Why union doesn't work properly but struct does?

Question

Why union doesn't work properly but struct does?

Debojit Acharjee 455

In the following program the values of the variable "orange" is printed properly when "union" is changed to "struct", but why the int and float values don't print properly when "union" is used?

#include <stdio.h>

union fruit
{
  int number;
  float weight;
  char *color;
} orange;
  
int main()
{
  orange.number = 6;
  orange.weight = 1.3;
  orange.color = "orange";

  printf("There are %d %s colored oranges weighing %.2f pounds", orange.number, orange.color, orange.weight);

  return 0;
}

Output:

There are 4214884 orange colored oranges weighing 0.00 pounds

YujianYao-MSFT 4,296 Reputation points Microsoft External Staff

2023-06-05T06:14:50.4833333+00:00

Hi Debojit Acharjee,

In the case of using a union, all members share the same memory space. This means that after assignment to one member, the value of other members will be overwritten. This leads to incorrect results when printing the distinct members of the union.

In your example, after you assign orange.number , you assign orange.weight next. Since these two members share the same memory space, the value of orange.number is overwritten, which cannot be printed correctly.

If you change union to struct, each member will use its own memory space and will not affect each other, so the value of each member will be printed correctly.

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YujianYao-MSFT 4,296 Reputation points Microsoft External Staff

2023-06-05T06:14:50.4833333+00:00

Hi Debojit Acharjee,

In the case of using a union, all members share the same memory space. This means that after assignment to one member, the value of other members will be overwritten. This leads to incorrect results when printing the distinct members of the union.

In your example, after you assign orange.number , you assign orange.weight next. Since these two members share the same memory space, the value of orange.number is overwritten, which cannot be printed correctly.

If you change union to struct, each member will use its own memory space and will not affect each other, so the value of each member will be printed correctly.

Answer 1

Bruce (SqlWork.com) 78,316 Volunteer Moderator

The union uses the same address for all the properties. This allows the same address to be interpreted as different types. A union will always be the value of the last assignment. In your case the literal.”orange”. When seen as a int, it’s the address, when seen as a float, it’s the address bits as a float.

note. The sizeof of a union is the size of the largest property.

Debojit Acharjee 455 Reputation points

2023-06-03T08:24:18.0433333+00:00

But why it is nor showing the correct values of the integer and float variables? Even if the compiler shares a same location, it should show the values of every variable.
David Lowndes 2,640 Reputation points MVP

2023-06-03T09:02:59.3466667+00:00

It will be showing the correct interpretation of what was last put in the union - which is a pointer to the string "orange". The 1.3 & 6 will have been overwritten. Use the debugger to see what's in the union.
Debojit Acharjee 455 Reputation points

2023-06-04T07:19:50.6666667+00:00
This has nothing to do with the pointer because when I tried using only int types then also the result was not as expected. The values of this program shows only 1 and 1 instead of 1 and 6.

#include <stdio.h> union fruit { int number; int weight; } orange; int main() { int int_size, float_size; orange.number = 6; orange.weight = 1; int_size = sizeof(orange.number); float_size = sizeof(orange.weight); printf("There are %d oranges weighing %d pounds\n", orange.number, orange.weight); printf("int_size: %d, float_size: %d\n", int_size, float_size); return 0; }

Output:

There are 1 oranges weighing 1 pounds

int_size: 4, float_size: 4

Both the variables of same types and size but then why the result is not showing 1 and 1?
David Lowndes 2,640 Reputation points MVP

2023-06-04T10:48:17.3033333+00:00

Your result is showing 1 and 1 !

Output: There are 1 oranges weighing 1 pounds
Debojit Acharjee 455 Reputation points

2023-06-04T13:32:19.08+00:00

Why it's showing 1 and 1 instead of 1 and 6?
David Lowndes 2,640 Reputation points MVP

2023-06-04T13:39:17.6933333+00:00

You need to read up and understand what a union is.
Both weight and number are the same memory location, so whatever is written last to either of them makes them both appear the same.
Barry Schwarz 3,746 Reputation points

2023-06-04T18:21:43.12+00:00

As everyone has tried to tell you, number and weight occupy the same memory. When you assign 1 to weight, it replace the 6 that was in that memory. The 6 is no longer available to print.

No matter which member of the union you attempt to reference, you will always get the last value stored in ANY member of the union.

Answer 2

David Lowndes 4,726

You need to read up on what a union is.

Debojit Acharjee 455 Reputation points

2023-06-05T05:59:46.9733333+00:00
I know what union is but I didn't know that there is a rule for declaring the members of an union and precedence for defining the variables. I came to know from my program that pointers cannot be used as a member when using other types as members. When defining the variables, the largest types should be defined last in the main function.

The following program runs properly only when the int variable is defined before the float, but not in reverse.

#include <stdio.h> union fruit { char number; float weight; } orange; int main() { orange.number = 6; // This should be declared last because it's of 4 bytes orange.weight = 1.3; //This will print the incorrect results printf("There are %d colored oranges weighing %.2f pounds\n", orange.number, orange.weight); orange.weight = 1.3; // This should be declared first because it's of 1 bytes orange.number = 6; //This will print the correct results printf("There are %d colored oranges weighing %.2f pounds\n", orange.number, orange.weight); return 0; }

Output:

There are 102 colored oranges weighing 1.30 pounds

There are 6 colored oranges weighing 1.30 pounds
David Lowndes 2,640 Reputation points MVP

2023-06-05T08:58:30.11+00:00

I'm sorry, but you're clearly not understanding what a union is.
Debojit Acharjee 455 Reputation points

2023-06-05T11:40:15.1766667+00:00

What I haven't understood about union? Why do you think the variable declaration list order makes a difference? It's because values are sored to the assigned variable one at a time.

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Why union doesn't work properly but struct does?

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