CODEWITHSUNDEEP
iosobjective-ccocoaautomatic-ref-countingautorelease
Awesome-o
Awesome-o

Reputation: 2070

__autoreleasing for return value in ARC Objective-C

According to Cocoa's "Create Rule" a method that returns a newly instantiated object must begin with either new, create, or copy if ownership is transferred to the caller.

Let's say I have a class that implements a factory method called (NSObject *)getCorrectObject.

This method will return a new instance of the correct subclass of the callee (determined by some internal state of the callee). Technically this method does not follow the "Create Rule" and could lead to memory leaks in non-ARC environments.

Would it be possible to instead use (NSObject *__autoreleasing)getCorrectObject to avoid using new or create in this case?

In non-ARC I would return an autoreleased object, but I'm not entirely sure if __autoreleasing works for anything other than In/Out parameters.

Upvotes: 1

Views: 761

Answers (1)

Rob Napier
Rob Napier

Reputation: 299275

According to Cocoa's "Create Rule" a method that returns a newly instantiated object must begin with either new, create, or copy if ownership is transferred to the caller.

This isn't called the Create Rule (and isn't correct). The Create Rule is a Core Foundation rule related to the words Create and Copy. Cocoa has a different rule related to “alloc”, “new”, “copy”, or “mutableCopy”.

Let's say I have a class that implements a factory method called (NSObject *)getCorrectObject.

Then it would be incorrectly named. Starting a method with get indicates that it returns a value by reference. The correct signature would be:

+ (BOOL)getCorrectObject(NSObject**)result;

This method will return a new instance of the correct subclass of the callee (determined by some internal state of the callee). Technically this method does not follow the "Create Rule" and could lead to memory leaks in non-ARC environments.

That is not based on whether it is a new instance. It's based on whether it includes an unbalanced retain.

Methods that begin with “alloc”, “new”, “copy”, or “mutableCopy” should return an object with one unbalanced retain. Other methods should return an object that has an equal number of retain and autorelease attached to it.

If for some reason you have to violate these naming rules (and you really don't want to violate these rules), the correct way to indicate it to ARC is by using NS_RETURNS_RETAINED (see Foundation/NSObjCRuntime.h for all the macros). This only assists ARC in correctly fixing your memory management when dealing with mixed ARC/Retain code in cases where it is impossible to fix your incorrect naming. Pure ARC code doesn't need this; it'll balance out either way. Pure Retain code won't be helped by this. The caller has to do the right thing (and so has to just know that you did it wrong and compensate).

Note that as long as you're using ARC throughout the project (and you really, really should be), most of this won't matter a lot in practice. ARC will balance everything out for you pretty much no matter what you name things, since you're not in charge of adding the retains, releases, and autoreleases anyway.

Upvotes: 2

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