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From: daniel@terra.ucsc.edu (Daniel Edelson)
Newsgroups: comp.lang.c++
Subject: Re: pointer arithmitic on a void*
Keywords: pointer arithmatic void*
Message-ID: <6746@darkstar.ucsc.edu>
Date: 10 Sep 90 18:05:13 GMT
References: <1990Sep7.144048.2049@gli.com>
Sender: usenet@darkstar.ucsc.edu
Reply-To: daniel@cis.ucsc.edu (Daniel Edelson)
Distribution: comp.std.c
Organization: University of California, Santa Cruz
Lines: 45

In article <1990Sep7.144048.2049@gli.com> bobj@gli.com (Robert Jacobs) writes:
>
>Is the following syntax allowable in c++. The reason I ask is because a version
>of my compiler used to allow it but the UPGRADE forbids it.
>
>int foo(void *p,int width)
>{
>	void *q;
>	q = p+width;
>}
>
>The compiler complains about not knowing the size information.
>In order to get this to work I have to recast p to either an int or a char*.

>Note I am not trying to dereference a void. I know that is illegal.
>If pointer arithmitic is not allowed on a void* then its usefullness is much 
>reduced.

>Robert Jacobs
>bobj@gli.com


Pointer arithmetic on void * is illegal. This does not
reduce the usefullness of the void * type because arithmetic 
on void * would mean ``add an unknown quantity to this pointer.''

According to the ARM section 5.7 additive operators can
only be applied to a pointer when the pointer points
to an array element. This is the only legal pointer
arithmetic. According to 8.2.4 an array may be
constructed from any built-in type except void. 
Therefore no pointer arithmetic with void* is legal.

This must be the case because there's no way a void* pointer could
be correctly incremented. At the raw machine level, what value
should the compiler add to the pointer value? If the pointer
pointed at an integer, p+1 would add sizeof(int) to the pointer.
If it pointed at a char then sizeof(char) would be added. In
general, because of how K&R first defined pointer arithmetic
the size of the referenced object must be known for a pointer
to be arithmetiked

Daniel Edelson               | Don't just garbage collect, recycle.
daniel@cis.ucsc.edu, or,     | 
...!sun!practic!peren!daniel |