Classes

Previously, a :NEW message was used to create an object. The message used to see what is in an object is the :SHOW message.

 ________________________________________________________________
|
|       > (send my-class :show)
|       Object is #<Object: #27756>, Class is #<Object: #23fe2>
|         MESSAGES = NIL
|         IVARS = NIL
|         CVARS = NIL
|         CVALS = NIL
|         SUPERCLASS = #<Object: #23fd8>
|         IVARCNT = 0
|         IVARTOTAL = 0
|       #<Object: #27756>
|________________________________________________________________

From the display of the MY-CLASS object you can see there are a variety of components. The components of a class are:

Class Pointer

This pointer shows to what class the object (instance or class) belongs. For a class, this always points to the built-in object CLASS. This is also true of the CLASS object, its class pointer points to itself.

Superclass Pointer

This pointer shows what the next class up the class hierarchy is. If the user does not specify what class is the superclass, it will point to the built-in class OBJECT.

Messages

This component shows what messages are allowed for the class, and the description of the method that will be used. If the method is system-defined, it will show up in the form of '#<Subr-: #18b98>'. Remember that the class hierarchy (through the Superclass Pointer) is searched if the requested message is not found in the class.

Instance Variables

This component lists what instance variables will be created when an object instance is created. If no instances of the class exist, there are no Instance Variables. If there are 5 instances of a class, there are 5 complete and different groups of the Instance Variables.

Class Variables and Values

The CLASS VARIABLES (CVAR) component lists what class variables exist within the class. The Class Values (CVAL) component shows what the current values of the variables are. Class Variables are used to hold state information about a class. There will be one of each of the Class Variables, independent of the number of instances of the class created.