Post By: Hanan Mannan
Contact Number: Pak (+92)-321-59-95-634
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LISP -Common Lisp Object System (CLOS)

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))

When you execute the code, it returns the following result:

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

Defining a Class Method

The defmethod macro allows you to define a method inside the class. The following example extends our Box class to include a method named volume.

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)
   (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))

When you execute the code, it returns the following result:

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

Inheritance

LISP allows you to define an object in terms of another object. This is called inheritance. You can create a derived class by adding features that are new or different. The derived class inherits the functionalities of the parent class.

The following example explains this:

Example

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)
   (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))

When you execute the code, it returns the following result:

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