Re: Forward declaration of enum iterator_selection?

From: Ramsay Jones <ramsay@ramsayjones.plus.com>
Date: Aug 8, 2016, 18:28 UTC
Message-ID: <c1ef8a65-844f-b676-cae0-abba87930063@ramsayjones.plus.com>
In-Reply-To: <57A8B3BD.1000002@kdbg.org>

On 08/08/16 17:30, Johannes Sixt wrote:

> Am 07.08.2016 um 22:34 schrieb Ramsay Jones:
>> On 05/08/16 23:26, Johannes Sixt wrote:

[snip]

Show 7 quoted lines

>> 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.

Ah, well spotted!

You prompted me to look at the C99 (and C11) standards, in particular sections 6.7.2.2 (Enumeration specifiers) and 6.7.2.3 (Tags).

So, while (technically) enumeration types are incomplete prior to the closing } in its definition, the constraint imposed in 6.7.2.3-2 states:

	"A type specifier of the form
		enum identifier
	without an enumerator list shall only appear after
	the type it specifies is complete"

which pretty much rules out its use here.

ATB, Ramsay Jones