Classes

Classes
You are encouraged to solve this task according to the task description, using any language you may know.

In object-oriented programming class is a set (a transitive closure) of types bound by the relation of inheritance. It is said that all types derived from some base type T and the type T itself form a class T. The first type T from the class T sometimes is called the root type of the class.

A class of types itself, as a type, has the values and operations of its own. The operations of are usually called methods of the root type. Both operations and values are called polymorphic.

A polymorphic operation (method) selects an implementation depending on the actual specific type of the polymorphic argument. The action of choice the type-specific implementation of a polymorphic operation is called dispatch. Correspondingly, polymorphic operations are often called dispatching or virtual. Operations with multiple arguments and/or the results of the class are called multi-methods. A further generalization of is the operation with arguments and/or results from different classes.

• single-dispatch languages are those that allow only one argument or result to control the dispatch. Usually it is the first parameter, often hidden, so that a prefix notation x.f() is used instead of mathematical f(x).
• multiple-dispatch languages allow many arguments and/or results to control the dispatch.

A polymorphic value has a type tag indicating its specific type from the class and the corresponding specific value of that type. This type is sometimes called the most specific type of a [polymorphic] value. The type tag of the value is used in order to resolve the dispatch. The set of polymorphic values of a class is a transitive closure of the sets of values of all types from that class.

In many OO languages the type of the class of T and T itself are considered equivalent. In some languages they are distinct (like in Ada). When class T and T are equivalent, there is no way to distinguish polymorphic and specific values.

The purpose of this task is to create a basic class with a method, a constructor, an instance variable and how to instantiate it.

ActionScript

<lang actionscript>package {

   public class MyClass {

       private var myVariable:int;  // Note: instance variables are usually "private"

       /**
        * The constructor
        */
       public function MyClass() {
           // creates a new instance
       }

       /**
        * A method
        */
       public function someMethod():void {
           this.myVariable = 1; // Note: "this." is optional
           // myVariable = 1; works also
       }
   }

}</lang>

Ada

Class is used in many languages to provide both encapsulation, or grouping of data and actions, and type definition. Ada packages provide encapsulation or grouping while type definitions are done using the type reserved word. Types participating in inheritance are named tagged record types.

A package specification has the following form: <lang ada>package My_Package is

  type My_Type is tagged private;
  procedure Some_Procedure(Item : out My_Type);
   function Set(Value : in Integer) return My_Type;

private

  type My_Type is tagged record
     Variable : Integer := -12;
  end record;

end My_Package;</lang> The type declaration at the top of the package gives public visibility to the private tagged type My_Type. Since My_Type is declared to be private, the public has no visibility of its structure. The type must be treated as a black box. The private section of the package specification includes the actual tagged record definition. Note that the data member Variable is initialized to -12. This corresponds to a default constructor for the type.

The package body must contain the implementation of the procedures and functions declared in the package specification. <lang ada> package body My_Package is

   procedure Some_Procedure(Item : out My_Type) is
   begin
      Item := 2 * Item;
   end Some_Procedure;

   function Set(Value : Integer) return My_Type is
      Temp : My_Type;
   begin
      Temp.Variable := Value;
      return Temp;
   end Set;

end My_Package;</lang> The Set function acts as a conversion constructor for My_Type.

An instance is typically created outside the package: <lang ada>with My_Package; use My_Package;

procedure Main is

  Foo : My_Type; -- Foo is created and initialized to -12

begin

  Some_Procedure(Foo); -- Foo is doubled
  Foo := Set(2007); -- Foo.Variable is set to 2007

end Main;</lang>

AmigaE

<lang amigae>OBJECT a_class

 varA, varP

ENDOBJECT

-> this could be used like a constructor PROC init() OF a_class

 self.varP := 10
 self.varA := 2

ENDPROC

-> the special proc end() is for destructor PROC end() OF a_class -> nothing to do here... ENDPROC

-> a not so useful getter PROC getP() OF a_class IS self.varP

PROC main()

 DEF obj : PTR TO a_class
 NEW obj.init()
 WriteF('\d\n', obj.varA)   -> this can be done, while
                            -> varP can't be accessed directly
 WriteF('\d\n', obj.varP)   -> or
 WriteF('\d\n', obj.getP())
 END obj

ENDPROC</lang>

BASIC

Works with: QuickBasic version 4.5

 DECLARE SUB MyClassDelete (pthis AS MyClass)
 DECLARE SUB MyClassSomeMethod (pthis AS MyClass)
 DECLARE SUB MyClassInit (pthis AS MyClass)

