extern(C) {
    struct ab;
}

ab*[int] map;

void main() {
    map.clear();
}


Cannot be compiled. Why?

Thanks.

Cheng Wei:

> extern(C) {
>     struct ab;
> }
> 
> ab*[int] map;
> 
> void main() {
>     map.clear();
> }
> 
> 
> Cannot be compiled. Why?

It's not specific of associative arrays:

extern(C) {
    struct AB;
}
AB*[] arr;
void main() {
    arr.length += 1;
}

Bye,
bearophile

what is the error message ?

Am 29.09.2011, 06:51 Uhr, schrieb Cheng Wei <rivercheng@gmail.com>:

> extern(C) {
>     struct ab;
> }
>
> ab*[int] map;
>
> void main() {
>     map.clear();
> }
>
>
> Cannot be compiled. Why?
>
> Thanks.

Just use void* for opaque pointers in D.

On 09/29/2011 01:28 PM, Trass3r wrote:
> Am 29.09.2011, 06:51 Uhr, schrieb Cheng Wei <rivercheng@gmail.com>:
>
>> extern(C) {
>> struct ab;
>> }
>>
>> ab*[int] map;
>>
>> void main() {
>> map.clear();
>> }
>>
>>
>> Cannot be compiled. Why?
>>
>> Thanks.
>
> Just use void* for opaque pointers in D.

Or an empty struct.

struct ab{}

The problem is that the void* cannot convert back to AB* when we want to use it in c library.

Just don't understand why the cast(AB*)p (p is void *) needs to know the
size of AB. Is there any unsafe cast which can blindly cast the
pointers?

Thanks a lot. This solves the problem.

However, it breaks the abstractness. Now in D side, we can call
auto v = ab(). This does not make sense, because then &v cannot be used
in the C library.

I don't understand why when we manipulate AB*, D compiler needs to know the size of struct ab. Moreover, when we use AB*[int], the D compiler complains about there's no opHash defined for AB. I don't think they are necessary at all.

Cheng Wei , dans le message (digitalmars.D.learn:29865), a écrit :
> Thanks a lot. This solves the problem.
> 
> However, it breaks the abstractness. Now in D side, we can call
> auto v = ab(). This does not make sense, because then &v cannot be used
> in the C library.
> 
> I don't understand why when we manipulate AB*, D compiler needs to know the size of struct ab. Moreover, when we use AB*[int], the D compiler complains about there's no opHash defined for AB. I don't think they are necessary at all.

I guess D is not designed to use abstract classes because they are not
needed in the language: the compiler reads all the symbols in the file
before doing the real compilation, but there may be no real issue for
the compiler, as long as you do not use the ab* for anything else than
passing it to C code.

You could file an enhancement request to support abstract pointer for
the sake of interoperability with C.

-- 
Christophe