Hi. I'm an objective-c newbie, and I have a question about a point
mentioned in Apple's documentation: http://developer.apple.com/documentation/Cocoa/Conceptual/
ObjectiveC/LanguageOverview/chapter_4_section_6.html#//apple_ref/
doc/uid/20001424-BAJHIAGB
The end of the section says, "In fact, rather than sending the
alloc message to self in a class method, it’s often better to send
alloc to [self class]. This way, if the class is subclassed, and
the rectangleOfColor: message is received by a subclass, the
instance returned will be the same type as the subclass." They
list a GOOD version and an EXCELLENT version of the
rectangleOfColor: class method. The only difference is that the
GOOD version does "id newInstance = [[self alloc] init];", while
the EXELLENT version does "id newInstance = [[[self class] alloc]
init];".
I'm hoping that someone can explain this point to me, because I
can't seem to wrap my mind around it. I don't see why there would
be a difference between the two versions. It seems to me that
[self class] would return the same object that self points to,
namely, your subclass object. After all, self should point to the
same object in either version of the method (when the receiver is
the subclass). If self didn't point to the subclass object in the
GOOD version (suppose, for example, that it pointed to the
subclass's superclass, which contains the definition of the
rectangleOfColor: method), then [self class] wouldn't return the
correct object in the EXCELLENT version. Moreover, if [self alloc]
created an instance of the superclass rather than of the subclass,
then the GOOD method would actually be disastrous, not just less
than excellent, because you'd eventually be trying to assign values
to non-existent instance variables of the subclass.
I'd appreciate any clarification here. Thanks!
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