6.3: Every Object Is an Instance of a Class

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Every object has a class; you can find out which one by sending it the message class.

1 class
>>> SmallInteger

20 factorial class
>>> LargePositiveInteger

'hello' class
>>> ByteString

(4@5) class
>>> Point

Object new class
>>> Object

A class defines the structure of its instances via instance variables, and the behavior of its instances via methods. Each method has a name, called its selector, which is unique within the class.

Since classes are objects, and every object is an instance of a class, it follows that classes must also be instances of classes. A class whose instances are classes is called a metaclass. Whenever you create a class, the system automatically creates a metaclass for you. The metaclass defines the structure and behavior of the class that is its instance. 99% of the time you will not need to think about metaclasses, and may happily ignore them. (We will have a closer look at metaclasses in Chapter : Classes and Metaclasses.)