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Re: Casting function pointers to different signatures

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On 04/23/2015 05:39 PM, samuelallenalexander@gmail.com wrote:
> On Thursday, April 23, 2015 at 10:10:51 PM UTC+1, James Kuyper wrote:
>> On 04/23/2015 04:39 PM, Sam wrote:
...
>> "A pointer to a function of one type may be converted to a pointer to a
>> function of another type and back again; the result shall compare equal
>> to the original pointer. If a converted pointer is used to call a
>> function whose type is not compatible with the referenced type, the
>> behavior is undefined." (6.3.2.3p8)
>>
>> Passing it as an argument to print_array() is not a problem. However,
>> calling it inside print_array() through a pointer that a function type
>> that is incompatible with the actual type of print_with_brackets() has
>> undefined behavior. "void" is not compatible with "char", so "void*" is
>> not compatible with "char*"= (6.7.6.1p2), so void (*)(void*) is not
>> compatible with void (*)(char*) (6.7.6.3p15).
>>
>> Note that buf in print_abc has the type char**, while array in
>> print_array() has the type void**. Those are incompatible types, too.
>>
>> Now, void* and char* are required to have the same representation and
>> same alignment requirement (6.2.5p28). Footnote 47 notes that "The same
>> representation and alignment requirements are meant to imply
>> interchangeability as arguments to functions, return values from
>> functions, and members of unions.". However, that's merely a
>> non-normative suggestion. Nothing in the standard actually requires that
>> the types be interchangeable. They could, for example, be passed between
>> function using different registers.
>>
>> As a practical matter, you can generally count on void* and char* to be
>> interchangeable - but I think you'd be doing better to make sure that
>> that the declared type for print_with_brackets() actually matches the
>> declared type for *printer in print_array(), and similarly for buf in
>> print_abc() and array in print_array(). Do the conversion from void* to
>> char* somewhere else (such as in the body of print_with_brackets()).
> 
> 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.

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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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