Automatic Reference Counting: Pointer to non-const type 'NSError *' with no explicit ownership

Question

In updating some of my code to be in compatible with the iOS 5 SDK, I attempted to refactor my code by using "Convert to Objective-C ARC" in Xcode and received an error. The error occurs on an instance variable in my .h file.

NSError **_error;

The error says "Pointer to non-const type 'NSError *' with no explicit ownership." How might I fix this?

This question (see http://stackoverflow.com/questions/6907726/setting-nserror-within-a-block-using-arc) is the closest thing I can find on the subject. — Ziminji, Oct 18 '11 at 12:14
Why are you using double indirection in an instance variable? You should only need that for method parameters so that NSErrors can be passed back up. — Brad Larson, Oct 18 '11 at 14:37
@BradLarson ok, so what's the correct way (in ARC) to initialize an `NSError` instance when calling a method that takes `(NSError**):error` as an argument? — raffian, Sep 23 '12 at 18:01
@SAFX - Same as you did before ARC, `NSError *error = nil;` and then you'll just pass `&error` into your method. — Brad Larson, Sep 24 '12 at 14:54

score 60 · Answer 1 · edited Jul 31 '12 at 12:59

60

You can use one of the lifetime qualifiers described in the Transitioning to ARC Release Notes

For NSError ** you'd use __autoreleasing so the example provided by Nikolai would then look like this

@interface Foo : NSObject
{
    NSError * __autoreleasing *error;
}

The syntax is a little weird and you should follow Nikolais advice, but if you're sure NSError ** is what you want this should work.

edited Jul 31 '12 at 12:59

John Conde

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answered Apr 25 '12 at 16:38

tapi

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2

This doesn't compile, try `NSError * __autoreleasing *_error;` – Luke Mcneice Jul 10 '12 at 16:34
2

This is the correct answer. It is not wrong to do this as the answer by @NikolaiRuhe states. – Paul de Lange Jul 31 '12 at 12:58
3

Thanks for this answer! I'd like to add that I've also used other lifetime qualifiers with this same syntax, like `NSArray * __strong *array`, etc. – Mr. T Jan 31 '14 at 19:39

score 18 · Accepted Answer · answered Oct 18 '11 at 14:32

When storing NSError objects in an instance variable you have to declare it as a simple pointer:

@interface Foo : NSObject
{
    NSError *_errror;
}

NSError ** is only used to indirectly return NSError objects from a method to the caller. It is (probably) an error to store it in an instance variable.

Automatic Reference Counting: Pointer to non-const type 'NSError *' with no explicit ownership

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