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Re: “ChattyG takes a college freshman C/C++ programming exam”

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c++, comp.lang.c
Subject Re: “ChattyG takes a college freshman C/C++ programming exam”
Followup-To comp.lang.c
Date 2023-10-08 13:54 +0100
Organization A noiseless patient Spider
Message-ID <87edi58eb2.fsf@bsb.me.uk> (permalink)
References <ufne0r$15ple$1@dont-email.me> <20231007235448.25c4986f123c88ff65d2f103@gmail.moc> <uftmh9$2vieq$1@dont-email.me> <20231008125543.518cab44cdefed7e8af393c8@gmail.moc>

Cross-posted to 2 groups.

Followups directed to: comp.lang.c

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Anton Shepelev <anton.txt@gmail.moc> writes:

> jak to Anton Shepelev:
>
>> > AS: Can a C function return a pointer to itself?
>> > CG: Yes, a C function can return a pointer to itself.
>>
>> The explanation is certainly wrong but it is true that it
>> is possible:
>
> I disagree.  A function returns the value of its declared
> return type.  And although C allows the definition of self-
> referential structs, it does [not] allow the declaration of self-
> referencial functions.

C does not have any way to express the /type/ of a function that returns
a pointer to itself.  But self-referential functions (as most people
would use the term) are commonplace, many of them having very dull and
ordinary types!

>> #include <stdio.h>
>>
>> typedef void (* fptr)(int) ;
>>
>> fptr foo(int n)
>> {
>>      printf("Call n. %d\n", n);
>>      return (fptr)foo;
>> }
>>
>> int main()
>> {
>>      fptr p = foo(1);
>>      p(2);
>>      return 0;
>> }

> You are converting void to function pointer fptr.  This is not
> supported in C.

Not in the fragment shown.  The conversion is from one
pointer-to-function type (void (*(*)(int))(int)) to another (void
(*)(int)) and that's allowed.

The trouble is that foo and foo(1) have different pointer-to-function
types.  If C could express the right type, one could write

  foo(1)(2)(3)(4);

but is it stands we can only write

  foo(1)(2);

before the types go wrong.  A third chained call would try to "call" a
void result.

> You might have taken advantage of
> void*, which is silently convertible to any pointer, but
> then again it does not work with function pointer, and C has
> not analog of void* for functions pointers, so an explicit
> conversion is seem inevitable.

I think it would be reasonable to say that a function /can/ return a
pointer to itself.  It's just that the type will be wrong.  After all,
we are happy to say that (void *)&some_int_var is a pointer to that int
object even though the type is wrong.  To use that pointer to get at the
int object, a conversion is required.  Similarly, to use the pointer
returned by a function like foo, a conversion is required.  I would do
it like this:

  #include <stdio.h>
   
  typedef void (*generic_function_ptr)(void);
  typedef generic_function_ptr (*fintp)(int);
   
  generic_function_ptr foo(int n)
  {
       printf("Call n. %d\n", n);
       return foo;
  }
   
  int main(void)
  {
       ((fintp)((fintp)foo(1))(2))(3);
  }


-- 
Ben.

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“ChattyG takes a college freshman C/C++ programming exam” Lynn McGuire <lynnmcguire5@gmail.com> - 2023-10-05 17:40 -0500
  Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-05 15:57 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Christian Gollwitzer <auriocus@gmx.de> - 2023-10-06 07:36 +0200
    Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-06 11:59 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-07 23:54 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” jak <nospam@please.ty> - 2023-10-08 09:42 +0200
      Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 04:27 -0400
      Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 12:55 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 13:06 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” candycanearter07 <no@thanks.net> - 2023-10-08 06:22 -0500
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 15:03 +0300
            Re: “ChattyG takes a college freshman C/C++ programming exam” Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-10-15 15:51 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <richard@damon-family.org> - 2023-10-15 17:53 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-16 00:07 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2023-10-16 22:01 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <richard@damon-family.org> - 2023-10-16 22:41 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-17 12:22 -0400
          Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-08 12:31 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Richard Damon <news.x.richarddamon@xoxy.net> - 2023-10-08 21:17 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:13 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-10 12:18 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” MarioCCCP <NoliMihiFrangereMentulam@libero.it> - 2023-10-13 11:22 +0200
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-13 10:04 -0700
        Re: “ChattyG takes a college freshman C/C++ programming exam” Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-08 13:54 +0100
        Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 11:25 -0400
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 20:07 +0300
        Re: “ChattyG takes a college freshman C/C++ programming exam” Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-10-08 16:37 +0000
          Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-08 20:05 +0300
            Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:13 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-09 13:30 -0700
                Re: Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 20:33 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-10 12:04 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2023-10-09 12:17 +0000
              Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:03 +0300
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 20:30 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:45 +0300
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 21:21 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-09 21:26 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 22:41 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-09 21:47 +0100
          Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-08 15:31 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-09 01:12 -0400
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-10 18:46 -0700
                Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-11 01:14 -0400
                Re: “ChattyG takes a college freshman C/C++ programming exam” scott@slp53.sl.home (Scott Lurndal) - 2023-10-11 13:47 +0000
                Re: “ChattyG takes a college freshman C/C++ programming exam” Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-10-11 07:56 -0700
            Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@gmail.moc> - 2023-10-09 23:06 +0300
              Re: “ChattyG takes a college freshman C/C++ programming exam” Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-10-09 17:15 -0700
      Re: “ChattyG takes a college freshman C/C++ programming exam” James Kuyper <jameskuyper@alumni.caltech.edu> - 2023-10-08 11:08 -0400
  Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-11 21:06 -0700
    Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-11 21:10 -0700
    Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:56 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:57 +0300
      Re: “ChattyG takes a college freshman C/C++ programming exam” "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-12 12:59 -0700
  Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-12 10:53 +0300
    Re: “ChattyG takes a college freshman C/C++ programming exam” Phil Carmody <pc+usenet@asdf.org> - 2023-10-23 15:39 +0300
      Re: “ChattyG takes a college freshman C/C++ programming exam” Anton Shepelev <anton.txt@g{oogle}mail.com> - 2023-10-23 16:45 +0300

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