Re: why does this method return an id?
Re: why does this method return an id?
- Subject: Re: why does this method return an id?
- From: Matt Neuburg <email@hidden>
- Date: Mon, 12 Dec 2011 22:39:20 -0800
On Sun, 11 Dec 2011 12:34:57 -0600, Charles Srstka <email@hidden> said:
>On Dec 11, 2011, at 10:58 AM, Matt Neuburg wrote:
>
>> There seems to be a small hole in my understanding of Objective-C. I sort of understand why alloc-init returns an id, but why do so many class method convenience instantiators also return an id? For example:
>>
>> [NSSortDescriptor sortDescriptorWithKey:@"indexOrig" ascending:YES]
>>
>> That method is declared as returning an id, which means you can use it accidentally anywhere, assign the result to the wrong thing, and get a mysterious crash. (Guess how I know that?) Now, I think I know why [NSString string] is declared as returning an id - it's because it's a class cluster, right? But NSSortDescriptor isn't a class cluster; the result really is an NSSortDescriptor. So why isn't it *declared* as an NSSortDescriptor? Thx - m.
>
>Most likely it’s to accommodate subclasses. If it weren’t declared to return an id, then doing something like this:
>
>MyFancySortDescriptorSubclass *sortDescriptor = [MyFancySortDescriptorSubclass sortDescriptorWithKey:@“Foo” ascending:YES];
>
>would cause a compiler warning.
But if you subclassed NSSortDescriptor and didn't override sortDescriptorWithKey:ascending: and expected [MyFancyEtc. sortDescriptorEtc.] to magically produce a MyFancyEtc. instead of an NSSortDescriptor, you'd *deserve* that warning. In fact, having that warning would be *good*. Why are we turning type checking *off* at a crucial moment like this?
Also, if that's the right answer (i.e. if it's all about subclassing), then I don't get why *every* convenience constructor isn't typed as an id. Are the counterexamples all things that one is expected never to subclass?
m.
--
matt neuburg, phd = email@hidden, <http://www.apeth.net/matt/>
A fool + a tool + an autorelease pool = cool!
Programming iOS 4!
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