Re: Forward declaration of enum iterator_selection?

From: Johannes Sixt <j6t@kdbg.org>
Date: Aug 8, 2016, 16:30 UTC
Message-ID: <57A8B3BD.1000002@kdbg.org>
In-Reply-To: <0604cf0a-2b94-93b3-3a01-213ea5b9849b@ramsayjones.plus.com>

Am 07.08.2016 um 22:34 schrieb Ramsay Jones:

Show 18 quoted lines

> On 05/08/16 23:26, Johannes Sixt wrote:
>> When refs.c is being compiled, the only mention of enum
>> iterator_selection is in this piece of code pulled in from
>> refs-internal.h(have a look at the preprocessed code):
>>
>> typedef enum iterator_selection ref_iterator_select_fn(
>>          struct ref_iterator *iter0, struct ref_iterator *iter1,
>>          void *cb_data);
>>
>> This looks like a forward declarations of an enumeration type name,
>> something that I thought is illegal in C. Am I wrong? (That may well be
>> the case, my C-foo is quite rusty.)
>
> At this point 'enum iterator_selection' is an incomplete type and may
> be used when the size of the object is not required. It is not needed,
> for example, when a typedef name is being declared as a pointer to, or
> as a function returning such a type. However, such a type must be
> complete before such a function is called or defined.

All you say is true when it is a struct type, of course. But I doubt that there exists such a thing called "incomplete enumeration type" in C. In fact, with these keywords I found https://gcc.gnu.org/onlinedocs/gcc/Incomplete-Enums.html which indicates that this is a GCC extension.

Show 6 quoted lines

> [...] I would rather the 'enum iterator_selection' be defined
> before this declaration. One solution could be to #include "iterator.h"
> prior to _all_ #include "refs/refs-internal.h" in all compilation units
> (Note it is in the opposite order in refs/iterator.c). Alternatively, you
> could put the #include "../iterator.h" into refs/refs-internal.h directly
> (some people would object to this).

I concur. Which one is the correct way to do, I do not know, either. It's a matter how the interface is intended to be used. Perhaps the typedef must be moved to iterator.h?

-- Hannes