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Re: Casting function pointers to different signatures

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On 04/24/2015 01:42 PM, samuelallenalexander@gmail.com wrote:
> On Friday, April 24, 2015 at 12:22:30 AM UTC+1, James Kuyper wrote:
>> On 04/23/2015 06:49 PM, Sam wrote:
>>> On Thursday, April 23, 2015 at 11:04:53 PM UTC+1, James Kuyper wrote:
>>>> On 04/23/2015 05:39 PM, Sam wrote:
>> ...
>>>>> Thanks for the very detailed answer with references and everything.
>>>>>
>>>>> It's really too bad the behavior is undefined since it would open the door for porting some genuinely useful OOP/FP idioms into C.
>>>>
>>>> How is the door closed? It's trivial to make the function types
>>>> compatible, and if you do, the code will work. Could you give an example
>>>> of code using one of those idioms where it's excessively difficult to
>>>> avoid the problem? Your current example does NOT qualify.
>>>
>>> Yes, you could make the individual printer functions have signature
>>> void (*) (void*), but that makes those individual printer functions a
>>> lot more opaque on their own, and it's very repetitive that you have
>>> to repeat the casts over and over.
>>
>> That comment suggests a possible misunderstanding. If the function types
>> were, in fact, compatible, there would be no need for a cast, the
>> function would already have the correct type.
> 
> Suppose I have structures of various sizes, and functions to print them.  For example,
> 
> struct an_int
> {
>   int x;
> };
> 
> struct two_ints
> {
>   int x;
>   iny y;
> };
> 
> void print_an_int( struct an_int *p )
> {
>   printf( "%d\n", p->x );
> }
> 
> void print_two_ints( struct two_ints *p )
> {
>   printf( "%d %d\n", p->x, p->y );
> }
> 
> If buf is a NULL-terminated array of struct an_int's, I'd like to be able to do
> 
> print_array( print_an_int, buf );
> 
> If buf is a NULL-terminated array of struct two_ints's, I'd like to be able to do
> 
> print_array( print_two_ints, buf );
> 
> 
> Is there a way to do this without changing print_an_int or print_two_ints themselves?

No - my suggestion involved changing the corresponding functions:

void print_an_int(void *pv)
{
     struct an_int *p = pv;
...
}

If there's a reason why you can't do that, you'll need to explain that
reason. If it's a valid reason, we might be able to suggest an
alternative approach that will work.

Note that in contexts like these, it's more typical for object-oriented
C code to access the appropriate function through a pointer stored with
the data it's designed to work on, so it's easier to make sure that it's
the correct function. Implementations of C++ based on C typically use a
vtable pointer for this purpose, though there are other ways to do it.
If you're familiar with the concept, I recommend considering using it in
this context. If you're unfamiliar with the concept, I can explain further.

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Casting function pointers to different signatures samuelallenalexander@gmail.com - 2015-04-23 13:39 -0700
  Re: Casting function pointers to different signatures Barry Schwarz <schwarzb@dqel.com> - 2015-04-23 14:01 -0700
  Re: Casting function pointers to different signatures James Kuyper <jameskuyper@verizon.net> - 2015-04-23 17:10 -0400
    Re: Casting function pointers to different signatures samuelallenalexander@gmail.com - 2015-04-23 14:39 -0700
      Re: Casting function pointers to different signatures James Kuyper <jameskuyper@verizon.net> - 2015-04-23 18:04 -0400
        Re: Casting function pointers to different signatures samuelallenalexander@gmail.com - 2015-04-23 15:49 -0700
          Re: Casting function pointers to different signatures James Kuyper <jameskuyper@verizon.net> - 2015-04-23 19:22 -0400
            Re: Casting function pointers to different signatures samuelallenalexander@gmail.com - 2015-04-24 10:42 -0700
              Re: Casting function pointers to different signatures James Kuyper <jameskuyper@verizon.net> - 2015-04-24 14:18 -0400
                Re: Casting function pointers to different signatures samuelallenalexander@gmail.com - 2015-04-24 12:36 -0700
                Re: Casting function pointers to different signatures James Kuyper <jameskuyper@verizon.net> - 2015-04-24 15:50 -0400
                Re: Casting function pointers to different signatures Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-04-24 13:18 -0700
    Re: Casting function pointers to different signatures Siri Cruz <chine.bleu@yahoo.com> - 2015-04-23 16:08 -0700
      Re: Casting function pointers to different signatures Keith Thompson <kst-u@mib.org> - 2015-04-23 18:36 -0700
        Re: Casting function pointers to different signatures Siri Cruz <chine.bleu@yahoo.com> - 2015-04-23 19:00 -0700
          Re: Casting function pointers to different signatures raltbos@xs4all.nl (Richard Bos) - 2015-04-25 10:48 +0000
      Re: Casting function pointers to different signatures Tim Rentsch <txr@alumni.caltech.edu> - 2015-04-25 04:05 -0700
  Re: Casting function pointers to different signatures Lőrinczy Zsigmond <nospam@for.me> - 2015-04-24 06:51 +0200
  Re: Casting function pointers to different signatures Richard Damon <Richard@Damon-Family.org> - 2015-04-25 10:57 -0400

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