Article: Q47693
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Operating System(s):
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 14-NOV-2001
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The information in this article applies to:
- Microsoft C for MS-DOS
- Microsoft C/C++ for MS-DOS
- Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52, 2.0, 2.1, 4.0
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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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:
Q39910 PRB: Initializing Non-Primary Union Element Produces Errors
Additional query words:
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Keywords : kbcode kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Technology : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC152 kbVC500Search
Version : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type : kbinfo
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