Q47693: INFO: Initializing Unions Initializes First Member of the Union

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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