Re: Forward declaration of enum iterator_selection?

From: Ramsay Jones <ramsay@ramsayjones.plus.com>
Date: Aug 7, 2016, 20:34 UTC
Message-ID: <0604cf0a-2b94-93b3-3a01-213ea5b9849b@ramsayjones.plus.com>
In-Reply-To: <933f540f-7752-cfce-5785-b67728fea987@kdbg.org>

On 05/08/16 23:26, Johannes Sixt wrote:

Show 7 quoted lines

> 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.

> My compiler does not complain (it's gcc 4.8), but I thought I mention it before someone with a pickier compiler stumbles over it...

So, I think this is correct.

Having said that, 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).

ATB, Ramsay Jones