Abstract type

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

Abstract type is a type without instances or without definition.

For example in object-oriented programming using some languages, abstract types can be partial implementations of other types, which are to be derived there-from. An abstract type may provide implementation of some operations and/or components. Abstract types without any implementation are called interfaces. In the languages that do not support multiple inheritance (Ada, Java), classes can, nonetheless, inherit from multiple interfaces. The languages with multiple inheritance (like C++) usually make no distinction between partially implementable abstract types and interfaces. Because the abstract type's implementation is incomplete, OO languages normally prevent instantiation from them (instantiation must derived from one of their descendant classes).

The term abstract datatype also may denote a type, with an implementation provided by the programmer rather than directly by the language (a built-in or an inferred type). Here the word abstract means that the implementation is abstracted away, irrelevant for the user of the type. Such implementation can and should be hidden if the language supports separation of implementation and specification. This hides complexity while allowing the implementation to change without repercussions on the usage. The corresponding software design practice is said to follow the information hiding principle.

It is important not to confuse this abstractness (of implementation) with one of the abstract type. The latter is abstract in the sense that the set of its values is empty. In the sense of implementation abstracted away, all user-defined types are abstract.

In some languages, like for example in Objective Caml which is strongly statically typed, it is also possible to have abstract types that are not OO related and are not an abstractness too. These are pure abstract types without any definition even in the implementation and can be used for example for the type algebra, or for some consistence of the type inference. For example in this area, an abstract type can be used as a phantom type to augment another type as its parameter.

Task: show how an abstract type can be declared in the language. If the language makes a distinction between interfaces and partially implemented types illustrate both.


[edit] ABAP

[edit] Abstract Class

class abs definition abstract.
public section.
methods method1 abstract importing iv_value type f exporting ev_ret type i.
protected section.
methods method2 abstract importing iv_name type string exporting ev_ret type i.
methods add importing iv_a type i iv_b type i exporting ev_ret type i.
class abs implementation.
method add.
ev_ret = iv_a + iv_b.

[edit] Interfaces

Interfaces in ABAP are classes with the following restrictions: 1. All methods must be abstract instance methods (Static methods aren't allowed). 2. Variables must be static final. The values may be computed at run time. 3. No static initialiser blockers. No static initialiser helper methods.

interface inter.
methods: method1 importing iv_value type f exporting ev_ret type i,
method2 importing iv_name type string exporting ev_ret type i,
add importing iv_a type i iv_b type i exporting ev_ret type i.

[edit] ActionScript

While ActionScript does not support explicit abstract classes, it does have interfaces. Interfaces in ActionScript may not implement any methods and all methods are public and implicitly abstract. Interfaces can extend other interfaces, and interfaces may be multiply inherited.

public interface IInterface
function method1():void;
function method2(arg1:Array, arg2:Boolean):uint;

Abstract types can also be simulated using the built-in flash.utils.getQualifiedClassName() function in the constructor to check that the runtime type is an inhertied class, and throwing exceptions from "abstract" methods which can be overridden by inheritors to disable them. If any inheriting class does not implement an abstract method, the error will not be thrown until the non-implemented method is called.

package {
import flash.utils.getQualifiedClassName;
public class AbstractClass {
private static const FULLY_QUALIFIED_NAME:String = "AbstractClass";
// For classes in a package, the fully qualified name should be in the form "package.name::class_name"
// Note that a double colon and not a dot is used before the class name. This is the format returned
// by the getQualifiedClassName() function.
public function AbstractClass() {
if ( getQualifiedClassName(this) == FULLY_QUALIFIED_NAME )
throw new Error("Class " + FULLY_QUALIFIED_NAME + " is abstract.");
public function abstractMethod(a:int, b:int):void {
throw new Error("abstractMethod is not implemented.");

Inheriting this class:

package {
public class Example extends AbstractClass {
override public function abstractMethod(a:int, b:int):void {
trace(a + b);

[edit] Ada

[edit] Interface

Interfaces in Ada may have no components or implemented operation except for ones implemented as null operations. Interfaces can be multiply inherited.

