From: Johannes Sixt Date: Mon, 08 Aug 2016 16:30:53 GMT Subject: Re: Forward declaration of enum iterator_selection? 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: > 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. > [...] 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