Union C++, first value ambigous.

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Hi,
I wanna ask these code :

C++

#include <stdio.h>

union MyUnion
{
	int iVal;
	float fVal;
	char cVal[5];
	//char c[9];
};

void main()
{
	union MyUnion u={10};
	u.fVal = 100;
	printf_s("%d\n%f\n%d\n%d\n", u.iVal, u.fVal, u.cVal[0], 
		sizeof(u)); 
	getchar();
}

The output was this like :

VB

But I think in the line of

C++

void main(){
union MyUnion u={10};
..
..
}

, I'd declared that first union member is 10.
Why it doesn't?

Thanks in advances

Posted 17-Apr-12 19:50pm

satrio_budidharmawan

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Comments

CPallini 18-Apr-12 4:05am

Because it is a union. Check out the documentation!

2 solutions

Solution 2

You have used a union where, I think, you actually want a structure. See here[^] for more information.

Posted 17-Apr-12 21:58pm

Richard MacCutchan

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nv3 · Accepted Answer · 2012-04-17T20:16:00

The intent of a union is overlay all members in the same storage space. This is usually done if only one of the members gets assigned a value at a time.

When you assigned 100.0 to u.fVal you thereby reused the storage of iVal and overwrote the 10 that was in there. What you later see in iVal is the floating point formatted value of fVal, interpreted as an int.

If you don't want that behaviour of multiple members sharing the same space, then use struct instead of union.