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.
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:
You can specify accessors for each slot when you define a class. For example, take our Box class:
It has the following syntax:
Create a new source code file named main.lisp and type the following code in it.
Create a new source code file named main.lisp and type the following code in it.
The following example explains this:
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
(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
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