INFO: Initializing Unions Initializes First Member of the Union

Last reviewed: September 2, 1997
Article ID: Q47693

The information in this article applies to:

Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
Microsoft C/C++ for MS-DOS, version 7.0
Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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:

   ARTICLE-ID: Q39910
   TITLE     : PRB: Initializing Non-Primary Union Element Produces Errors

Keywords          : CLngIss kbcode kbfasttip
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52;  WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo

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Last reviewed: September 2, 1997
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