Re: Anonymous union members problem

[llewelly@xmission.com]

Mathieu Fluhr <mfluhr@nero.com> writes:

> Hello
> 
> I am porting VC++ source code to Linux using g++ 3.3 and I am currently
> getting some errors with C++ code like this sample program:
> 
> 
> struct foo
> {
>   struct

Anonymous structs are never described in the ISO C++
    standard. Therefor they are an extension or an error. GCC supports
    anonymous structs with -fms-extensions. 

>   {
>     typedef int structType;

However, gcc does not support typedefs within anonymous structs, even
    with -fms-extensions. I don't know why. (Probabably because
    standard C++ does not allow typedefs within anonymous unions.) If
    MS supports typedefs within anonymous unions, you might consider
    submiting a feature request.

Better, email MS and ask them to stop encouraging people to use
    extensions. :-)

>     union
>     { 
>       int test;
>     };
>   };
> };
> 
> int main()
> {
>   return 0;
> }
> 
> When I try to compile this program, I get the following message:
> 
> mathieu@c-l-175:~/tests/bitfields$ g++-3.3 -Wall bitfields.cpp
> bitfields.cpp:5: error: `typedef int foo::<anonymous
> struct>::structType'
>    invalid; an anonymous union can only have non-static data members
> 
> Is this invalid C++ code ?

Yes. Anonymous structs are mentioned nowhere in the
    standard. (Structs, and classes in general are covered in 
    ch 9. Anonymous unions are in 9.5/2 .)

> If yes is there a way to make it valid

struct foo
{
    typedef int structType;
    union { int test; };
};

> and
> compilable under Linux ?

struct foo
{
    typedef int structType;
    //Need -fms-extensions for anonymous struct.
    struct { union { int test; }; };
};

Now I have a question for you: What is an anonymous struct good for?