On Dec 11, 2011, at 12:39 PM, Ben Kennedy wrote:
> On 11 Dec 2011, at 10:34 am, Charles Srstka wrote:
>
>> 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.
>
> No it wouldn't. Precisely because it's a subclass.
>
> (On the other hand, if NSSortDescriptor tried returning a
> MyFancySortDescriptorSubclass, you would get a warning.)
Yes it would, because the compiler has no way of knowing that something defined
to return a generic NSSortDescriptor * is the right subclass to assign to a
variable typed MyFancySortDescriptorSubclass * or not.
#import <Foundation/Foundation.h>
@interface Foo : NSObject
+ (Foo *)foo;
@end
@implementation Foo
+ (Foo *)foo {
return [[[self alloc] init] autorelease];
}
@end
@interface Bar : Foo
@end
@implementation Bar
@end
int main (int argc, const char * argv[]) {
@autoreleasepool {
Bar *bar = [Bar foo];
NSLog(@"class of bar: %@", NSStringFromClass([bar class]));
}
return 0;
}
When building this, the compiler warns:
$ clang -framework Foundation main.m
main.m:35:14: warning: incompatible pointer types initializing 'Bar *' with an
expression of type 'Foo *' [-Wincompatible-pointer-types]
Bar *bar = [Bar foo];
^ ~~~~~~~~~
1 warning generated.
However, when running it, the result is correct:
2011-12-11 12:52:26.489 a.out[17461:707] class of bar: Bar
showing us that this would have been just fine if +foo had returned an id.
Charles_______________________________________________
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