 TYPE MyClass
   Variable AS INTEGER
 END TYPE

 DIM obj AS MyClass
 MyClassInit obj
 MyClassSomeMethod obj

 SUB MyClassInit (pthis AS MyClass)
   pthis.Variable = 0
 END SUB

 SUB MyClassSomeMethod (pthis AS MyClass)
   pthis.Variable = 1
 END SUB

ALGOL 68

The following code is experimental. Basically ALGOL 68 is not object oriented, so the task to create (and use of) objects is tedious due to the lack of certain constructs, especially the lack of OO syntactic sugar. For further details:

• Ross, D.T., "Features Essential for a Workable Algol X," ALGOL Bulletin, No. 26, August 1967, pp. 6-12, and ACM SIGPLAN Notices, Vol.2, No.11, November 1967.

Other examples of this experimental approach are located at pages: Life in two dimensions, Playing Cards and Stack.

<lang algol68>MODE MYDATA = STRUCT(

   INT name1 

); STRUCT(

   INT name2,
   PROC (REF MYDATA)REF MYDATA new,
   PROC (REF MYDATA)VOID init,
   PROC (REF MYDATA)VOID some method

) class my data := (

 # name2 := # 2, # Class attribute #

 # PROC new := # (REF MYDATA new)REF MYDATA:(
       (init OF class my data)(new);
       new
  ),

 # PROC init := # (REF MYDATA self)VOID:(
       """ Constructor  (Technically an initializer rather than a true 'constructor') """;
       name1 OF self := 0 # Instance attribute #
   ),

 # PROC some method := # (REF MYDATA self)VOID:(
       """ Method """;
       name1 OF self := 1;
       name2 OF class my data := 3
   )

);

1. class name, invoked as a function is the constructor syntax #

REF MYDATA my data = (new OF class my data)(LOC MYDATA);

MODE GENDEROPT = UNION(STRING, VOID); MODE AGEOPT = UNION(INT, VOID);

MODE MYOTHERDATA = STRUCT(

   STRING name,
   GENDEROPT gender,
   AGEOPT age

); STRUCT (

   INT count,
   PROC (REF MYOTHERDATA, STRING, GENDEROPT, AGEOPT)REF MYOTHERDATA new,
   PROC (REF MYOTHERDATA, STRING, GENDEROPT, AGEOPT)VOID init,
   PROC (REF MYOTHERDATA)VOID del

) class my other data := (

 # count := # 0,  # Population of "(init OF class my other data)" objects #

 # PROC new := # (REF MYOTHERDATA new, STRING name, GENDEROPT gender, AGEOPT age)REF MYOTHERDATA:(
         (init OF class my other data)(new, name, gender, age);
         new
     ),

 # PROC init := # (REF MYOTHERDATA self, STRING name, GENDEROPT gender, AGEOPT age)VOID:(
       """ One initializer required, others are optional (with different defaults) """;
       count OF class my other data +:= 1;
       name OF self := name;
       gender OF self := gender;
       CASE gender OF self IN
           (VOID):gender OF self := "Male"
       ESAC;
       age OF self := age
   ),

 # PROC del := # (REF MYOTHERDATA self)VOID:(
       count OF class my other data -:= 1
   )

);

PROC attribute error := STRING: error char; # mend the error with the "error char" #

1. Allocate the instance from HEAP #

REF MYOTHERDATA person1 = (new OF class my other data)(HEAP MYOTHERDATA, "John", EMPTY, EMPTY); print (((name OF person1), space,

       (gender OF person1|(STRING gender):gender|attribute error), new line));  # "John Male" #

print (((age OF person1|(INT age):age|attribute error), new line)); # Raises AttributeError exception! #

1. Allocate the instance from LOC (stack) #

REF MYOTHERDATA person2 = (new OF class my other data)(LOC MYOTHERDATA, "Jane", "Female", 23); print (((name OF person2), space,

       (gender OF person2|(STRING gender):gender|attribute error), space, 
       (age OF person2|(INT age):age|attribute error), new line))  # "Jane Female 23" #</lang>

Output:

John Male
*
Jane Female         +23

C

Works with: gcc version 4.0.2

<lang c>typedef struct MyClass {

 int variable;

