Re: Stupid objective-c question
Re: Stupid objective-c question
- Subject: Re: Stupid objective-c question
- From: Doug Hill <email@hidden>
- Date: Wed, 21 Sep 2016 17:02:35 -0700
> On Sep 21, 2016, at 4:52 PM, Steve Mills <email@hidden> wrote:
>
>> On Sep 21, 2016, at 18:44, Gabriel Zachmann <email@hidden> wrote:
>>
>> I've got a stupid, curious question regarding a code snippet that I have found on the net (I tried it, it works).
>>
>> Here is the code snippet:
>>
>> - (void) observeValueForKeyPath: (NSString *) keyPath ofObject: (id) object
>> change: (NSDictionary *) change context: (void *) context
>> {
>> if ( context == (__bridge void *) @"mediaLibraryLoaded" )
>> {
>> // ...
>>
>>
>> My question is: how can the compiler know that '==' in this case is a NSString comparison?
>> Or is some other magic going on here? if so, which?
>> Does the compiler know it should perform some kind of dynamic method dispatch?
>
> My guess, without seeing the code that set up the observer, is that it was also set up with @"mediaLibraryLoaded", and the compiler collects and reuses string constants, so the address is the same. I'd guess that if you ensure that the string is a unique variable, it won't work.
>
> NSString* s = [NSString stringWithFormat:@"%@%@%@", @"media", @"Library", @"Loaded"];
> if(context == (__bridge void*)s)
>
> Steve via iPad
For the above test, you could also try turning off the LLVM code-gen setting "gcc_reuse_strings".
(Which parenthetically, you probably wouldn't want to do in shipping code, particularly if you have a lot of strings.)
But yeah, as everyone says, it's generally not a good thing to rely upon this behavior, and just use -[NSString isEqual:]
Doug Hill
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