Re: ObjC BOOL and boolean c expressions

FROM : John Stiles
DATE : Fri Aug 31 19:02:09 2007

I *think* that the C spec doesn't specify the result of a boolean 
expression other than "zero means false, nonzero means true." So from 
a "compliance with the spec" standpoint, the first version might 
return different results from the second version.

In practice, I've never seen a compiler which didn't generate 1 for a 
true result when computing a boolean expression. And YES == 1.

I am sure that an exception exists out there somewhere, on some weird 
hardware/compiler combination, where a true boolean expression 
returns a value other than one, but I doubt you will see this 
becoming an issue in your code if you keep using standard tools.


On Aug 30, 2007, at 11:19 PM, Charles Steinman wrote:

>
> --- Eyal Redler <<email_removed>> wrote:
>

>> Hi,
>>
>> I'm not sure where and when but I recall reading
>> something about ObjC
>> BOOL type values YES and NO not being the same as
>> whet a boolean
>> expression might produce.
>> I can't recall the exact details but there was some
>> kind of gotcha
>> related to this. For example, the expression
>>
>> BOOL myBool=(myInt==1);
>>
>> might set myBool differently then
>>
>> BOOL myBool=(myInt==1)?YES:NO;

>
> The two expressions are equivalent. YES is defined as
> (BOOL)1, while NO is defined as (BOOL)0.
>

>>
>> Or maybe it was that
>>
>> if (myBool)
>>     [foo bar];
>>
>> will perform differently then
>> if (myBool==YES)
>>     [foo bar];

>
> I wouldn't say it will work differently, but it might.
> There's no language constraint that says a BOOL can
> only be set to YES or NO (though I wouldn't
> intentionally set a BOOL to some other value). So if a
> method returns a nonzero value that is not 1, if
> (myBool) will evaluate to true while if (myBool ==
> YES) will evaluate to false.
>
> Cheers,
> Chuck
>
>
>
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