PRB: Nested Nameless Structs Can Cause C2020 Error in C

ID: Q64686

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
- Microsoft C for OS/2, versions 6.0, 6.0a
- Microsoft C/C++ for MS-DOS, version 7.0
- 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

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

Keywords : kbcode kbCompiler kbVC100 kbVC150 kbVC200 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600
Version : MS- DOS:6.0,6.00a,6.00ax,7.0;OS/2:6.0,6.00a;WINDOWS:1.0,1.5;WINDOWS NT:1.0,2.0, 4.0,4.1,4.2,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbprb