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From the author of Copying and Mutability

Copying and Mutability

All Foundation classes do one very simple thing to improve efficiency, and people often fail to consider this fact in their own code: When you send a -copy message to an immutable object, it returns self.

Most imperative languages encourage the idea of defensive copying. If you're not sure whether you want a reference to an object or a copy of it, you should take a copy. This scheme prevents other parts of the program from modifying things that you don't want them to change.

In various bits of Cocoa, you'll see copy methods implemented like this:

- (id)copyWithZone: (NSZone*)aZone
    return [self retain];

If you send a -copy or -copyWithZone: message to the object, you won't actually get a copy; instead, you'll get a new owned reference. From your perspective, there's no difference, because a copy of an immutable object is semantically equivalent to an identical new object.

Remembering to do this is very important if, for example, you create a custom object type to use as keys in a dictionary. NSDictionary copies objects that are used as keys, so you end up with a lot of redundant copies if your immutable objects are performing real copies.

Another important thing to remember is to copy early. If you send a -copy message to a mutable object, you'll get an immutable copy. If you then send a -copy message to that object, you'll just increase its reference count.

If you keep sending -copy messages to a mutable object, however, you'll keep getting new copies. This approach is a lot more expensive, in terms of both CPU time and memory. Instead, you can make the immutable copy early and keep using it, which is a lot cheaper.

You can find places where you're copying redundantly by looking for -copyWithZone: messages being sent to mutable objects.

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