Re: Struct Error

Richard Damon <richard@damon-family.org> writes:

On 1/29/25 5:59 AM, Tim Rentsch wrote:
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bart <bc@freeuk.com> writes:
>

On 24/01/2025 14:37, Michael S wrote:
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On Thu, 23 Jan 2025 10:54:10 +0000
bart <bc@freeuk.com> wrote:
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On 23/01/2025 01:05, James Kuyper wrote:
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On 2025-01-22, bart <bc@freeuk.com> wrote:
>

Gcc 14.1 gives me an error compiling this code:
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      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:
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      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:
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   struct bar *ptr;
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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?

>
My guess is that it makes the simplest implementation of a C compiler
much more complicated.  While I don't think it has been explicited
stated, one goal the original language, and apparently kept by the
Standards Comittee, has been to make the language fairly simple to
proceess to get working code.  To optimize to make fast, might take
more work, but to make your first complier for a system should be
straight forward.  I believe a C compiler can still be done with a
single pass through the source code, with limited look ahead, and only
the final "link" step needs to be able to handle large chunks of the
program.
>
Allowing the pointer to array time to be based on an incomplete type
might make this goal harder.

Possibly.  I suspect the question was never considered, simply
because it never came up.  It's unusual even to have a pointer to
an array with unknown extent, and an array with an incomplete
element type is an even weirder beast.  It's easy to believe that
the peculiar circumstances of the situation being asked about
just never occurred to anyone.  Given that, the simple rule in
the C standard has an obvious and natural appeal.