type Queue is limited interface;
procedure Enqueue (Lounge : in out Queue; Item : in out Element) is abstract;
procedure Dequeue (Lounge : in out Queue; Item : in out Element) is abstract;

Interfaces can be declared synchronized or task when intended implementations are to be provided by protected objects or tasks. For example:

type Scheduler is task interface;
procedure Plan (Manager : in out Scheduler; Activity : in out Job) is abstract;

[edit] Abstract type

Abstract types may provide components and implementation of their operations. Abstract types are singly inherited.

with Ada.Finalization;
type Node is abstract new Ada.Finalization.Limited_Controlled and Queue with record
Previous : not null access Node'Class := Node'Unchecked_Access;
Next  : not null access Node'Class := Node'Unchecked_Access;
end record;
overriding procedure Finalize (X : in out Node); -- Removes the node from its list if any
overriding procedure Dequeue (Lounge : in out Node; Item : in out Element);
overriding procedure Enqueue (Lounge : in out Node; Item : in out Element);
procedure Process (X : in out Node) is abstract; -- To be implemented

Here Node is an abstract type that is inherited from Limited_Controlled and implements a node of a doubly linked list. It also implements the interface of a queue described above, because any node can be considered a head of the queue of linked elements. For the operation Finalize an implementation is provided to ensure that the element of a list is removed from there upon its finalization. The operation itself is inherited from the parent type Limited_Controlled and then overridden. The operations Dequeue and Enqueue of the Queue interface are also implemented.

[edit] Agda

Using records for storing the interface methods and instance arguments (which are similar to Haskell type classes) for overloading:

module AbstractInterfaceExample where
open import Function
open import Data.Bool
open import Data.String
-- * One-parameter interface for the type `a' with only one method.
record VoiceInterface (a : Set) : Set where
constructor voice-interface
field say-method-of : a → String
open VoiceInterface
-- * An overloaded method.
say : {a : Set} → ⦃ _ : VoiceInterface a ⦄ → a → String
say ⦃ instance ⦄ = say-method-of instance
-- * Some data types.
data Cat : Set where
cat : Bool → Cat
crazy! = true
plain-cat = false
-- | This cat is crazy?
crazy? : Cat → Bool
crazy? (cat x) = x
-- | A 'plain' dog.
data Dog : Set where
dog : Dog
-- * Implementation of the interface (and method).
instance-for-cat : VoiceInterface Cat
instance-for-cat = voice-interface case where
case : Cat → String
case x with crazy? x
... | true = "meeeoooowwwww!!!"
... | false = "meow!"
instance-for-dog : VoiceInterface Dog
instance-for-dog = voice-interface $ const "woof!"
-- * and then:
-- say dog => "woof!"
-- say (cat crazy!) => "meeeoooowwwww!!!"
-- say (cat plain-cat) => "meow!"

There is dog and cat is objects of different types for which the interface method is implemented.

[edit] Aikido

An abstract class contains functions that have no body defined. You cannot instantiate a class that contains abstract functions.

class Abs {
public function method1...
public function method2...

Interfaces in Aikido define a set of functions, operators, classes, interfaces, monitors or threads (but no variables) that must be implemented by a class implementing the interface.

interface Inter {
function isFatal : integer
function operate (para : integer = 0)
operator -> (stream, isout)

[edit] AmigaE

In AmigaE, abstract methods are supported but interfaces are not.

OBJECT fruit
PROC color OF fruit IS EMPTY
OBJECT apple OF fruit
PROC color OF apple IS WriteF('red ')
OBJECT orange OF fruit
PROC color OF orange IS WriteF('orange ')
PROC main()
DEF a:PTR TO apple,o:PTR TO orange,x:PTR TO fruit
FORALL({x},[NEW a, NEW o],`x.color())

prints to the console:

