在C中，我应该被允许使用指向不完整类型数组的指针吗？

Question

The IAR Embedded C compiler is happy with this, and I assumed it was correct C code:

struct incomplete;
typedef struct incomplete (*why_not)[2];
struct incomplete {struct incomplete *known_to_work;} array[2];
why_not ok = &array;

However, gcc and clang choke on the definition of why_not :

incomplete.c:2:29: error: array type has incomplete element type ‘struct incomplete’
 typedef struct incomplete (*why_not)[2];
                             ^

Technically, there is no reason to reject a definition of a "pointer to array of incomplete" type. After all, the structure definition is needed only where such a variable is dereferenced, or some pointer arithmetics is performed.

I'm eager to hide structure definitions where possible.

What does the C standard say about this?

Answer 1

The code uses an array declarator where the element type is incomplete. This a constraint violation in ISO C, according to 6.7.6.2/1 Array declarators :

Constraints

In addition to optional type qualifiers and the keyword static, the [ and ] may delimit an expression or * . If they delimit an expression (which specifies the size of an array), the expression shall have an integer type. If the expression is a constant expression, it shall have a value greater than zero. The element type shall not be an incomplete or function type.

Answer 2

The following statement

typedef struct incomplete (*why_not)[2];

is a pointer to an array of two struct incomplete . The compiler does not know the size of struct incomplete yet, since it is not yet defined.

The following will however work:

typedef struct incomplete *why_not[2];

ie: an array of pointers to struct incomplete . The compiler does know the size of a pointer to an incomplete type (ie: it is just the size needed to store an address).

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Edit:

The compiler does not need to know the size of struct incomplete in either case (as long as no pointer arithmetic is taking place), since both declarations are just declaring pointers.