Can NSUUID be extended by inheritance? How?

Recently (reviewing some code) I stumbled upon an oddity that results in a bug in our program.

An API we are using has the following implementation (that I am going to write in Swift, even though the original code is in Objective-C)

internal class MyUUID: NSUUID { }

Which is completely useless as it always returns an empty instance.

I am going to paste the code from my playground here for explanation purposes.

For example: creating a simple NSUUID would be something like this:

let a = NSUUID()
a.description //this creates a valid uuid

While creating a MyUUID should be similar

let b = MyUUID()
b.description //it returns an instance, but is completely empty.

But it doesn't work.

Inspecting a little bit more, reveals the NSUUID initialiser creates a __NSConcreteUUID instance, while MyUUID doesn't and it doesn't matter what I try to do, it won't create an appropriate UUID.

So, my question: Is it possible to be able to create a child implementation of NSUUID?

Solution

You could use class_setSuperclass to change the superclass of MyUUID at runtime. This approach would be illegal in Swift, due to type safety, but you could still do it in Objective-C.

Depending on your actual goals you may be able to use CFUUIDRef instead.

As requested, here's an example of the class_setSuperclass approach. Just drop this in to a new single view project.

#import <objc/runtime.h>

@interface MyUUID : NSUUID

- (void) UUIDWithHello;

@end

@implementation MyUUID

- (void) UUIDWithHello {
    NSLog(@"Hello! %@", self.UUIDString);
}

@end

@interface ViewController ()


@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // Make a UUID that you want to subclass
    NSUUID *uuid = [[NSUUID alloc] init];
    NSLog(@"Initial UUID: %@", uuid.UUIDString);

    // Ignore deprecation warnings, since class_setSuperclass is deprecated
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"

    // Change MyUUID to inherit from the NSUUID's hidden subclass instead of NSUUID
    class_setSuperclass([MyUUID class], [uuid class]);  // [uuid class] is __NSConcreteUUID

    // Turn deprecation warnings back on
#pragma GCC diagnostic pop

    // Make a new myUUID and print it
    MyUUID *myUuid = [[MyUUID alloc] init];
    [myUuid UUIDWithHello];
}

@end

Note that this is a bit dangerous. If whatever secret subclass NSUUID has additional instance variables, it will require more memory, which [MyUUID alloc] won't request. This could cause a crash later when something requests these instance variables.

To get around this, you could instead instantiate your MyUUID instance like this:

NSLog(@"Initial UUID's class: %@", NSStringFromClass(uuid.class));
Class topSecretUUIDSubclass = uuid.class; // __NSConcreteUUID
MyUUID *myUuid2 = [[topSecretUUIDSubclass alloc] init];
[myUuid2 UUIDWithHello];
object_setClass(myUuid2, [MyUUID class]);

Basically this will make myUuid2 a __NSConcreteUUID and then change it to a MyUUID. However, this will only work if MyUUID doesn't add any instance variables.

If MyUUID does need to add its own instance variables, it will need to override +alloc to provide additional memory for these instance variables, using class_createInstance().