NSCopying, copyWithZone and NSDictionary

Question

Firstly I would like confirmation that I have understood NSCopying correctly ...

In order to use a simple NSObject subclass as a key in an NSDictionary I must have it implement the NSCopying protocol. In the copied instance's copyWithZone method I must alloc/init a new instance of my class, set its properties to be identical to the copied instance and return it.

Secondly, why does an NSDictionary use a copy of the instance added to it rather than the instance itself?

Keys in a dictionary are copied. The objects are retained. Usually, strings are used as keys, and since they do conform to NSCopying, there's no issues. If for some reason you wanted to use some other kind of object, you'd have to make certain it conformed to NSCopying as well. — Extra Savoir-Faire, Sep 26 '11 at 14:55
Look at http://stackoverflow.com/questions/2394083/cocoas-nsdictionary-why-are-keys-copied — Extra Savoir-Faire, Sep 26 '11 at 15:29

score 3 · Accepted Answer · answered Sep 26 '11 at 14:52

3

The subclass does not need to implement NSCopying if it is the object, the key should usually be NSStrings, which are copied.

answered Sep 26 '11 at 14:52

hypercrypt

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Right: Only keys need to implement `NSCopying`. – Wevah Sep 26 '11 at 14:54
Sorry I was unclear. The object will be used as a key. I've amended the question to reflect this. So why does it need a copy of the object rather than the object itself? – Undistraction Sep 26 '11 at 15:05
Also, if you need to have custom behaviour on your keys / objects, have a look at [CFDictionary](http://developer.apple.com/library/ios/#documentation/CoreFoundation/Reference/CFDictionaryRef/Reference/reference.html) and [CFMutableDictionary](http://developer.apple.com/library/ios/#documentation/CoreFoundation/Reference/CFMutableDictionaryRef/Reference/reference.html) which are toll-free bridged to `NSDictionary` and `NSMutableDictionary` – hypercrypt Sep 26 '11 at 15:13
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Thanks for the links. Missed that first one somehow. Just found this as well which explains how to correctly implement an object for use as a key (and so you can use it for comparison) http://bynomial.com/blog/?p=73 – Undistraction Sep 26 '11 at 15:38

score 1 · Answer 2 · answered Sep 26 '11 at 14:56

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You have to implement NSCopying protocol for every custom class if you want make them copyable
It must use a copy because if you will modify an object contained in the original dictionary it's copy won't be affected by the change

answered Sep 26 '11 at 14:56

daveoncode

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NSCopying, copyWithZone and NSDictionary

2 Answers2