Objective-C Nullability: Qualifying Constant Strings

Question

I have gotten into a pretty good habit of declaring and using constant strings for things like NSNotification names. I declare them like so:

extern NSString * const ABCAwesomeThingHappenedNotification;

With the introduction of Xcode 6.3 and Swift 1.2, I'm going back and auditing Objective-C classes that interop with Swift using the new nonnull, nullable, and null_unspecified qualifiers.

When adding the qualifiers to a header that also has externally visible static strings, I receive the following warning:

warning: pointer is missing a nullability type specifier (__nonnull or __nullable)

Hmm. That's confusing / interesting. Can someone explain the reasoning behind this message? When using ABCAwesomeThingHappenedNotification in Swift, it never suggests that it's an optional String or implicitly unwrapped String.

Different question, but may also be interesting in this context: Swift 1.2 optional external variable?. — Martin R
– Martin R, Commented Apr 14, 2015 at 16:51
Related: How to create class methods that conform to a protocol shared between Swift and Objective-C? (but this isn't a duplicate) — nhgrif
– nhgrif, Commented May 18, 2015 at 0:27
April, 2018. Still need to declare the __nonnull for extern * to avoid the error. — lal
– lal, Commented Apr 15, 2018 at 21:40

Josip B. · Accepted Answer · 2015-05-07 09:14:34Z

34

I agree that having this specifier shouldn't be required but here is syntax

extern NSString * const MyConstant; extern NSString * __nonnull const MyConstant;

answered May 7, 2015 at 9:14

Josip B.

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4 Comments

SwiftArchitect Over a year ago

@stackoverflow.com/users/2791584/lneuner: actually, since we are talking about the interface, this is the correct answer.

Douglas Hill Over a year ago

The question asks for the reasoning behind the warning, not the syntax to fix it.

Josip B. Over a year ago

I agree but when you don't know the answer then fix is like an answer.

lal Over a year ago

April, 2018. Still need to declare the __nonnull for extern * to avoid the error.

Douglas Hill · Accepted Answer · 2015-04-18 20:58:58Z

In your implementation, you could define:

NSString * const ABCAwesomeThingHappenedNotification = @"ABCAwesomeThingHappenedNotification";

in which case the pointer is clearly nonnull. However, this is also valid:

NSString * const ABCAwesomeThingHappenedNotification = nil;

which must be considered nullable because the pointer is always a null pointer.

(The explicit initialisation to nil is redundant since this happens implicitly if no initial value is provided, but clarifies this example.)

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