Common LISP predated the advance of object-oriented programming by couple of decades. However, it object-orientation was incorporated into it at a later stage.

Defining Classes

The defclass macro allows creating user-defined classes. It establishes a class as a data type. It has the following syntax −

(defclass class-name (superclass-name*)
   (slot-description*)
   class-option*))

The slots are variables that store data, or fields.

A slot-description has the form (slot-name slot-option*), where each option is a keyword followed by a name, expression and other options. Most commonly used slot options are −

• :accessor function-name

• :initform expression

• :initarg symbol

For example, let us define a Box class, with three slots length, breadth, and height.

(defclass Box () 
   (length 
   breadth 
   height)
)

Providing Access and Read/Write Control to a Slot

Unless the slots have values that can be accessed, read or written to, classes are pretty useless.

You can specify accessors for each slot when you define a class. For example, take our Box class −

(defclass Box ()
   ((length :accessor length)
      (breadth :accessor breadth)
      (height :accessor height)
   )
)

You can also specify separate accessor names for reading and writing a slot.

(defclass Box ()
   ((length :reader get-length :writer set-length)
      (breadth :reader get-breadth :writer set-breadth)
      (height :reader get-height :writer set-height)
   )
)

Creating Instance of a Class

The generic function make-instance creates and returns a new instance of a class.

It has the following syntax −

(make-instance class {initarg value}*)

Example

Let us create a Box class, with three slots, length, breadth and height. We will use three slot accessors to set the values in these fields.

Create a new source code file named main.lisp and type the following code in it.

(defclass box ()
   ((length :accessor box-length)
      (breadth :accessor box-breadth)
      (height :accessor box-height)
   )
)
(setf item (make-instance ''box))
(setf (box-length item) 10)
(setf (box-breadth item) 10)
(setf (box-height item) 5)
(format t "Length of the Box is ~d~%" (box-length item))
(format t "Breadth of the Box is ~d~%" (box-breadth item))
(format t "Height of the Box is ~d~%" (box-height item))

Length of the Box is 10
Breadth of the Box is 10
Height of the Box is 5

(defclass box ()
   ((length :accessor box-length)
      (breadth :accessor box-breadth)
      (height :accessor box-height)
      (volume :reader volume)
   )
)

; method calculating volume   

(defmethod volume ((object box))
   (* (box-length object) (box-breadth object)(box-height object))
)

 ;setting the values 

(setf item (make-instance ''box))
(setf (box-length item) 10)
(setf (box-breadth item) 10)
(setf (box-height item) 5)

; displaying values

(format t "Length of the Box is ~d~%" (box-length item))
(format t "Breadth of the Box is ~d~%" (box-breadth item))
(format t "Height of the Box is ~d~%" (box-height item))
(format t "Volume of the Box is ~d~%" (volume item))

Length of the Box is 10
Breadth of the Box is 10
Height of the Box is 5
Volume of the Box is 500

(defclass box ()
   ((length :accessor box-length)
      (breadth :accessor box-breadth)
      (height :accessor box-height)
      (volume :reader volume)
   )
)

; method calculating volume   
(defmethod volume ((object box))
   (* (box-length object) (box-breadth object)(box-height object))
)
  
;wooden-box class inherits the box class  
(defclass wooden-box (box)
((price :accessor box-price)))

;setting the values 
(setf item (make-instance ''wooden-box))
(setf (box-length item) 10)
(setf (box-breadth item) 10)
(setf (box-height item) 5)
(setf (box-price item) 1000)

; displaying values
(format t "Length of the Wooden Box is ~d~%" (box-length item))
(format t "Breadth of the Wooden Box is ~d~%" (box-breadth item))
(format t "Height of the Wooden Box is ~d~%" (box-height item))
(format t "Volume of the Wooden Box is ~d~%" (volume item))
(format t "Price of the Wooden Box is ~d~%" (box-price item))

Length of the Wooden Box is 10
Breadth of the Wooden Box is 10
Height of the Wooden Box is 5
Volume of the Wooden Box is 500
Price of the Wooden Box is 1000