Re: Type-safe typecasts

[Paul Eggert]

On 4/6/21 12:13 PM, Marc Nieper-Wißkirchen wrote:
> gl_list_iterator_next has to return two things: An element (represented by
> a const void *) and a boolean value. As elements may be NULL

Ah, OK, then that's the problem. The API shouldn't allow null elements. 
They're not that useful anyway. If they really are needed for some 
lists, I suppose one could have a more-complicated API to return them 
(by setting a bool elsewhere); but usually they aren't.

> So to make my original code portable C, I would have to code
>
> ...
> const void *e;
> while (gl_list_iterator_next (&i, &e, NULL))
>   {
>     struct foo *foo = (void *) e;
>     ++foo->bar;
>   }
> ...
>
> The const typecast is, unfortunately, still needed to silence compiler
> warnings

Yes, that would be portable. But that cast indicates another problem 
with the API. It should return void *, not void const * (think of strchr 
as the model here). The API should discourage type-casts, since they're 
more dangerous than the alternatives.