Re: Struct Error

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?
It just doesn't make sense.
Is it just because such pointers HAVE to work, otherwise self-referential structs become impossible? That would make it a hack, in which case why not apply it to arrays too?

The case of the recursive structure is special only in a sense that it's
o.k. in C++, because [unlike C] in C++ struct considered complete within
its own body.

For non-recursive, you can choose to declare the pointer-to-array after the struct has been fully defined.