Re: Anonymous structs with duplicate members within unions

[Mark Mitchell <mark at codesourcery dot com>]

Steven L. Zook wrote:
> Consider these declarations:
>
> union uFred { struct { unsigned char A;
>                        unsigned      B;
>                      };
>               struct { unsigned char C;
>                        long          D;
>                      };
>             };
> union uDerf { struct { unsigned char A;
>                        unsigned      B;
>                      };
>               struct { unsigned char A;
>                        long          D;
>                      };
>             };

uDerf is invalid ISO C++; there is no such thing as an anonymous struct 
in that language.  As a GNU extension, the anonymous structs are treated 
like the standard anonymous unions; the fields are directly accessible 
as uDerf::A.  As such, they are ambiguous.  Thus, if you replaced 
"struct" with "union" in the above code, your code would be invalid ISO 
C++.  That's why it is also rejected in GNU C++.

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304