Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!cornell!batcomputer!olson
From: olson@batcomputer.tn.cornell.edu (olson)
Newsgroups: comp.std.c
Subject: Re: function returning pointer to itself
Message-ID: <5489@batcomputer.tn.cornell.edu>
Date: 13 Jul 88 06:41:26 GMT
References: <5485@batcomputer.tn.cornell.edu>
Reply-To: olson@tcgould.tn.cornell.edu (olson)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 79

In article <5485@batcomputer.tn.cornell.edu> olson@tcgould.tn.cornell.edu (olson) writes:
>Does the new C standard have a natural way to declare a
>function that returns a pointer to itself
>????

Thanks to all the people that replied to my question.
Everyone said effectively, "NO, this can not be done NATURALLY in C"
(which is the response I expected)
BUT you can mock it up by
  1) using type cast
  2) encapsulation in a data structure

That C can't do this naturally bugs me.
What do people think of the following proposal?

Define a new primative type  'self'   (not a very good name ...)
  self *p;	means  p points to something that 
                         points to something that ...
                   or  p points to itself
		       p->p->p->p->p->...
  self *f();	means  f is a function returning a pointer to a 
                              function returning a pointer to a ...
                    or f is a function returning a pointer to itself
		       f()->f()->f()->...
  self **f()	means  f is a function returning a pointer to a pointer to a
                              function returning a pointer to a pointer to a ...
                   or  ???
                       f()->p->f()->p->f()->p->...
  self *(*p)() 	means  p is a pointer to a function returning a pointer to a
                              pointer to a function returning a pointer to a ...
		   or  ???
		       p->f()->p->f()->p->f()->...
That is, 'self' sets up a definition that is "solved" by iteration
(but that is NOT how you implement it)
self *(*p)() =>  p=*(*p)() = *(*(*(*p)()))() = ...

Note that   self  must always involve pointers,  self p  is not allowed.

Further one could write
  p->f()->f()->f()->f()->...
as
 typedef self *state();
 state *p;
Without typedefs this would not be possible to express,
unless further syntax modifications were made, such as
  self *<*p>();		means p->f()->f()->f()->...
  self *<f()>;		means f()->p->p->p->...
where the <> indicate the part that is to be iterated on
ie    self *<f()>    =>    <f()>=*<f()>      rather than  f = *<f()>

SIDE ISSUE: (independent of self)
Note in the same way that it is impossible to declare a function
using the typedef
typedef int myfunc();
because 
myfunc f(){};   means   int  (f())(){};
which is illegal,
it is impossible to declare a function
state f(){};
This is something that bugs me too.
Would it screw up the intuitiveness of typedefs too much
to define  myfunc f(){}; to mean  int f(){}; 
since the only other meaning is illegal????????


COMMENTS
1) I don't see how 'self' would pose any implementation problems.
   (on the other hand I have never written a compiler, only thought about it)
2) There have been enough occasions where I could have used this primative
   that I would call it a useful feature.
3) It might be opening a pandora's box.  Consider, someone might say, well
   I want  f()->p->f()->p->p->f()->p->p->p->f()->...
   or one where the number of p's are the digits of PI.
   Well, what can I say.  These would be a lot harder to implement.
   I haven't been able to conceive of a use for them.
4) It is probably to late to add this primative to the new C standard.

Todd Olson
olson@helios.tn.cornell.edu
Physics Dept/ Cornell Univ/ Ithaca NY 14853