} MyClass;

struct MyClass* MyClass_new() {

 struct MyClass* pthis = (struct MyClass*)malloc( sizeof(struct MyClass) );
 //memset(pthis, 0, sizeof(struct MyClass) );
 pthis->variable = 0;
 return pthis;

}

void MyClass_delete(struct MyClass** pthis) {

 if(pthis && *pthis)
 {
   free(*pthis);
   *pthis = NULL;
 }

}

struct void MyClass_someMethod(struct MyClass* pthis) {

 pthis->variable = 1;

}

struct MyClass* obj = MyClass_new(); MyClass_someMethod(obj); MyClass_delete(&obj);</lang>

C++

Works with: g++ version 4.0.2

<lang cpp>class MyClass { public:

 void someMethod(); // member function = method
 MyClass(); // constructor

private:

 int variable; // member variable = instance variable

};

// implementation of constructor MyClass::MyClass():

 variable(0)

{

 // here could be more code

}

// implementation of member function void MyClass::someMethod() {

 variable = 1; // alternatively: this->variable = 1

}

// Create an instance as variable MyClass instance;

// Create an instance on free store MyClass* pInstance = new MyClass; // Instances allocated with new must be explicitly destroyed when not needed any more: delete pInstance;</lang>

Note: MyClass instance(); would not define an instance, but declare a function returning an instance. Accidentally declaring functions when object definitions are wanted is a rather common bug in C++.

Functions can also be defined inline:

<lang cpp>class MyClass { public:

 MyClass(): variable(0) {}
 void someMethod() { variable = 1; }

private:

 int variable;

};</lang>

Note that member functions in C++ by default are not polymorphic; if you want a polymorphic member function, you have to mark it as virtual. In that case, you should also add a virtual destructor, even if that is empty. Example:

<lang cpp>class MyClass { public:

 virtual void someMethod(); // this is polymorphic
 virtual ~MyClass(); // destructor

};</lang>

C#

<lang csharp>public class MyClass {

   public MyClass()
   {
   }
   public void SomeMethod()
   {
   }
   private int _variable;
   public int Variable
   {
       get { return _variable; }
       set { _variable = value; }
   }
   public static void Main()
   {
       // instantiate it
       MyClass instance = new MyClass();
       // invoke the method
       instance.SomeMethod();
       // set the variable
       instance.Variable = 99;
       // get the variable
       System.Console.WriteLine( "Variable=" + instance.Variable.ToString() );
   }

}</lang>

Common Lisp

<lang lisp>(defclass circle ()

 ((radius :initarg :radius
          :initform 1.0
          :type number
          :reader radius)))

(defmethod area ((shape circle))

 (* pi (expt (radius shape) 2)))

> (defvar *c* (make-instance 'circle :radius 2)) > (area *c*) 12.566370614359172d0</lang>

D

<lang d>module Class;

import std.stdio;

class MyClass {

   //constructor
   this()
   {
   }

   void someMethod()
   {
       variable = 1;
   }

   private int _variable;

   // getter method
   int variable()
   {
       return _variable;
   }

   // setter method
   int variable(int new_variable)
   {
       return _variable = new_variable;
   }

}

void main() {

   // scope instances are allocated on the heap
   // this is not really necessary because unneeded objects will be GC'ed
   scope instance = new MyClass();

   // prints 'variable=0' because ints are initialized to 0 by default
   writefln("variable=", instance.variable);

   // invoke the method
   instance.someMethod;

   // prints 'variable=1'
   writefln("variable=", instance.variable);

   // set the variable using setter method
   instance.variable = 99;

   // prints 'variable=99'
   writefln("variable=", instance.variable);

}</lang>

E

In E, classes, constructors, and instance variables are not built into the language. This is an example of the basic convention; different cases may call for objects built in different ways.

<lang e>def makeColor(name :String) {

   def color {
       to colorize(thing :String) {
         return `$name $thing`
       }
   }
   return color

}</lang>

Example interactive session creating and using it:

<lang e>? def red := makeColor("red")

1. value: <color>

? red.colorize("apple")

1. value: "red apple"</lang>

Factor

<lang factor>TUPLE: my-class foo bar baz ; M: my-class quux foo>> 20 + ; C: <my-class> my-class 10 20 30 <my-class> quux ! result: 30 TUPLE: my-child-class < my-class quxx ; C: <my-child-class> my-child-class M: my-child-class foobar 20 >>quux ; 20 20 30 <my-child-class> foobar quux ! result: 30</lang>

Forth

Works with: Win32Forth

ANSI Forth has no object oriented features, but as Forth is a very easy language to extend, many object oriented programming systems have been implemented for it over the years. WinForth has one such system, which is described here.