red orange

[edit] Argile

Works with: Argile version 1.0.0
use std
(: abstract class :)
class Abs
text name
AbsIface iface
class AbsIface
function(Abs)(int)->int method
let Abs_Iface = Cdata AbsIface@ {.method = nil}
.: new Abs :. -> Abs {let a = new(Abs); a.iface = Abs_Iface; a}
=: <Abs self>.method <int i> := -> int
(self.iface.method is nil) ? 0 , (call self.iface.method with self i)
(: implementation :)
class Sub <- Abs { int value }
let Sub_Iface = Cdata AbsIface@ {.method = (code of (nil the Sub).method 0)}
.: new Sub (<int value = -1>) :. -> Sub
let s = new (Sub)
s.iface = Sub_Iface
s.value = value
.: <Sub this>.method <int i> :. -> int {this.value + i}
(: example use :)
.:foobar<Abs a>:. {print a.method 12 ; del a}
foobar (new Sub 34) (: prints 46 :)
foobar (new Sub) (: prints 11 :)
foobar (new Abs) (: prints 0 :)

[edit] AutoHotkey

Works with: AutoHotkey_L
color(r, g, b){ 
static color
If !color
color := Object("base", Object("R", r, "G", g, "B", b
,"GetRGB", "Color_GetRGB"))
return Object("base", Color)
Color_GetRGB(clr) {
return "not implemented"
waterColor(r, g, b){
static waterColor
If !waterColor
waterColor := Object("base", color(r, g, b),"GetRGB", "WaterColor_GetRGB")
return Object("base", WaterColor)
return clr.R << 16 | clr.G << 8 | clr.B
blue := color(0, 0, 255)
msgbox % blue.GetRGB() ; displays "not implemented"
blue := waterColor(0, 0, 255)
msgbox % blue.GetRGB() ; displays 255

[edit] BBC BASIC

BBC BASIC is a procedural language with no built-in OO features. The CLASSLIB library implements simple Object Classes with multiple inheritance; an abstract class may be created without any instantiation, the sole purpose of which is for other classes to inherit from it. At least one member or method must be declared, but no error will be generated if there is no implementation:

      INSTALL @lib$+"CLASSLIB"
REM Declare a class with no implementation:
DIM abstract{method}
REM Inherit from the abstract class:
DIM derived{member%}
PROC_inherit(derived{}, abstract{})
REM Provide an implementation for the derived class:
DEF derived.method : PRINT "Hello world!" : ENDPROC
REM Instantiate the derived class:
PROC_new(instance{}, derived{})
REM Test by calling the method:

[edit] C

Doing abstract types in C is not particularly trivial as C doesn't really support classes. The following series will show an abstract type, followed by a realizable class that provides the abstract interface, and finally followed by an example of usage.

The header file for the abstract class, interfaceAbs.h

typedef struct sAbstractCls *AbsCls;
typedef struct sAbstractMethods {
int (*method1)(AbsCls c, int a);
const char *(*method2)(AbsCls c, int b);
void (*method3)(AbsCls c, double d);
} *AbstractMethods, sAbsMethods;
struct sAbstractCls {
AbstractMethods klass;
void *instData;
#define ABSTRACT_METHODS( cName, m1, m2, m3 ) \
static sAbsMethods cName ## _Iface = { &m1, &m2, &m3 }; \
AbsCls cName ## _Instance( void *clInst) { \
AbsCls ac = malloc(sizeof(struct sAbstractCls)); \
if (ac) { \
ac->klass = &cName ## _Iface; \
ac->instData = clInst; \
return ac; }

#define Abs_Method1( c, a) (c)->klass->method1(c, a)
#define Abs_Method2( c, b) (c)->klass->method2(c, b)
#define Abs_Method3( c, d) (c)->klass->method3(c, d)
#define Abs_Free(c) \
do { if (c) { free((c)->instData); free(c); } } while(0);


That will define the abstract class. The next section declares a public interface for a class providing the interface of the abstract class. This class is Silly and the code is in file silly.h. Note the actual structure of the class is not provided here. We don't want it visible.

#ifndef SILLY_H
#define SILLY_H
#include "intefaceAbs.h"
typedef struct sillyStruct *Silly;
extern Silly NewSilly( double, const char *);
extern AbsCls Silly_Instance(void *);

Ok. Now it is necessary to provide the implementation of the realizable class. This code should be in silly.c.

#include "silly.h"
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
struct sillyStruct {
double v1;
char str[32];
Silly NewSilly(double vInit, const char *strInit)
Silly sily = malloc(sizeof( struct sillyStruct ));
sily->v1 = vInit;
sily->str[0] = '\0';
strncat(sily->str, strInit, 31);
return sily;
int MyMethod1( AbsCls c, int a)
Silly s = (Silly)(c->instData);
return a+strlen(s->str);
const char *MyMethod2(AbsCls c, int b)
Silly s = (Silly)(c->instData);
sprintf(s->str, "%d", b);
return s->str;
void MyMethod3(AbsCls c, double d)
Silly s = (Silly)(c->instData);
printf("InMyMethod3, %f\n",s->v1 * d);
ABSTRACT_METHODS( Silly, MyMethod1, MyMethod2, MyMethod3)

That last macro, ABSTRACT_METHODS may need a little explanation. First note that macros do a string substitution of the parameter values into the arguments of the defined macro, with a little hitch. In the macro definition the ' ## ' expression is special. Here cName ## _Iface gets converted to Silly_Iface, as 'Silly' replaces cName. So the macro call declares an instance of the class record, and defines a constructor named Silly_Instance, which takes a Silly structure as an arguments and uses the class record it previously set up as well.

The methods MyMethod1, MyMethod2, and MyMethod3 are called through the abstract class interface and do not need to be visible outside this file. Hence, they are declared static.

Now all's left is some example code that uses all this stuff.

#include <stdio.h>
#include "silly.h"
int main()
AbsCls abster = Silly_Instance(NewSilly( 10.1, "Green Tomato"));
printf("AbsMethod1: %d\n", Abs_Method1(abster, 5));
printf("AbsMethod2: %s\n", Abs_Method2(abster, 4));
Abs_Method3(abster, 21.55);
return 0;

[edit] C#

abstract class Class1
public abstract void method1();
public int method2()
return 0;

[edit] C++

You can declare a virtual function to not have an implementation (called "pure virtual function") by the following "= 0" syntax after the method declaration. A class containing at least one pure virtual function (or inheriting one and not overriding it) cannot be instantiated.

class Abs {
virtual int method1(double value) = 0;
virtual int add(int a, int b){
return a+b;

Because C++ allows multiple inheritance of classes, no distinction is made between interfaces and abstract classes.

[edit] Caché ObjectScript

In Caché, abstract and data type classes cannot be instantiated directly - there must be a 'concrete subclass' that extends them as well as the '%RegisteredObject' class in order to instantiate an object, see example below.

Class Abstract.Class.Shape [ Abstract ]
Parameter SHAPE = 1;
Property Name As %String;
Method Description() {}
Class Abstract.Class.Square Extends (%RegisteredObject, Shape)
Method Description()
Write "SHAPE=", ..#SHAPE, !
Write ..%ClassName()_$Case(..%Extends(..%PackageName()_".Shape"), 1: " is a ", : " is not a ")_"shape"

Data type classes differ because they cannot contain properties, see example below.

Class Abstract.DataType.Shape [ ClassType = datatype ]
Parameter SHAPE = 1;
Method Description() {}
Class Abstract.DataType.Square Extends (%RegisteredObject, Shape)
Method Description()
Write "SHAPE=", ..#SHAPE, !
Write ..%ClassName()_$Case(..%Extends(..%PackageName()_".Shape"), 1: " is a ", : " is not a ")_"shape"

Both class types can contain implementation code. Caché allows multiple inheritance of classes, so no distinction is made between abstract classes and interfaces.

USER>Do ##class(Abstract.Class.Square).%New().Description()
Square is a shape

USER>Do ##class(Abstract.DataType.Square).%New().Description()
Square is a shape

[edit] Clojure

Using defprotocol, we can define what is essentially an interface.

(defprotocol Foo (foo [this]))

[edit] COBOL

[edit] Interface

Translation of: F#
       INTERFACE-ID. Shape.
METHOD-ID. perimeter.
END METHOD perimeter.
METHOD-ID. shape-area.
END METHOD shape-area.
CLASS-ID. Rectangle.
METHOD-ID. perimeter.
COMPUTE ret = width * 2.0 + height * 2.0
END METHOD perimeter.
METHOD-ID. shape-area.
COMPUTE ret = width * height
END METHOD shape-area.
END CLASS Rectangle.

