指向 C 中前向声明结构数组的指针

Question

I have a question about forward declarations in C.

Code

typedef struct yhash_s t_yhash;// forward declaration
struct yhash_s {
    size_t  size_h; 
    t_yhash (*yhash)[];// pointer to array of structures
};

Once the code is compiled with gcc, it complains:

error: array type has incomplete element type 't_yhash' {aka 'struct yhash_s'}

I do understand that t_yhash is not know (yet) and size of array can't be computed, but I am asking about pointer to an array of unknown yet size, which should be perfectly resolvable IMHO.

How do I fix that forward declaration and struct itself?

Answer 1

The trouble is that array declarators may not have an incomplete type as the element type (C11 6.7.6.2/1). And t_yash (ie struct yhash_s ) is not complete until the closing brace of the struct definition.

This rule is also responsible for another piece of trivia; it's legal to have (before the struct definition is complete):

void func( t_yhash *a );

but not legal to have:

void func( t_yhash a[] );

even though the adjustment rule would work just fine if not for the incomplete element type rule.

Probably the language design could be improved slightly by refining this rule to allow some cases like the function prototype, but it clearly wasn't something that arose with the language design committee.

But even without this rule, your use case might have another problem; the size of the pointer might not be known. It would be legal (although unlikely in practice) for "pointer to array of struct X" to have a different size than "pointer to array of struct Y". There is a rule that all pointers to struct must have the same size, but no such rule for pointers to array.

Answer 2

In reply to this part of your post:

How do I fix that forward declaration and struct itself?

You can use void * to stash your array, and then convert it back later.

typedef struct yhash_s t_yhash;
struct yhash_s {
               size_t  size_h;
               void *yhash;
               };

static inline t_yhash (*yhash(t_yhash y))[] {
    return y.yhash;
}

If the function syntax is too obtuse:

typedef t_yhash t_yhash_array[];

static inline t_yhash_array *yhash(t_yhash y) {
    return y.yhash;
}

For example:

t_yhash x[10];
t_yhash y = { 10, &x };
assert(yhash(y) == &x);