Re: !foo vs foo == nil

[Negm-Awad Amin]

Am Do,21.08.2008 um 10:47 schrieb Thomas Engelmeier:

Am 21.08.2008 um 05:03 schrieb Michael Ash:

There was a common perception that NULL is not really the same as  
nil. But
seems like in the end it really is (void*)0.

They differ in type, not in value.

"NULL" is (void *) 0.
"nil" is (id) 0.
"Nil" is (Class) 0.

This is true conceptually but not as far as their actual definition.
NULL can be either 0 or (void *)0.

Let's be a little bit more precise, I'll try to paraphrase correctly  
from my memory:

a.)
(int) NULL  is NOT required or guaranteed  0x0 by the standard. This  
is why one should never use
  if( anPtr == (void *) 0 );
instead of
  if( anPtr == NULL );
On modern machines, it usually is.

b.) At least the C++ standard mandates for type conversion of an  
pointer to an boolean, that e.g. ( !anPtr )  is equivalent  to ( ! 
(anPtr == NULL )). I didn't read the C99 spec but I'm sure there is  
something similar specified.

This implementation detail does probably not matter on most  
platforms today as even microcontrollers have AFAIK linear address  
spaces.
When I was writing my first C app on a DOS machine, there were  
segmented addresses. That meant  it could happen that

((somePtr + offset) - offset) == somePtr

evaluated to false - (somePtr + offset) changed the segment,  
anotherPtr - offset did not. Big debug surprise for poor little  
68020 fanboy ;-)

First: Thanks a lot! I wanted to write somthing like this, but …

I always say, that theoretically it is dangerous to relay on NO = 0  
and nil = 0 (The assumption YES = 1 is simply wrong on some machines).  
In practice of course this always works.

But in the documentation of  Objective-C 2 from Apple, the definitions  
of NO and nil are diclosed. So I think, now it is for (and only for)  
Objective-C 2 "official" that NO and nil equals to 0.

Sometimes you see in source code something like this:
if( booleanVarOrExpression == YES )
(I think, Rentzsch does it that way, IIRC) and Ithink, that this is  
simply correct.

Cheers,
Amin

Regards,
        Tom_E
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Amin Negm-Awad
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