In Objective-C, why can a class extension add instance variables, but a class class cannot?

Question

I know that we cannot add instance variables to the class category, but we can add to the continuation class, can someone tell me why or give me some details about this. I know that we can use the associated element in a category, I want to know that implementation details lead to this. Many thanks.

Answer 1

TL; DR

Class extensions only modify the classes you own, while categories can modify any existing class.

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http://www.sealiesoftware.com/blog/archive/2009/01/27/objc_explain_Non-fragile_ivars.html http://www.cocoawithlove.com/2010/03/dynamic-ivars-solving-fragile-base.html

Answer 2

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@interface SimpleClass : NSObject
@property int someProperty;
@end

@interface Foo()
@property float aNewProperty; // added through class extension 
@end

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