Passing class instance as void* and casting back

[Benjamin Thaut]

Am 05.11.2011 09:38, schrieb Maxim Fomin:
> Hello. I use curl library and need to overwrite default behavior of
> data/headers writing.
> I use
>
>     curl_easy_setopt(curl, CurlOption.headerfunction,&header_func);
>     curl_easy_setopt(curl, CurlOption.writeheader,&this);
>
> where "this" is a class Inet instance, and header_func is a static
> function of the class
> (should be a callback of fwrite() args semantics). The class has
> buffer for header_func
> which should copy data from char *ptr to the buffer.
> In
>     private static size_t header_func(char *ptr, size_t size, size_t
> nmemb, void *userdata)
> object is casted back:
>     Inet *net = cast(Inet*)userdata;
> However, I receive segfault at the beginning of header_func().
> I don't know how dmd compiler treats object references.
> I tried to pass just "this" (without taking address) and cast without
> * and all 4 combinations
> of&this/this, cast(Inet*)/cast(Inet); none of them works (segfault on
> access to Inet class).
>
> So, how can I pass an object through void* or may be there is another
> way to tell
> header_func() which object should be used?
>
> Thanks.
>
> P.S. D is a nice language, it's a pleasant to work with)

If you cast the void* to a INet when recieving it you should also cast 
it to a INet when passing it to curl. Pointers to classes point at the 
same address as any parent class they inherit from. Interfaces however 
might point into the middle of a class, thus the address of a interface 
reference is different then the address of a class reference. Classes 
and interfaces are references types in D so there is no need to use the 
address operator.

Correct would be something like:

INet callback = this;
curl_easy_setopt(curl, CurlOption.headerfunction, &header_func);
curl_easy_setopt(curl, CurlOption.writeheader, callback);

...

void header_func(...)
{
  INet callback = cast(INet)userdata;
  ...
}

-- 
Kind Regards
Benjamin Thaut