How to add a variable to category in Objective-C?

Question

It's requied to create a category with a new variable (of type NSArray).

OriginalClass+Extension.h:

@interface OriginalClass (Extension) {
    NSArray *_array;
}

@property (nonatomic, retain) NSArray *array;

@end

But I got the error: Cannot declare variable inside @interface or @protocol.

Please help to solve the problem.

score 4 · Accepted Answer · edited May 23 '17 at 12:14

4

As the other stated, you can't. Although has H2CO3 pointed out, you can use associative references. On Apple Documents:

Note that a category can’t declare additional instance variables for the class; it includes only methods. However, all instance variables within the scope of the class are also within the scope of the category. That includes all instance variables declared by the class, even ones declared @private.

If you want to go for associated object, you can use this answer. Moreover, you can use this post by Ole Begemann.

edited May 23 '17 at 12:14

Community

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answered Oct 21 '12 at 18:41

Rui Peres

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@Altaveron The same workaround that H2CO3 proposed, earlier ... (StackOverflow is strange :-) ) – aleroot Oct 21 '12 at 19:06
This answer has a link to exact code. – Dmitry Oct 21 '12 at 19:08
@Altaveron mine has the exact same link. – Oct 21 '12 at 19:58
No! http://oleb.net/blog/2011/05/faking-ivars-in-objc-categories-with-associative-references/ – Dmitry Oct 21 '12 at 22:14

score 3 · Answer 2 · answered Oct 21 '12 at 18:16

3

You can't, a category can't declare additional instance variables ...

Reference : here .

answered Oct 21 '12 at 18:16

aleroot

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What should I use to add a new data variable to the class? – Dmitry Oct 21 '12 at 18:17
You will have to subclass though – aleroot Oct 21 '12 at 18:18
I want to use the modified class with original class name. – Dmitry Oct 21 '12 at 18:20
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You can't add an instance variable in a category, period . – aleroot Oct 21 '12 at 18:20
http://oleb.net/blog/2011/05/faking-ivars-in-objc-categories-with-associative-references/ – Dmitry Oct 21 '12 at 19:06
@Altaveron it is a workaround ... – aleroot Oct 21 '12 at 19:07

score 2 · Answer 3 · answered Oct 21 '12 at 18:15

2

Simple: you can't add instance variables to a class using a category.

If you need to store additional data: use associated objects.

answered Oct 21 '12 at 18:15

1

@Altaveron perhaps associated objects can help you. (See my edited answer.) – Oct 21 '12 at 18:17
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A category allows you to add methods to an existing class. Period. See http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/objectivec/chapters/occategories.html – kamprath Oct 21 '12 at 18:17

How to add a variable to category in Objective-C?

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