Path: utzoo!attcan!uunet!mcsun!ukc!strath-cs!cs.glasgow.ac.uk!jack
From: jack@cs.glasgow.ac.uk (Jack Campin)
Newsgroups: comp.lang.misc
Subject: Re: Relationship between C and C++
Message-ID: <4869@vanuata.cs.glasgow.ac.uk>
Date: 20 Mar 90 15:56:08 GMT
References: <8432@hubcap.clemson.edu> <5200048@m.cs.uiuc.edu>
Reply-To: jack@cs.glasgow.ac.uk (Jack Campin)
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robison@m.cs.uiuc.edu wrote:

> Why is pointer arithmetic denigrated so much?  The C-style pointer
> arithmetic seems to me to implement a fairly simple abstraction,
> roughly equivalent to ``a tape and a read/write head.''
> Most C implementations do not do bounds checking on pointers, 
> but I fail to see why pointer arithmetic is inherently evil.

> Can anyone clue me as to the basis for pointer paranoia?

Garbage collection, for one.  If you try to implement a BCPL-family
language on a machine/OS with garbage-collected object memory [i.e. a
storage model a bit more sophisticated than a Turing machine], you have
real problems identifying what objects are still being referenced when
pointers can be incremented.

Secondly, real machines are sufficiently different from the Turing tape
that the result of a pointer arithmetic operation is not merely wordsize-
dependent like normal computer "arithmetic", it's OS-dependent.  Or do C
fans think it's a good idea to have operator semantics vary with the host
segmentation scheme?

Thirdly, the aliasing it enables is a semantic mess.  If anyone claims
different, let's see your denotational model of ANSI C.

-- 
--  Jack Campin   Computing Science Department, Glasgow University, 17 Lilybank
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