.name.
Advantage of union over structure
It occupies less memory because it occupies the memory of largest member only.
Disadvantage of union over structure
It can store data in one member only.
Difference between Structure and Union
| Structure | Union | |
|---|---|---|
| 1 | For defining structure use struct keyword. | For defining union we use union keyword |
| 2 | Structure occupies more memory space than union. | Union occupies less memory space than Structure. |
| 3 | In Structure we can access all members of structure at a time. | In union we can access only one member of union at a time. |
| 4 | Structure allocates separate storage space for its every members. | Union allocates one common storage space for its all members. Union find which member need more memory than other member, then it allocate that much space |
Memory Allocation in Structure and Union
Structure allocates separate storage space for its every members. Union allocates one common storage space for its all members. Union find which member need more memory than other member, then it allocate that much space
In case of Structure
Syntax
struct emp { int ID; char name[10]; double salary; };
Example
For above structure, memory allocation like below. int ID -- 2B char name[10] -- 10B double salary -- 8B Total memory allocation = 2+6+8 = 16 Bytes
In case of Union
Syntax
union emp { int ID; char name[10]; double salary; };
For above union, only 8 bytes of memory will be allocated because double data type will occupy maximum space of memory over other data types.
Total memory allocation = 8 Bytes
Total memory allocation = 8 Bytes
When use Structure and Union
When need to manipulate the data for all member variables then use structure. When need to manipulate only one member then use union.
Note:
- All the properties of the structure are same for union, except initialization process.
- In case of structure initialize all data members at a time because memory location are different but in case of union only one member need to be initialize.
- In case of union if we initializing multiple member then compiler will gives an error.
Example of Union in C
#include<stdio.h> #include<conio.h> union emp { int ID; char name[10]; double salary; }u; // reference of union void main() { clrscr(); printf("Enter emp Id: "); scanf("%d",&u.ID); printf("Enter emp Name: "); scanf("%s",&u.name); printf("Enter emp Salary: "); scanf("%f",&u.salary); printf("Emp ID: %d",u.ID); printf("Emp Name: %s",u.name); printf("Emp Salary: %f",u.salary); getch(); }
Output
Output: Emp ID: 100 Emp Name: Porter Emp Salary: 20000
