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Union

What is Union

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The ones who are crazy enough to think they can change the world are the ones who do.
- Steve Jobs

C Union

Union is similar to the structures, it also contains number of members like structures. In C, union is identified by the keyword union The members of union share the same memory location within the computer, whereas each structure member has its own memory location within the computer. At any time of compilation, only one member of the union can reside in that memory location. The union require bytes that are equal to the bytes required by largest member within union. The union allocates fixed bytes. Thus, unions are used to conserve or save memory.

Declaration of Union

The declaration of union is similar to structure, except the keyword union instead of struct.

Syntax - Union

Syntax
union unionname { 
data type member1;
data type member2;
}variable1, variable2;

C program - Difference Between Structure and Union

diff.c
#include <stdio.h>
int main()
{
//structures declartion
struct sample{
    double d1;     //occupies 8 bytes in memory
    float f1;     //occupies 4 bytes in memory
}s1;
//Union declartion
union samp{
    double d2;     //occupies 8 bytes in memory
    float f2;     //occupies 4 bytes in memory
}u1;
printf("\nSize of Structure : %d ", sizeof(s1));
printf("\nSize of Union : %d ", sizeof(u1));
return 0;
}
  • Size of Structure : 16
  • Size of Union : 8

Note:

The program illustrates that the union allocate bytes, that is equal to its largest member. The size may vary from compilers.

Accessing a Union Member

A union member can be accessed similar to structure member, that by using (.)dot or period operator. The general format is as follows,

Syntax
unionvariablename.membername;
union-member-decl.c
#include <stdio.h>
int main()
{
/structures declartion
struct sample{
    int a;
    int b;
}s1;
//Union declartion
union samp{
    int c;
    int d;
}u1;
s1.a = 10;
s1.b = 20;
printf("\nThe Structure member values a and b are : %d %d ", s1. a, s1.b);
u1.c = 30;
u1.d = 40;
printf("\nThe Union member values c and d are : %d %d ", u1. c, u1.d);
return 0;
}
  • The Structure member values a and b are : 10 20
  • The Union member values c and d are : 40 40

Note:

The program illustrates that the existing data of union member is replaced by new data, by this, unions can store one data at any time.

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