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A union is a particular data type in C that allows several data types to be stored in the same memory address. A union can have numerous members, but only one member can have value at any given time. Unions are a convenient approach to use the same memory region for numerous purposes.

Both Structures and unions are used to group several variables. But while a structure enables us to treat a number of different variables stored at different places in memory, a union enables us to treat the same space in memory as a number of different variables. That is, a union offers a way for a section of memory to be treated as a variable of one type on one occasion, and as a different variable of a different type on another occasion.

Defining a union:

The union keyword define unions.

For instance,

union car
{
  char name[50];
  int price;
};

Creating union variables

When a union is defined, a user-defined type is created. However, no memory is set aside. Variables must be created to allocate memory for a given union type and work with it.

union car
{
  char name[50];
  int price;
};

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

Members of a union can gain access.

We employ the ‘.‘ operator to gain access to union members The ‘->‘ operator is used to access pointer variables.

In the preceding example,

• Car1.price is used to get the price of car1.
• To get the price of a car3, use (*car3).price or car3->price.

Note: the size of a union variable will always be the size of its largest element.

Union of structure

Just as one structure can be nested within another, a union too can be nested in another union. There can be a union in a structure or a structure in a union.

Example:

// Online C compiler to run C program online
#include <stdio.h>

int main() {
  struct a
  {
      int i;
      char c[2];
  };
  struct b
  {
      int j;
      char d[2];
  };
  union z
  {
      struct a key;
      struct b data;
  };
  union z strange;
  
  strange.key.i=512;
  strange.data.d[0]=0;
  strange.data.d[1]=32;
  
  printf("\n%d",strange.key.i);
   printf("\n%d",strange.data.j);
    printf("\n%d",strange.key.c[0]);
   printf("\n%d",strange.data.d[0]);
    printf("\n%d",strange.key.c[1]);
    printf("\n%d",strange.data.d[1]);
    return 0;
}

Output:

512
512
0
0
32
32

here, we access the elements of the union in this program using the ‘.‘ operator twice. Thus,

strange.key.i

refers to the variable i in the structure key in the union strange.

Note: also read about the Typedef in C

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