Re: Type-safe typecasts

["Marc Nieper-Wißkirchen" <marc.nieper+gnu@gmail.com>]

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Hi Paul,

thanks!

By the way, the snippet you gave is not portable C code, as it assumes
> that 'void *' and 'struct foo *' have the same machine representation.
> This is not necessarily true on (admittedly now-rare) machines that have
> different flavors of pointers. I suspect the main problem here is either
> in the calling code, or in the API for gl_list_iterator_next: if it
> returned a possibly-null value of type 'const void *' instead of storing
> the result through a 'const void **' pointer, the calling code wouldn't
> have gotten into this portability mess.
>

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, the boolean
value is really needed. Of course, one could switch the actual return
parameter with the one returned through a pointer but then it wouldn't look
as nice in while or for-loops.

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 as the Gnulib list API suffers a bit from const-poisoning when the
actual elements are pointers actually non-const objects.

Marc

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