A while ago, I came across a problematic piece of code that turned out to be an interesting one:

extension UIViewController {
    static var screenName = String(describing: self)
}

The value of screenName is always equal to (Function). But what I needed was the type of UIViewController instance as a String. For example:

class ReaderViewController: UIViewController { ... }
print(ReaderViewController)

// Expected Output: ReaderViewController
// Actual Output: (Function)

But why is that so?

I tried to simplify the problematic piece of code and reduce the variables that were involved. In the first step, I took out screenName from the extension and put it inside a class:

class ReaderViewController: UIViewController {
    static var screenName = String(describing: self)
}

The result was the same.

What if ReaderViewController doesn’t inherit from UIViewController?

class ReaderViewController {
    static var screenName = String(describing: self)
}

Boom! Compiler Error:

Use of unresolved identifier 'self'.

By trial and error I managed to narrow down the problem to the point that led me to this conclusion:

• If the root class of ReaderViewController is NSObject, the value of screenName is (Function).
• Otherwise, we get the compiler error

It looks like we have two different problems here that need to be investigated separately. But actually, when I was looking for the root cause of the former one, I found an explanation for the later one too.

When I jumped to the definition of NSObject, I faced this (summarized) code:

public protocol NSObjectProtocol {

    //...

    func `self`() -> Self

    //...

}

@available(iOS 2.0, *)
open class NSObject : NSObjectProtocol {
    //...
}

self is a method that returns the object which it is called on. It explains why String(describing: self) returns (Function) within a subclass of NSObject. But it doesn’t explain why within the initializer expression of a stored type property, self isn’t available.

I replaced the stored type property with a computed one, and it silenced the compiler-error problem:

class ReaderViewController {
    static var screenName: String {
        return String(describing: self)
    }
}

Still, I don’t know why referring to self within type property initializer expression caused that compiler error and I have asked it here.

By the way, at least now I know that self within type property initializer expression is not available, so it is an unresolved identifier. If the class is a subclass of NSObject, in the absence of self, NSObject’s self ( which is a method ) is available and String(describing: self) returns the type of it which is (Function).

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P.S. While I was writing this post, I found this related question on Swift Forum: