Q64686: PRB: Nested Nameless Structs Can Cause C2020 Error in C

Article: Q64686
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Last Modified: 27-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C for OS/2 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 4.1, 4.2, 5.0, 6.0 
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SYMPTOMS
========

In Microsoft C, using a nameless structure as a member of a structure causes a
compiler error if the two structures both have members with the same name. C
versions 6.x generate the error:

  C2030: 'varname': struct/union member redefinition.

In C/C++ versions 7.0 and later, the error is:

  error C2020: 'varname' : 'struct' member redefinition

The code sample below, when built as a C source file, reproduces the error. The
same sample, built as a C++ source file, will not display this behavior.

CAUSE
=====

This is expected behavior, not a bug. When a nameless structure is used within
another structure, the members of the nameless structure become members of the
new structure. In the C language, the member names must be unique.

MORE INFORMATION
================

Sample Code
-----------

  /* compile options needed: none /Tc */ 

  void main(void)
  {

    struct s1
      {
        int a,b,c;
      };

    struct s2
      {
        struct s1;  /* nameless struct */ 
        float z;
        char a[10];  /* error occurs here */ 
      } *p_s2;
  }

The second structure (s2) is effectively the following:

  struct s2
    {
      int a,b,c;  /* Was nameless struct s1 */ 
      float z;
      char a[10];  /* Error occurs here */ 
    } *p_s2;

This illustrates that the member variable "a" is being redefined. Correctly used
nameless structures can provide a good technique for building structures with
similar data structure without having the complexity of truly nested structures.

Additional query words: 8.0 8.0c 9.0 10.0 11.0 /Tp

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Keywords          : kbcode kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC410 kbVC420 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC16bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,4.1,4.2,5.0,6.0
Issue type        : kbprb

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