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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Struct Error
Date: Wed, 29 Jan 2025 02:59:50 -0800
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bart <bc@freeuk.com> writes:

> On 24/01/2025 14:37, Michael S wrote:
>
>> On Thu, 23 Jan 2025 10:54:10 +0000
>> bart <bc@freeuk.com> wrote:
>>
>>> On 23/01/2025 01:05, James Kuyper wrote:
>>>
>>>> On 2025-01-22, bart <bc@freeuk.com> wrote:
>>>>
>>>>> Gcc 14.1 gives me an error compiling this code:
>>>>>
>>>>>      struct vector;
>>>>>      struct scenet;
>>>>>
>>>>>      struct vector {
>>>>>          double x;
>>>>>          double y;
>>>>>          double z;
>>>>>      };
>>>>>
>>>>>      struct scenet {
>>>>>          struct vector center;
>>>>>          double radius;
>>>>>          struct scenet (*child)[];
>>>>>      };
>>>>
>>>> 6.7.6.2p2:  "The element type shall not be an incomplete or
>>>> function type."
>>>>
>>>> I have many draft versions of the C standard.  n2912.pdf, dated
>>>> 2022-06-08, says in 6.7.2.1.p3 about struct types that "... the
>>>> type is incomplete144) until immediately after the closing brace
>>>> of the list defining the content, and complete thereafter."
>>>>
>>>> Therefore, struct scenet is not a complete type until the closing
>>>> brace of it's declaration.
>>>
>>> Wouldn't this also be the case here:
>>>
>>>      struct scenet *child;
>>>     };
>>
>> Just to point out if it was not said already:  the problem is not
>> related specifically to recursive structures.  It applies to arrays
>> of incomplete types in all circumstances.
>>
>> struct bar;
>> struct bar (*bag)[]; // error
>> typedef struct bar (*bat)[]; // error
>
> I don't think anyone has yet explained why that is an error (other
> than C says it is), but not this:
>
>   struct bar *ptr;
>
> This is a pointer to an incomplete type.  Attempts to do ++ptr
> for example will fail later on if that struct has not yet been
> defined.
>
> So why not the same for the pointer-to-array versions?

The question you should be asking is why did the original C
standards body make the rule they did?

The answer might be because this exception to a simple and
general rule is almost never useful, and never necessary.

Considering that it has been 35 years since that original rule
was made, and 2025 is the first time the question has come up,
the indications are that the original decision was a good one.