C Programming - Structures, Unions, Enums - Discussion

David Rajesh your explanation is good

Union allocate memory for the variables which has highest.

Size only once, and all other variables are over write in it.

Hence the above program memory for int allocate once, first it store 'a' variable value later it over write with recent initialize 'b'.

Value, so the out put will be 'b' value.

Here we came to know that, union allocates memory of 2 bytes for an integer.

In 1st prg we have assigned as 20 and in 2nd prg we have assigned as 10. i.e., when you print the address of both we. A and we. B the address are same, because union allocates the largest memory size for 2bytes only.

How it will print 20?

#include<stdio.h>

int main()
{
union var
{
int a, b;
};
union var v;
//v.a=10;
v.b=20;
printf("%d\n", v.a);

return 0;
}
output for the above program is :20.
can u guess? v.a is not assigned any value instead the value is printed...

Good Explanation...

I think it will give compile error...
cause
v.a=10;
v.b=20;
printf("%d\n", v.a);
since v.b=20 overwrites the value v.a=10
but program is trying to print the value which doesnot exist.
...
..
.
correct me if i m wrong...

The difference between union and structure is

Structure used as prototype in creating objects with same memory, it can not compress the memory.

struct a{ int a; float b; }obj;
Here obj is having 6 bytes in memory.

Where as for union, it allocates largest memory among all datatypes defined in the union.

union a{ int a; float b;}obj;
Here obj is having 4 bytes instead of 6 Bytes.

So, union utilizes largest memory assigned to variable among all variables and it will be used for all its operations.

Good Logic Bro.
But it also in structure?

If in above question a is initiallised with 20 and b with 10 then
o/p becomes 10(the last updated value is 10)..... ..!!!!