The following lines declare a new minimal Circle class in Swift. The code file for the sample is included in the swift_3_oop_chapter_02_01 folder:
class Circle {
}
The class keyword, followed by the class name (Circle), composes the header of the class definition. In this case, the class doesn't have a parent class or superclass; therefore, there are neither superclasses listed after the class name nor a colon (:). A pair of curly braces ({}) encloses the class body after the class header. In the forthcoming chapters, we will declare classes that inherit from another class, and therefore, they will have a superclass. In this case, the class body is empty. The Circle class is the simplest possible class we can declare in Swift.
Tip
Any new class you create that doesn't specify a superclass is considered a base class. Whenever you declare a class without a subclass, the class doesn't inherit from a universal base class, as happens in other programming languages such as C#. Thus, the Circle class is known as a base class in Swift.