Declare a class

<lang forth>:class MyClass <super Object

 int memvar

 :m ClassInit: ( -- )
      ClassInit: super
      1 to memvar ;m

 :m ~: ( -- )  ." Final " show: [ Self ] ;m

 :m set: ( n -- )  to memvar ;m
 :m show: ( -- ) ." Memvar = " memvar . ;m

class</lang>

Allocate a static object

<lang forth>MyClass newInstance</lang>

Allocate a dynamic object, saving its pointer in a global variable.

<lang forth>New> MyClass value newInstance</lang>

Call member functions

<lang forth>10 set: newInstance show: newInstance</lang>

Free a dynamically allocated object

<lang forth>newInstance dispose 0 to newInstance \ no dangling pointers!</lang>

Example of dynamic allocation and local variable use"

<lang forth>: test { \ obj -- }

   New> MyClass to obj
     show: obj
     1000 set: obj
   obj dispose ;</lang>

Groovy

A class: <lang groovy>/** Ye olde classe declaration */ class Stuff {

   /** Heare bee anne instance variable declared */
   def guts
   
   /** This constuctor converts bits into Stuff */
   Stuff(injectedGuts) {
       guts = injectedGuts
   }
   
   /** Brethren and sistren, let us flangulate with this fine flangulating method */
   def flangulate() {
       println "This stuff is flangulating its guts: ${guts}"
   }

}</lang>

A demonstration: <lang groovy>def stuff = new Stuff( I have made mistakes in the past. I have made mistakes in the future.

   -- Vice President Dan Quayle

)

stuff.flangulate()

stuff.guts = Our enemies are innovative and resourceful, and so are we. They never stop thinking about new ways to harm our country and our people, and neither do we.

   -- President George W. Bush

stuff.flangulate()</lang>

Output:

This stuff is flangulating its guts: 
I have made mistakes in the past.
I have made mistakes in the future.
    -- Vice President Dan Quayle

This stuff is flangulating its guts: 
Our enemies are innovative and resourceful, and so are we.
They never stop thinking about new ways to harm our country and our people,
and neither do we.
    -- President George W. Bush

J

Class definition: <lang j>coclass 'exampleClass'

exampleMethod=: monad define

1+exampleInstanceVariable

)

create=: monad define

'this is the constructor'

)

exampleInstanceVariable=: 0</lang>

Instantiation: <lang j> exampleObject=: conew 'exampleClass'</lang>

Java

<lang java>public class MyClass {

 // instance variable
 private int variable;  // Note: instance variables are usually "private"

 /**
 * The constructor
 */
 public MyClass() {
   // creates a new instance
 }

 /**
 * A method
 */
 public void someMethod() {
  this.variable = 1;
 }

}</lang> Note: "this." in someMethod is optional. "variable = 1;" works also. If a parameter also named "variable" came into someMethod, using "this" specifies using the instance variable rather than the local method variable.

JavaScript

Works with: Firefox version 2.0

<lang javascript>//constructor function MyClass(initVal) {

   //instance variable
   if(initVal == undefined) {
       this.number = 1;
   }
   else {
       this.number = initVal;
   }

}

//method of MyClass MyClass.prototype.getDouble = function() {

   return this.number * 2;

};

var instance1 = new MyClass; //or "new MyClass();" instance1.number = 5; alert( instance1.getDouble() ); //10

var instance2 = new MyClass(3); alert( instance2.getDouble() ); //6</lang>

Replace the alert calls with WScript.Echo, and this code

Works with: JScript version 5.7

Lisaac

<lang Lisaac>Section Header

+ name := SAMPLE;

Section Inherit

- parent : OBJECT := OBJECT;

Section Private

+ variable : INTEGER <- 0;

Section Public

- some_method <- (

 variable := 1;

);

- main <- (

 + sample : SAMPLE;

 sample := SAMPLE.clone;
 sample.some_method;

);</lang>

Oberon-2

Works with: OO2C version 2.1.11

<lang oberon2>MODULE M;

  TYPE
     T = POINTER TO TDesc;
     TDesc = RECORD
        x: INTEGER
     END;

  PROCEDURE New*(): T;
     VAR t: T;
  BEGIN
     NEW(t); t.x := 0;
     RETURN t
  END New;

  PROCEDURE (t: T) Increment*;
  BEGIN
     INC(t.x)
  END Increment;

END M.</lang>

Exported procedures are marked with an asterisk (*). There is nothing special about the constructor New, it is just a function that returns a new object of type T. The name of the method receiver can also be chosen freely. INC is a predeclared procedure that increments its argument.

