PRB: Initializing Non-Primary Union Element Produces Errors

• The Microsoft C/C++ Compiler (CL.EXE) included with: - 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

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'

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 */

   ARTICLE-ID: Q47693
   TITLE     : Initializing Unions Initializes First Member of the Union

Keywords          : CLIss
Version           : MS-DOS:6.0,6.0a,6.0ax,7.0; WINDOWS:1.0,1.5,1.52; WINDOWS  NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbprb