How do I know if it is an NSObject?

2 minute read

While I spend most of my time trying to write Swift code these days, there is at least one thing Swift can’t do: Interop with C++. For most of you, that isn’t a problem and you don’t need to worry about it. Unfortunately for me, that is my day job. I work in integrating a C++ library into an app that is mainly Swift code, and therefore spend a lot of time writing Objective-C++ code.

Yesterday I was looking at a delegate which returned an item from a cache. Going into the cache it had to conform to the NSCopying protocol, but coming out, I could say that the type is going to be id<NSCopying> but Swift was just treating this as AnyObject? so I thought why not just leave it as id? Then, in a particular use of this method, I knew that the data I returned was going to be a UIImage. Actually, I knew that it should be a UIImage. In order to avoid bugs down the road, I checked the type first:

id myObject = [myCache getItem:@"someKey"];
if (![myObject isKindOfClass:[UIImage class]]) 
    NSLog(@"Things went wrong!");

Now, if it’s not a UIImage we can detect it. Right? Well, no.

One of the things about Objective-C that isn’t obvious to everyone is the various differences between id, id<NSObject> and NSObject. As it turns out, NSObject is the root class of most Objective-C classes. Most. Classes don’t have to inherit from NSObject and are free to choose what they like to inherit from. The most common example is NSProxy.

OK, so if it’s an object which inherits from NSProxy then the above check won’t work? Actually, it will. While NSProxy isn’t a subclass of NSObject, it does implement the NSObject protocol. This protocol actually declares that an object implementing it must have the methods we would expect to find when trying to check the type of the object, such as isKindOfClass:, isMemberOfClass:, respondsToSelector: and conformsToProtocol:.

As far as I am aware, there are no other root classes available in Foundation so we are going to have to go somewhere else for one. That is a little beyond the scope of this blog post, so if you don’t already know how this works, you basically have to do it all manually. Allocating memory, clearing it up, the whole thing. Let’s not do that and imagine we have our own root class and various other classes can inherit from it.

Back to that variable myObject, what happens if it is not something which implements the NSObject protocol? Using isKindOfClass: isn’t going to work as the selector won’t be recognised (and that’s if you can even get it to compile). Neither will any of the other methods defined in the NSObject protocol. We need a way to check what the class is without relying on these methods.

Here is where the Objective-C runtime comes to the rescue! In the runtime there is a method named object_getClassName(). This takes the instance of a class (anything with type id) and returns the name of the class as a const char*. While it makes checking inheritance a little awkward, we can definitely use it on unknown types to make our check look something more like:

id myObject = [myCache getItem:@"someKey"];
const char* objectName = object_getClassName(myObject);
if (strcmp(objectName, "MyRootClass") != 0)
    NSLog(@"Things went wrong"!);

On a final note, please don’t use custom base classes. You basically don’t ever have a good reason. I’m not saying that good reasons don’t exist, I’m just saying yours almost certainly isn’t one of them.