Object Pascal

Works with: Turbo Pascal version 6.0

Note: This is not part of standard Pascal, but Turbo Pascal specific

<lang objectpascal>type

MyClass = object
           variable: integer;
           constructor init;
           destructor done;
           procedure someMethod;
          end;

constructor MyClass.init;

begin
 variable := 0;
end;

procedure MyClass.someMethod;

begin
 variable := 1;
end;

var

instance: MyClass; { as variable }
pInstance: ^MyClass; { on free store }

begin

{ create instances }
instance.init;
new(pInstance, init); { alternatively: pInstance := new(MyClass, init); }

{ call method }
instance.someMethod;
pInstance^.someMethod;

{ get rid of the objects }
instance.done;
dispose(pInstance, done);

end;</lang>

Objective-C

Works with: GCC

Works with: Cocoa

Works with: GNUstep

Interface:

<lang objc>// There are no class variables, so static variables are used. static int myClassVariable = 0;

@interface MyClass : NSObject {

   int variable; // instance variable

}

- (int)variable; // Typical accessor - you should use the same name as the variable

@end</lang>

Implementation:

<lang objc>@implementation MyClass

// Was not declared because init is defined in NSObject - init {

   if (self = [super init])
       variable = 0;
   return self;

}

- (int)variable {

   return variable;

}

@end</lang>

Using the class:

<lang objc>// Creating an instance MyClass *mc = [[MyClass alloc] init];

// Sending a message [mc variable];

// Releasing it. When its reference count goes to zero, it will be deallocated [mc release];</lang>

OCaml

<lang ocaml>class my_class =

 object (self)
   val mutable variable = 0
   method some_method = variable <- 1
 end</lang>

Using the class:

# let instance = new my_class;;
val instance : my_class = <obj>
# instance#some_method;;
- : unit = ()

Perl

Works with: Perl version 5.8.6

The implementation (there are no declarations): <lang perl>{

    # a class is a package (i.e. a namespace) with methods in it
   package MyClass;

    # a constructor is a function that returns a blessed reference
   sub new {
       my $class = shift;
       bless {variable => 0}, $class;
        # the instance object is a hashref in disguise.
        # (it can be a ref to anything.)
   };

    # an instance method is a function that takes an object as first argument.
    # the -> invocation syntax takes care of that nicely, see Usage paragraph below.
   sub someMethod {
       my $self = shift;
       $self->{variable} = 1;
   };

};</lang> Using the class: <lang perl>my $instance = MyClass->new; # invoke constructor method

$instance->someMethod; # invoke method on object instance

# instance deallocates when the last reference falls out of scope</lang>

Perl 6

Works with: Rakudo version #22 "Thousand Oaks"

<lang perl6>class Camel { has Int $.humps = 1; }

my Camel $a .= new; say $a.humps; # Automatically generated accessor method.

my Camel $b .= new humps => 2; say $b.humps;</lang>

A more complex example:

<lang perl6>class Butterfly {

   has Int $!age;    # The ! twigil makes it private.
   has Str $.name;
   has Str $.color;
   has Bool $.wings;

   multi method new (Str $s) {
       Butterfly.bless(*, name => $s, color => 'blue')
   }
   multi method new (Int $n) {
       Butterfly.bless(*, age => $n, name => "Butterfly aged $n")
   }

   submethod BUILD (:$!name = 'Camelia', :$!age = 2, :$!color = 'pink') {
   # BUILD is called by bless. Its primary use is to to control
   # object initialization.
       $!wings = $!age > 1;
   }

   method flap () {
       say ($.wings
         ?? 'Watch out for that hurricane!'
         !! 'No wings to flap.');
   }

}

my Butterfly $a .= new 5; say "Name: {$a.name}, Color: {$a.color}"; $a.flap;

my Butterfly $b .= new 'Osgood', age => 4; say "Name: {$b.name}, Color: {$b.color}"; $b.flap;</lang>

PHP

<lang php>class MyClass {

   public static $classVar;
   public $instanceVar; // can also initialize it here
   function __construct() {
       $this->instanceVar = 0;
   }
   function someMethod() {
       $this->instanceVar = 1;
       self::$classVar = 3;
   }

} $myObj = new MyClass();</lang>

Pop11

Object system is implemented as a library, so we must first load it.

