Q39910: PRB: Initializing Non-Primary Union Element Produces Errors

Article: Q39910
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): kbCompiler kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 25-JUL-2001

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The information in this article applies to:

- Microsoft C/C++ Compiler (CL.EXE), used with:
   - Microsoft C for MS-DOS 
   - Microsoft C/C++ for MS-DOS 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0 
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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'

CAUSE
=====

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.

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.

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.

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 */ 

REFERENCES
==========

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

  Q47693 Initializing Unions Initializes First Member of the Union

Additional query words:

======================================================================
Keywords          : kbCompiler kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbprb

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