INFO: Initializing Unions Initializes First Member of the Union

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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.

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MORE INFORMATION

The following example demonstrates the issue:

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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

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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 (http://support.microsoft.com/kb/39910/EN-US/ ) PRB: Initializing Non-Primary Union Element Produces Errors

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