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From: James Dennett <jdennett@cox.net>
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Subject: Re: private types
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Date: Mon, 20 Mar 2006 08:47:13 -0800
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Date: 2006-03-20T08:47:13-08:00
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Robert A Duff wrote:
> Maciej Sobczak <no.spam@no.spam.com> writes:
> 
>> but still, (void*)0 == NULL will always evaluate to true, even if the
>> internal representation of the null pointer is not zero.
> 
> Are you sure?  Are you talking about C or C++ or both?

It's true for both, given the right interpretation of
(void*)0.

> Does (void*)x always return NULL if x is an appropriate-sized integer
> whose value is zero? 

Not necessarily.  But if x is a null pointer constant,
which to C++ means an integral constant expression with
value zero, then it will.  (C also allows an ICE of zero
cast to void*, so you have a NPC already.)

> That would require run-time overhead if
> NULL is not represented as all-zero-bits.

Yes, but that's not required.  You can get different
behaviour from

int i(0);
void *p((void*)i);

than from

int const i(0);
void *p((void*)i);

because for C++ the latter i is a valid integral constant
expression, while the former is not.

> On the other hand, it's pretty confusing if casting zero to (void*)
> is sometimes guaranteed to return NULL, and sometimes not, depending
> on whether the zero value is known at compile time.

Afraid so; generally C++ is arranged so that removing const,
where legal, should not change the meaning of code -- but
this is an exception, in theory at least.

-- James