Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!att!ihlpb!TSfR!usenet
From: usenet@TSfR.UUCP (usenet)
Newsgroups: comp.lang.c
Subject: Re: When is a cast not a cast?
Summary: the orc runs, shrieking, from the room.
Message-ID: <549@TSfR.UUCP>
Date: 19 May 89 07:50:34 GMT
References: <2747@buengc.BU.EDU> <10191@smoke.BRL.MIL> <406@skye.ed.ac.uk> <10276@smoke.BRL.MIL> <2890@buengc.BU.EDU> <546@TSfR.UUCP> <2908@buengc.BU.EDU>
Reply-To: orc@TSfR.UUCP (David L. Parsons)
Followup-To: comp.lang.c
Organization: This Space for Rent - Lisle, IL
Lines: 57
Expires:

In article <2908@buengc.BU.EDU> bph@buengc.bu.edu (Blair P. Houghton) writes:
>In article <546@TSfR.UUCP> orc@TSfR.UUCP (David L. Parsons) writes:
>>In article <2890@buengc.BU.EDU> bph@buengc.bu.edu (Blair P. Houghton) writes:
>>[more discussion of the properties of pointer+pointer]
>>>
>>>>What can one legally do with it?
>>>
>>>What one can legally do between (consenting) integers?
>
>[...uses the fact that pointer-pointer now returns int to show how
>associativity of pointer addition would break if we allowed pointer+pointer...]
>
>So, like, make pointer-pointer return the pointer.

   So, what will happen is that a pointer will just be another scalar,
but with some additional properties that aren't subject to the operators
that you'd use with a scalar? But wouldn't that kill architectures that
don't have a flat address space? Building the language around a non-linear
address space is a bad thing, yes, but isn't building it around a linear
address space just as bad?

>>  What does pointer+pointer give you, aside from clever loop optimizations?
>
>Hey.  It's arcane.  So is calloc().  Then you find somewhere to use it,
>and you're hopping with joy.  Jeez.

   I'm sorry, I said that wrong.  Your clever loop optimisation was _nice_
and I liked it, but my question is what other things are given, and _must_
you have pointer+pointer yields pointer to get it?   In the case you gave
there, pointer+pointer yields pointer_thing would do just as well, and you
don't lost pointer-pointer in the process.

>Allowing pointer addition and treating pointers as types would make
>pointer arithmetic a consistent thing.

   But they already are consistant.  They aren't consistant if you
think of them as a scalar, but a pointer isn't a scalar- it contains
an address (not a scalar) and some fieldwidth information.  The examples
given with latitude & longitute or street addresses show the difficulty
of treating the result as a pointer.

  Consider, also, << and >> and floating point numbers.
Does prohibiting these operators make floating-point arithmetic
inconsistant? (Bitshifting a float/double gives a result that's
_highly_ dependent on the FP representation used - and _what_ does
bitshifting an int a fractional amount mean?)

>So, one of these days I'll find
>a situation, some few examples of which we've seen already, where I
>will really want to do pointer+pointer...

   Yes.  But must it break the existing relationship between pointers
and integers?


   -david parsons
   -orc@pell.uucp