Re: error in using NSNumber's numberWithInt: method not flagged
Re: error in using NSNumber's numberWithInt: method not flagged
- Subject: Re: error in using NSNumber's numberWithInt: method not flagged
- From: John McCall <email@hidden>
- Date: Sun, 09 Jun 2013 13:45:52 -0700
On Jun 9, 2013, at 1:32 PM, Kyle Sluder <email@hidden> wrote:
> On Jun 9, 2013, at 1:26 PM, Boyd Collier <email@hidden> wrote:
>> Quite by chance, I stumbled on to what seems to be an error that ought to be caught by the compiler but wasn't. Here's an example:
>>
>> NSNumber *testNumber = [NSNumber numberWithInt:5];
>> int testInt = (int)testNumber;
>>
>> […snip…]
>>
>> Should I file a bug report or is this a well-known anomaly that I've just come across?
>
> Sitting on a runway right now, so I can't double-check the C spec, but I believe this is a consequence of C's use of integers for truth types.
>
> A pointer must be convertible to integer type, because it can be used in the form `if (expressionOfPointerType)`. It follows that casting to int must also be allowed. I'm not sure what the spec says about the actual value of the result.
Explicitly converting a pointer to an integer type (other than _Bool/bool) reinterprets the bit-representation of the pointer as an integer. There are a lot of low-level tricks you can then play with that. Of course, you need to use an integral type that's large enough to store the pointer value.
Playing with the bit-representation of Objective-C pointers specifically is ill-advised because the implementation reserves the right to play its own games with object pointers (such as the optimized representation of integer values when boxed as NSNumbers on 64-bit platforms), but nonetheless this behavior is inherited from C and cannot really be changed.
John.
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