Re: !foo vs foo == nil
Re: !foo vs foo == nil
- Subject: Re: !foo vs foo == nil
- From: James Montgomerie <email@hidden>
- Date: Thu, 21 Aug 2008 11:27:28 +0100
On 21 Aug 2008, at 09:06, Clark Cox wrote:
This is not a "hack" or a "coincidence", this is by design. A non-NULL
pointer *is* a boolean expression that evaluates to true, just as a
non-zero integer is. Again, it doesn't work by coincidence, it is a
guarantee of the language standard.
To go off on a bit of a tangent, if you read this without thinking of
all the implications of other parts of the language you /can/ still
run into problems. For example:
BOOL b = (BOOL)v;
could leave you with b == NO even if v != 0.
If BOOL is a shorter type than value's type (e.g. if BOOL is char and
v is long), and if the lower bits of v are all 0 but the higher bits
are not, the higher bits will just get truncated off in the conversion.
i.e. this:
---
#include <Foundation/Foundation.h>
int main() {
int myFlags = 0x01000200;
int myBitMask = 0x01000000;
BOOL isFlagSet = (BOOL)(myFlags & myBitMask);
if(isFlagSet == NO) { // Could (of course) use "if(!isMasked)"
here, I just used this for clarity
printf("Oh No!\n");
}
return 1;
}
---
gives:
---
Thing:tmp jamie$ gcc test.m -framework Foundation
Thing:tmp jamie$ ./a.out
Oh No!
---
This has puzzled me for a while in the past when I was wondering why
my bit masks were not evaluating correctly.
Jamie.
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