objective c - Category on NSObject also works on Class -


so have category on nsobject called customcategory, following:

#import <foundation/foundation.h>  @interface nsobject (customcategory)  -(bool)dosomething;  @end  #import "nsobject+customcategory.h"  @implementation nsobject (customcategory)  -(bool)dosomething {     nslog(@"done");     return no; }  @end

ideally, work on object this:

nsobject* object = [nsobject new]; [object dosomething];

however, found works in way no problem:

[nsobject dosomething];

so wondering, since instance method have added through category, why works on class?

instance methods on nsobject class methods on nsobject.

this works because of way objective-c dynamic dispatch. if send message object method implementation looked in objects class. if send message class sending regular message class object. there implementation looked in called meta-class. meta-classes automatically generated compiler. class methods instance methods of meta-class. handled transparently compiler.

inheritance works on meta-class level. meta-class class inherits meta-class of superclass. have 2 parallel inheritance hierarchies there. root classes nsobject handled differently. there meta-class can't inherit meta-class of superclass there no superclass. root classes meta-class inherits root class itself.

and since class methods of class instance methods of meta-class , nsobjects meta-class inherits nsobject instance methods on nsobject class methods on nsobject.


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