PRB: Nested Nameless Structs Can Cause C2020 Error in C (64686)

The information in this article applies to:

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

This article was previously published under Q64686

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.

Modification Type:	Minor	Last Reviewed:	7/5/2005
Keywords:	kbcode kbCompiler kbprb KB64686