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From: richard@aiai.ed.ac.uk (Richard Tobin)
Newsgroups: comp.lang.c
Subject: Re: When is a cast not a cast?
Message-ID: <464@skye.ed.ac.uk>
Date: 19 May 89 17:51:26 GMT
References: <406@skye.ed.ac.uk> <10276@smoke.BRL.MIL> <2890@buengc.BU.EDU> <334.nlhp3@oracle.nl>
Reply-To: richard@aiai.UUCP (Richard Tobin)
Organization: AIAI, University of Edinburgh, Scotland
Lines: 30

In article <334.nlhp3@oracle.nl> bengsig@oracle.nl (Bjorn Engsig) writes:

>>>In article <406@skye.ed.ac.uk> richard@aiai.UUCP (Richard Tobin) writes:
>>>>  t = p + q;

>char *p, *q;
>
>and let p==3 and q==5.  Then what is p+q?  Which type does it have?

I'm not of course suggesting that this should be added to C.  However,
I don't think it would be *impossible* to specify semantics for it.

p+q would have type "sum of 2 pointers to char".  Subtracting a pointer
to char would give a pointer to char.  Adding a pointer to char would
give "sum of 3 pointers to char".  Adding a pointer to int would give
"sum of 2 pointers to char and a pointer to int".  (We of course need
some canonicalisation here.)  Dereferencing would of course be illegal.

It might be an error to mix pointers to different aggregates.

Just to reiterate, I don't think this should really be done.  I would
hope that a compiler would generate good enough code for my original
example, but I haven't seen one that does.

-- Richard

-- 
Richard Tobin,                       JANET: R.Tobin@uk.ac.ed             
AI Applications Institute,           ARPA:  R.Tobin%uk.ac.ed@nsfnet-relay.ac.uk
Edinburgh University.                UUCP:  ...!ukc!ed.ac.uk!R.Tobin