PRB: Initializing Non-Primary Union Element Produces Errors

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This article was previously published under Q39910

SYMPTOMS

Attempting to initialize any element of a union except the first may generate any combination of the following compiler messages:

error C2078: too many initializers

warning C4047: 'initialization' - different levels of indirection

-or-

warning C4047: 'initializing' : 'type1' differs in levels of indirection from 'type2'

warning C4133: 'initializing' : incompatible types - from 'type1' to 'type2'

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The compiler issues these messages by design. According to the ANSI standard, initialization of a union must be directed to the first element. An attempt to initialize some other union member may be interpreted by the compiler as a syntactically incorrect initialization of the first element.

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RESOLUTION

If it is necessary to initialize a non-primary element of a union, change the order of your union members, so that the element that requires initialization appears as the first element of the union.

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MORE INFORMATION

Example1.c, in the sample code below, fails compilation with one C2078 error and three C4047 warnings. The error is generated because only one union element may be initialized (the first) and by the time the compiler processes the "{ch1,ch2,ch3,ch4}" initialization, it has already filled element one. The warnings are generated because the compiler expects to be initializing "union TEST tsty" with integer values, but it is finding char* (character pointers) instead.

Example2.c compiles but produces a C4133 warning. The compiler expects to initialize "union ONION anONION" with an integer pointer but finds a float pointer instead. Although, this situation is not serious enough to prohibit compilation, it does indicate that there may be a problem.

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Sample Code

/* Example1.c
 *
 * Compile options needed: none
 */ 

union TEST
{
    struct
    {
      int a;
      int b;
      int c;
    } one;             /* union element #1 */ 
    struct
    {
      char *ptr1;
      char *ptr2;
      char *ptr3;
      char *ptr4;
    } two;             /* union element #2 */ 
} u;

char ch1[10], ch2[10], ch3[10], ch4[10];

union TEST test ={ {1,2,3},                /* initialize element #1 */ 
                   {ch1,ch2,ch3,ch4} };    /* generates error C2078 */ 

void main(void)
{
   union TEST tsty ={ {ch1,ch2,ch3,ch4} }; /* generates 3 C4047
                                              warnings */ 
}

-------------
/* Example2.c
 *
 * Compile options needed: /c
 */ 

union  ONION  { int *iptr; float *fptr; };
float sam = 3.141592;
union ONION anONION = { &sam };  /* generates warning C4133 */

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REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:

47693 (http://support.microsoft.com/kb/47693/EN-US/ ) Initializing Unions Initializes First Member of the Union

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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 Standard Edition
Microsoft Visual C++ 6.0 Service Pack 5

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kbcompiler kbprb KB39910

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Retired KB Content Disclaimer

This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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