void, VOID, C and C++

Question

I have the following code:

typedef void VOID;
int f(void);
int g(VOID);

which compiles just fine in C (using gcc 4.3.2 on Fedora 10). The same code compiled as C++ gives me the following error:

void.c:3: error: ‘<anonymous>’ has incomplete type
void.c:3: error: invalid use of ‘VOID’

Now, this is something in external library and I would like the owner to fix that problem. So I have a question - does C++ standard forbids this construct? Could you give me a pointer/citation? The only thing I can recall is that function declaration with (void) to signal empty parameter list is deprecated in C++, but I don't understand why typedefed VOID does not work.

Answer 1

Both .c and .cpp compiles just fine under OS X and fedora core 10 for me (gcc/g++ 4.3.2 in both case)!

Answer 2

I just put your code in a .cpp file, and it compiled with no problems in VS2005, SUSE, Redhat, and Solaris, so I guess your specific gcc version does not approve of this. Gal

Answer 3

gcc bugs. See #32364 and #9278.

Answer 4

Works fine with gcc 4.0.1 on a Mac, even with the flags "-Wall -ansi -pedantic".

I don't remember seeing anywhere about the deprecation of void in an empty parameter list though...

Answer 5

What distribution are you using to compile this on? Also, the bugs that codelogic mentioned may already be fixed as the current stable version is 4.3.3, and I use Arch Linux, and their repositories have been updated to 4.3.3 already.

Edit: My bad, I see that you have Fedora 10, in that case, run this command as root:

yum check-update

And see if GCC was updated to 4.3.3 in the repository. If so then run:

yum update

Answer 6

Yes, as far as i know the second declaration is invalid in C++ and C89, but it is valid in C99.

From The C99 draft, TC2 (6.7.5.3/10):

The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.

It's explicitly talking about the type "void", not the keyword.

From The C++ Standard, 8.3.5/2:

If the parameter-declaration-clause is empty, the function takes no arguments. The parameter list (void) is equivalent to the empty parameter list.

That it means the actual keyword with "void", and not the general type "void" can also be seen from one of the cases where template argument deduction fails (14.8.2/2):

• Attempting to create a function type in which a parameter has a type of void.

It's put clear by others, notable in one core language issue report here and some GCC bugreports linked to by other answers.

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To recap, your GCC is right but earlier GCC versions were wrong. Thus that code might have been successfully compiled with it earlier. You should fix your code, so that it uses "void" for both functions, then it will compile also with other compilers (comeau also rejects the second declaration with that "VOID").