Anonymous union members problem

Anonymous union members problem

[Mathieu Fluhr]

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
  {
    typedef int structType; 
    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 ? If yes is there a way to make it valid and
compilable under Linux ?

Regards,
Mathieu

Re: Anonymous union members problem

[Eljay Love-Jensen]

Hi Mathieu,

 >Is this invalid C++ code ?

Yes, this is invalid C++ code.

 >If yes is there a way to make it valid and compilable under Linux ?

Take the typedef out of the anonymous struct.
--or--
Name the anonymous struct so it is no longer anonymous.

I favor the second.

HTH,
--Eljay

Re: Anonymous union members problem

[Mathieu Fluhr]

Hello Eljay

Thanks for answering so quiclkly. I still have some questions/remarks...

>  >Is this invalid C++ code ?
> 
> Yes, this is invalid C++ code.
> 

Humm... do you have any reference about that ?

>  >If yes is there a way to make it valid and compilable under Linux ?
> 
> Take the typedef out of the anonymous struct.
> --or--
> Name the anonymous struct so it is no longer anonymous.
> 
> I favor the second.
> 

I already thought about these 2 possibilities... but none of them is
actually suitable: this anonymous struct is generated by a macro that
take a type as parameter (aim of the typedef) which is particular to
this structure.

So any other idea ? 

Re: Anonymous union members problem

[Eljay Love-Jensen]

Hi Mathieu,

 >Humm... do you have any reference about that ?

Try "g++ -pedantic -Wall -W foo.cpp".

Anonymous structures and classes are not C++.  (Anonymous unions are C++.)

ISO 14882:1998 section 9.1

 >I already thought about these 2 possibilities... but none of them is 
actually suitable: this anonymous struct is generated by a macro that take 
a type as parameter (aim of the typedef) which is particular to this 
structure.  So any other idea ?

Then your code is not C++, it's merely C++-ish.

You'll need to use a compiler that is compatible with your source code's 
C++-ish variant.  There may be a GCC flag that relaxes the rules and would 
allow your code to compile with GCC 3.3.  I don't know of such a flag, off 
the top of my head.

HTH,
--Eljay

Re: Anonymous union members problem

[llewelly]

Mathieu Fluhr <mfluhr@nero.com> writes:
[snip]
> I already thought about these 2 possibilities... but none of them is
> actually suitable: this anonymous struct is generated by a macro that
> take a type as parameter (aim of the typedef) which is particular to
> this structure.
> 
> So any other idea ? 

You must fix the macro.

Re: Anonymous union members problem

[llewelly]

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?