[edit] Common Lisp

In Common Lisp, classes do not implement methods, but methods specialized for particular kinds of arguments may be defined for generic functions. Since we can programmatically determine whether methods are defined for a list of arguments, we can simulate a kind of abstract type. We define an abstract type kons to which an object belongs if methods for kar and kdr are defined for it. We define a type predicate konsp and a type kons in terms of the type predicate.

(defgeneric kar (kons)
(:documentation "Return the kar of a kons."))
(defgeneric kdr (kons)
(:documentation "Return the kdr of a kons."))
(defun konsp (object &aux (args (list object)))
"True if there are applicable methods for kar and kdr on object."
(not (or (endp (compute-applicable-methods #'kar args))
(endp (compute-applicable-methods #'kdr args)))))
(deftype kons ()
'(satisfies konsp))

We can make the built-in types cons and integer konses. We start with cons, using the obvious definitions.

(defmethod kar ((cons cons))
(car cons))
(defmethod kdr ((cons cons))
(cdr cons))
(konsp (cons 1 2)) ; => t
(typep (cons 1 2) 'kons) ; => t
(kar (cons 1 2)) ; => 1
(kdr (cons 1 2)) ; => 2

For integers, we'll define the kar of n to be 1 and the kdr of n to be n - 1. This means that for an integer n, n = (+ (kar n) (kdr n)).

(defmethod kar ((n integer))
(defmethod kdr ((n integer))
(if (zerop n) nil
(1- n)))
(konsp 45) ; => t
(typep 45 'kons) ; => t
(kar 45) ; => 1
(kdr 45) ; => 44

[edit] Component Pascal

(* Abstract type *)
(* Integer inherits Object *)
Integer = POINTER TO RECORD (Object)
(* Point inherits Object *)
Point = POINTER TO RECORD (Object)
x,y: REAL


(* Abstract method of Object *)
PROCEDURE (dn: Object) Show*, NEW, ABSTRACT;
(* Implementation of the abstract method Show() in class Integer *)
PROCEDURE (i: Integer) Show*;
END Show;
(* Implementation of the abstract method Show() in class Point *)
PROCEDURE (p: Point) Show*;
END Show;

For usage see tasks Stacks.

[edit] D

import std.stdio;
class Foo {
// abstract methods can have an implementation for
// use in super calls.
abstract void foo() {
interface Bar {
void bar();
// Final interface methods are allowed.
final int spam() { return 1; }
class Baz : Foo, Bar { // Super class must come first.
override void foo() {
void bar() {}
void main() {}

[edit] Delphi

Abstract Class introduced in Delphi 2006. An abstract class cannot be instantiated and must be derived from in order to be used.

TSomeClass = class abstract (TObject)

Abstract Methods can only be implemented in derived classes. A concrete class that contains abstract methods can be instantiated. A warning will be generated at compile time, and an EAbstractError exception will thrown if the method is called at run time.

TMyObject = class(TObject)
procedure AbstractFunction; virtual; abstract; // Your virtual abstract function to overwrite in descendant
procedure ConcreteFunction; virtual; // Concrete function calling the abstract function
procedure TMyObject.ConcreteFunction;
AbstractFunction; // Calling the abstract function

[edit] DWScript

DWScript has both abstract classes and abstract methods.

See Delphi.

[edit] E

In E, the implementation of an object is never used to determine type membership (except when dealing with the host platform's objects if it uses such distinctions, such as the JVM), so all types are abstract.

A simple abstract type without enforcement can be created using the interface expression:

interface Foo {
to bar(a :int, b :int)

With enforcement, a separate stamp is created which must be applied to the instances. This is analogous to a Java interface.

interface Foo guards FooStamp {
to bar(a :int, b :int)
def x implements FooStamp {
to bar(a :int, b :int) {
return a - b

[edit] Eiffel

deferred class
-- a feature whose implementation is left to a descendent
an_effective_feature: STRING
-- deferred (abstract) classes may still include effective features
Result := "I am implemented!"

