Article ID: 47693
Article Last Modified on 7/5/2005
APPLIES TO
- Microsoft C Professional Development System 6.0a
- Microsoft Visual C++ 1.0 Professional Edition
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 1.51
- Microsoft Visual C++ 1.52 Professional Edition
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 2.1
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition
This article was previously published under Q47693
SUMMARY
When initializing a union, the initialization value is applied to the first member of the union even if the type of the value matches a subsequent member. As stated in the ANSI Standard, Section 3.5.7:
A brace-enclosed initializer for a union object initializes the
member that appears first in the declaration list of the union
type.
Because you cannot initialize the value of any member of a union other than the first one, you must assign their values in a separate statement. Initializing a union with a value intended for a subsequent member causes that value to be converted to the type of the first member.
MORE INFORMATION
The following example demonstrates the issue:
Sample Code
/* Compile options needed: none
*/
#include <stdio.h>
union { int a;
float b;
} test = {3.6}; /* This is intended to initialize 'b' */
/* however, the value will be converted */
/* (first to a long and then to an int) */
/* in order to initialize 'a'. */
void main (void)
{
float dummy = 0.0; /* This causes the floating point */
/* math package to be initialized. */
/* Not necessary with VC++ for */
/* Windows NT. */
printf ("test.a = %d, test.b = %f\n", test.a, test.b);
}
The output from the example, though not what is intended, is as follows:
test.a = 3, test.b = 0.00000
To associate a value with "b", you can reverse the order of the members, as in the following:
union {
float b;
int a;
} test = {3.6};
Or, you can retain the order of the elements and assign the value in a separate statement, as in the following:
test.b = 3.6;
Either of these methods creates the following output:
test.a = 26214, test.b = 3.600000
Under Windows NT, the output would be as follows:
test.a = 1080452710, test.b = 3.600000
REFERENCES
For examples and explanation of possible compiler errors and warnings generated when attempting to initialize a non-primary union element, please see the following article in the Microsoft Knowledge Base:
39910 PRB: Initializing Non-Primary Union Element Produces Errors
Keywords: kbinfo kblangc kbcode KB47693
