Swift native base class or NSObject

Swift classes that are subclasses of NSObject:

• are Objective-C classes themselves
• use objc_msgSend() for calls to (most of) their methods
• provide Objective-C runtime metadata for (most of) their method implementations

Swift classes that are not subclasses of NSObject:

• are Objective-C classes, but implement only a handful of methods for NSObject compatibility
• do not use objc_msgSend() for calls to their methods (by default)
• do not provide Objective-C runtime metadata for their method implementations (by default)

Subclassing NSObject in Swift gets you Objective-C runtime flexibility but also Objective-C performance. Avoiding NSObject can improve performance if you don't need Objective-C's flexibility.

Edit:

With Xcode 6 beta 6, the dynamic attribute appears. This allows us to instruct Swift that a method should use dynamic dispatch, and will therefore support interception.

public dynamic func foobar() -> AnyObject {
}

I also found that if basing a Swift class on NSObject, I saw some unexpected run-time behaviour that could hide coding bugs. Here is an example.

In this example, where we don't base on NSObject, the compiler correctly spots the error in testIncorrect_CompilerShouldSpot, reporting "... 'MyClass' is not convertible to 'MirrorDisposition'"

class MyClass {
  let mString = "Test"

  func getAsString() -> String {
    return mString
  }

  func testIncorrect_CompilerShouldSpot() {
    var myString = "Compare to me"
      var myObject = MyClass()
      if (myObject == myString) {
        // Do something
      }
  }

  func testCorrect_CorrectlyWritten() {
    var myString = "Compare to me"
      var myObject = MyClass()
      if (myObject.getAsString() == myString) {
        // Do something
      }
  }
}

In this example, where we base on NSObject, the compiler doesn't spot the error in testIncorrect_CompilerShouldSpot:

class myClass : NSObject {
  let mString = "Test"

  func getAsString() -> String {
    return mString
  }

  func testIncorrect_CompilerShouldSpot() {
    var myString = "Compare to me"
      var myObject = MyClass()
      if (myObject == myString) {
        // Do something
      }
  }

  func testCorrect_CorrectlyWritten() {
    var myString = "Compare to me"
      var myObject = MyClass()
      if (myObject.getAsString() == myString) {
        // Do something
      }
  }
}

I guess the moral is, only base on NSObject where you really have to!

According to the language reference, there is no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed.

Note that omitting a superclass from the class declaration, doesn't assign a implicit base superclass of any kind. It defines a base class, which will effectively become the root for an independent class hierarchy.

From the language reference:

Swift classes do not inherit from a universal base class. Classes you define without specifying a superclass automatically become base classes for you to build upon.

Trying to reference super from a class without a super class (i.e. a base class) will result in a compile time error

'super' members cannot be referenced in a root class

The following is copied from Apple's Swift-eBook and gives an appropriate answer to your question:

Defining a Base-Class

Any class that does not inherit from another class is known as a base class.

Swift classes do not inherit from a universal base class. Classes you define without specifying a superclass automatically become base classes for you to build upon.

Reference