<lang pop11>uses objectclass; define :class MyClass;

   slot value = 1;

enddefine;</lang>

Defining class MyClass automatically defines two constructors, newMyClass and consMyClass and slot (instance variable) accessors, so we can immediately start using our new class:

<lang pop11>;;; Construct instance with default slot values lvars instance1 = newMyClass();

Construct instance with explicitely given slot values

lvars instance2 = consMyClass(15);

Print slot value using dot notation

instance1.value => instance2.value =>

Print slot value using funtional notation

value(instance1) =>

Change slot value

12 -> value(instance1);

Print it

value(instance1) =></lang>

We can add methods at any time (even after creating an instance):

<lang pop11>define :method reset(x : MyClass);

  0 -> value(x);

enddefine; reset(instance1);

Print it

instance1 =></lang>

Python

<lang python>class MyClass:

   name2 = 2 # Class attribute

   def __init__(self):
       """
       Constructor  (Technically an initializer rather than a true "constructor")
       """
       self.name1 = 0 # Instance attribute
 
   def someMethod(self):
       """
       Method
       """
       self.name1 = 1
       MyClass.name2 = 3
 
 

myclass = MyClass() # class name, invoked as a function is the constructor syntax.

class MyOtherClass:

   count = 0  # Population of "MyOtherClass" objects
   def __init__(self, name, gender="Male", age=None):
       """
       One initializer required, others are optional (with different defaults)
       """
       MyOtherClass.count += 1
       self.name = name
       self.gender = gender
       if age is not None:
           self.age = age
   def __del__(self):
       MyOtherClass.count -= 1

person1 = MyOtherClass("John") print person1.name, person1.gender # "John Male" print person1.age # Raises AttributeError exception! person2 = MyOtherClass("Jane", "Female", 23) print person2.name, person2.gender, person2.age # "Jane Female 23"</lang>

Python allows for very flexible argument passing including normal named parameters, defaulted/optional named parameters, up to one "varargs" tuple, and any number of keywords arguments (which are all passed in the form of a single dictionary (associative array), and any non-ambiguous combination of these). All types of argument passing for functions can also be used for object instantiation/initialization (passed to the special __init__() method) as shown.

New-style classes inherit from "object" or any descendant of the "object" class:

<lang python>class MyClass(object):

   ...</lang>

These "new-style" classes support some features which were unavailable in "classic classes". New features include a __new__() with lower level control over object instantiation, metaclass support, static methods, class methods, "properties" (managed attributes) and "slots" (attribute restrictions).

R

R has (at least) 5 different object oriented systems. S3 and S4 correspond to different versions of the S language, from which R was derived. See, for example, this presentation by Freidrich Leisch for a more thorough introduction to S3 and S4 classes. Both these class systems are in use, and ship with the standard R distribution. The OOP, R.oo and proto packages provide other systems.

S3

S3 provides a very simple class system designed to be easily used interactively. <lang R>#You define a class simply by setting the class attribute of an object circS3 <- list(radius=5.5, centre=c(3, 4.2)) class(circS3) <- "circle"

1. plot is a generic function, so we can define a class specific method by naming it plot.classname

plot.circle <- function(x, ...) {

  t <- seq(0, 2*pi, length.out=200)
  plot(x$centre[1] + x$radius*cos(t),
     x$centre[2] + x$radius*sin(t),
     type="l", ...)

} plot(circS3)</lang>

S4

S4 is a more formal class system that provides validity checking and a way to define different methods for different input signatures. <lang R>setClass("circle",

  representation(
     radius="numeric",
     centre="numeric"),
  prototype(
     radius=1,
     centre=c(0,0)))

1. Instantiate class with some arguments

circS4 <- new("circle", radius=5.5)

1. Set other data slots (properties)

circS4@centre <- c(3,4.2)

1. Define a method

setMethod("plot", #signature("circle"),

  signature(x="circle", y="missing"),
  function(x, ...)
  {
     t <- seq(0, 2*pi, length.out=200)
     #Note the use of @ instead of $
     plot(x@centre[1] + x@radius*cos(t),
        x@centre[2] + x@radius*sin(t),
        type="l", ...)
  })

plot(circS4)</lang>

RapidQ

<lang rapidq>TYPE MyClass EXTENDS QObject

   Variable AS INTEGER

   CONSTRUCTOR
       Variable = 0
   END CONSTRUCTOR

   SUB someMethod
       MyClass.Variable = 1
   END SUB

END TYPE

' create an instance DIM instance AS MyClass

' invoke the method instance.someMethod</lang>

Raven

Build classes:

<lang raven>class Alpha

   'I am Alpha.' as greeting
   define say_hello
       greeting print

class Beta extend Alpha

   'I am Beta!' as greeting</lang>

Execute classes to create objects:

<lang raven>Alpha as alpha Beta as beta</lang>

Call methods:

<lang raven>alpha.say_hello beta.say_hello</lang>

Result:

<lang raven>I am Alpha. I am Beta!</lang>

Ruby

<lang ruby>class MyClass

 @@class_var = []

 def initialize
   # 'initialize' is the constructor method invoked during 'MyClass.new'
   @instance_var = 0
 end

 def some_method
   @instance_var = 1
   @@class_var << Time.now
 end

 def self.class_method
   # ...
 end

end

myclass = MyClass.new</lang>

Slate

Slate objects operate as prototypes with multi-methods: <lang slate>prototypes define: #MyPrototype &parents: {Cloneable} &slots: #(instanceVar). MyPrototype traits addSlot: #classVar.

x@(MyPrototype traits) new [

 x clone `>> [instanceVar: 0. ]

].

x@(MyPrototype traits) someMethod [

 x instanceVar = 1 /\ (x classVar = 3)

].</lang>

Smalltalk

<lang smalltalk>Object subclass: #MyClass

 instanceVariableNames: 'instanceVar'
 classVariableNames: 'classVar'
 poolDictionaries: 
 category: 'Testing' !

!MyClass class methodsFor: 'instance creation'! new

 ^self basicNew  instanceVar := 0 ! !

!MyClass methodsFor: 'testing'! someMethod

 ^self instanceVar = 1; classVar = 3 ! !

MyClass new someMethod!</lang>

SuperCollider

<lang SuperCollider>MyClass {

   classvar someVar, <another, <>thirdVar;    // Class variables.
   var <>something, <>somethingElse;           // Instance variables.
                  // Note: variables are private by default. In the above, "<" enables getting, ">" enables setting

   *new {
       ^super.new.init         // constructor is a class method. typically calls some instance method to set up, here "init"
   }

   init {
         something = thirdVar.squared;
         somethingElse = this.class.name;
   }

   *aClassMethod {
        ^  someVar + thirdVar         // The "^" means to return the result. If not specified, then the object itself will be returned ("^this")
   }

   anInstanceMethod {
       something = something + 1;
   }

}</lang>

Tcl

Works with: Tcl version 8.6

Works with: Tcl version 8.5

and the TclOO package

<lang Tcl>oo::class create summation {

   variable v
   constructor {} {
       set v 0
   }
   method add x {
       incr v $x
   }
   method value {} {
       return $v
   }
   destructor {
       puts "Ended with value $v"
   }

} set sum [summation new] puts "Start with [$sum value]" for {set i 1} {$i <= 10} {incr i} {

   puts "Add $i to get [$sum add $i]"

} $sum destroy</lang>

Visual Basic .NET

Defining a class

<lang vbnet>Class Foo

  Private m_Bar As Integer

  Public Sub New()

  End Sub

  Public Sub New(ByVal bar As Integer)
      m_Bar = bar
  End Sub

  Public Property Bar() As Integer
      Get
          Return m_Bar
      End Get
      Set(ByVal value As Integer)
          m_Bar = value
      End Set
  End Property

  Public Sub DoubleBar()
      m_Bar *= 2
  End Sub

  Public Function MultiplyBar(ByVal x As Integer) As Integer
      Return x * Bar
  End Function

End Class</lang>

Using an object

<lang vbnet>'Declare and create separately Dim foo1 As Foo foo1 = New Foo

'Declare and create at the same time Dim foo2 As New Foo

'... while passing constructor parameters Dim foo3 As New Foo(5)

'... and them immediately set properties Dim foo4 As New Foo With {.Bar = 10}

'Calling a method that returns a value Console.WriteLine(foo4.MultiplyBar(20))

'Calling a method that performs an action foo4.DoubleBar()

'Reading/writing properties Console.WriteLine(foo4.Bar) foo4.Bar = 1000</lang>