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Monday, October 11, 2004
 
C/C++: Structure Vs Union

A structure is a collection of items of different types; and each data item will have its own memory location. Where as only one item within the union can be used at any time, because the memory allocated for each item inside the union is in a shared memory location i.e., only one memory location will be shared by the data items of union.

Size of union will be the size of the biggest variable.

Why do we need Union in the first place?

Sometimes we may not need the data of all the (related) data items of a complex data structure and be storing/accessing only one data item at a time. Union helps in such scenarios.

e.g.,
typedef union
{
int Wind_Chill;
char Heat_Index;
} Condition;

typedef struct
{
float temp;
Condition feels_like;
} Temperature;

Wind Chill is only calculated when it is cold and heat index is used only when it is hot. There is no need for both of them at the same time. So when we specify the temp, feels_like will have only one value - either wind chill or heat index, but not both.

The following simple program illustrate the above explanation:
% cat structunion.c
#include <stdio.h>
#include <stdlib.h>

typedef union
{
int Wind_Chill;
char Heat_Index;
} Condition;

typedef struct
{
float temp;
Condition feels_like;
} Temperature;

void main()
{
Temperature *tmp;

tmp = (Temperature *)malloc(sizeof(Temperature));

printf("\nAddress of Temperature = %u", tmp);
printf("\nAddress of temp = %u, feels_like = %u", &(*tmp).temp, &(*tmp).feels_like);
printf("\nWind_Chill = %u, Heat_Index= %u\n", &((*tmp).feels_like).Wind_Chill, &((*tmp).feels_like).Heat_Index);
}

% cc -o structunion structunion.c

% ./structunion
Address of Temperature = 165496
Address of temp = 165496, feels_like = 165500
Wind_Chill = 165500, Heat_Index= 165500



Comments:
Great Content in your Blog... helps a lot refreshing some basic concepts... Thanks a lot....
 
The explanation is easy to understand.
 
Simple but good explanation, thanks
 
simple,easy,good .
thanks
 
Simple and beneficial. Thanks.
 
yes its too good to revise
 
yes its too good to revise
 
Wonderful explanation, and great article!
I have bumped into this website after googling and was glad to find a detailed, comprehensible and yet easy to understand explanation. I think that the most important thing on this article is the example, that makes really clear why one should use the union anyhow. Thanks a lot!
 
thanks boss
 
The Best Explanation. Thanks a lot dude!!
 
Thanks for your detailed explanations! They really helped me understand! Thank you very much for your effort! Regards from Paul T.
 
KIS, GOOD
 
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