What is nonnull in objective C?

Question

Can someone elaborate why is nonnull introduced in iOS 9 ?

For example, the NSArray method + (instancetype)array; is now + (instancetype nonnull)array;

Refer to : https://developer.apple.com/library/prerelease/ios/releasenotes/General/iOS90APIDiffs/frameworks/Foundation.html

Is this an objective-c level feature, and how would this affect the existing apps ?

score 22 · Accepted Answer · edited Feb 20 '16 at 23:26

They have made sure that wherever the type is not-nullable it is now a nonnull type.

Like earlier NSMutableArray addObject method was

- (void)addObject:(ObjectType)anObject

and now it has been changed to

- (void)addObject:(ObjectType nonnull)anObject

So it means you cannot pass a null object (nil) to this method. Same way, in your case

+ (instancetype nonnull) array

method will never return nil.

Reference: https://developer.apple.com/swift/blog/?id=25

rounak · Answer 2 · 2015-06-23T12:18:37.797

11

nonnull is a keyword to tell the compiler that the return value (or parameter, or property) will never be nil. This was introduced in a previous version of Xcode to enable better inter operability between Obj-C and Swift's optional types.

You can learn more about it on the official Swift blog

edited Jun 23 '15 at 12:18

answered Jun 23 '15 at 11:53

rounak

9,217
3
42
59

9

You mean "will never *intentionally* be nil". – Hot Licks Jun 23 '15 at 12:14
1

It may be nil, but then your app is in DS anyway, and beyond hope. I wonder if anyone has tried what happens when you _do_ assign nil to a Swift object reference. – gnasher729 Jun 23 '15 at 13:48
2

@gnasher729 - Based on the questions I see here, people try that quite often. – Hot Licks Jun 24 '15 at 11:43

score 6 · Answer 3 · answered Jun 23 '15 at 13:22

nonnull is keyword which notify compiler that the value returned by object/parameters will never be nil.

In general, you should look at nullable and nonnull roughly the way you currently use assertions or exceptions: violating the contract is a programmer error. In particular, return values are something you control, so you should never return nil for a non-nullable return type unless it is for backwards-compatibility.

score 0 · Answer 4 · answered Jun 23 '15 at 12:37

nullable and nonnull has been introduced to make Objective C and Swift interoperability easier.

Objective C doesn't make any difference between optional and non-optional references. Then Swift compiler cannot be sure if a particular reference to Objective C code is optional or not.

nullable annotation is the same than optional in Swift. nonnull annotation is the same than non-optional in Swift.

The rule of thumb is any simple pointer type will be assumed to be nonnull (for more details read the official Swift blog)

I would also say that this new annotation will also improve code quality from Objective C point of view. I usually wonder would the app crash if a pass nil as parameter? For example:

id var;
NSMutableArray *a = [NSMutableArray new];
[a addObject:var];

Compiler doesn't say anything in this case and your application will crash in execution time! Now with this new annotation you will see a warning in compilation time. I know this example is stupid, but there are some cases that you don't know if you need to check whether or not a property is nil before calling a method unless you read the documentation.

In your example, I think it's important to note that the application only crashes due to the exception being thrown by NSMutableArray. The fact that the object argument is annotated as nonnull doesn't make the app crash with an exception (unfortunately). I tend to validate my inputs for nil still within my public methods and throw my own descriptive exceptions rather than it crashing later on for reasons harder to diagnose. — Dom Chapman, Dec 27 '15 at 14:18

What is nonnull in objective C?

4 Answers4

Linked