A more expressive view of an Abstract type in Eiffel:

title: "Prototype Person"
description: "Abstract notion of a {PERSON}."
synopsis: "[
Abstract Data Types as represented by any Eiffel class, fully or partially implemented, are
not just about the attribute and routine features of the class (deferred or implemented).
The class and each feature may also have specification rules expressed as preconditions,
post-conditions, and class invariants. Other assertion contracts may be applied to fully
implemented features as well.
In the example below, while `age' is deferred (i.e. "
abstract"), we have coded a rule which
states that any caller of `age' must only do so after a `birth_date' has been defined and
attached to that feature. Failing to do so will cause a contract violation. Moreover, the
class invariant makes two strong assertions that must always hold for any implemented version
of {PERSON}: The `birth_date' (if attached--that is--not Void or null) must be in the past
and never in the future. Also, if "
Years" are used to represent the age, the calculation of
`age' must always agree with "
current year - birth year = age".
This form of Abstract Data Type specification has very clear advantages in that not only
must client code or descendents conform statically, implementing what is deferred, but they
must also obey the rules of the assertions dynamically in a polymorphic run-time situation.

deferred class
feature -- Access
suffix: STRING
birth_date: detachable DATE
-- Date-of-Birth for Current {PERSON}.
feature -- Basic Operations
age: NATURAL_64
-- Age of Current {PERSON} in some undefined units.
has_birth_date: attached birth_date
age_units: STRING
-- Unit-of-Measure (UOM) of `age'.
Result := year_unit_string
year_unit_string: STRING = "Years"
not_future: attached birth_date as al_birth_date implies al_birth_date < (create {DATE}.make_now)
accurate_age: attached birth_date as al_birth_date and then age > 0 and then age_units.same_string (year_unit_string)
implies ((create {DATE}.make_now).year - al_birth_date.year) = age

[edit] Fantom

abstract class X
Void method1 ()
echo ("Method 1 in X")
abstract Void method2 ()
class Y : X
// Y must override the abstract method in X
override Void method2 ()
echo ("Method 2 in Y")
class Main
public static Void main ()
y := Y()

[edit] Forth

Works with: 4tH version 3.61.5
Translation of: Fantom

There are numerous, mutually incompatible object oriented frameworks for Forth. This one works with the FOOS preprocessor extension of 4tH.

include 4pp/lib/foos.4pp
:: X()
method: method1
method: method2
end-class {
 :method { ." Method 1 in X" cr } ; defines method1
:: Y()
extends X()
end-extends {
 :method { ." Method 2 in Y" cr } ; defines method2
: Main
static Y() y
y => method1
y => method2

Works with any ANS Forth

Needs the FMS-SI (single inheritance) library code located here: http://soton.mpeforth.com/flag/fms/index.html

include FMS-SI.f
The FMS object extension uses duck typing and so has no need for abstract types.

[edit] Fortran

Simple abstract derived type (i.e. abstract class) in Fortran 2008

! abstract derived type
type, abstract :: TFigure
real(rdp) :: area
! deferred method i.e. abstract method = must be overridden in extended type
procedure(calculate_area), deferred, pass :: calculate_area
end type TFigure
! only declaration of the abstract method/procedure for TFigure type
abstract interface
function calculate_area(this)
import TFigure !imports TFigure type from host scoping unit and makes it accessible here
implicit none
class(TFigure) :: this
real(rdp) :: calculate_area
end function calculate_area
end interface

[edit] F#

A type with only abstract members and without constructors is an interface (when not marked with the AbstractClass attribute). Example:

type Shape =
abstract Perimeter: unit -> float
abstract Area: unit -> float
type Rectangle(width, height) =
interface Shape with
member x.Perimeter() = 2.0 * width + 2.0 * height
member x.Area() = width * height

A type that leaves some or all members unimplemented, is an abstract class. It has to be marked with the AbstractClass attribute. Example:

type Bird() =
// an abstract (=virtual) method with default impl.
abstract Move : unit -> unit
default x.Move() = printfn "flying"
// a pure virtual method
abstract Sing: unit -> string
type Blackbird() =
inherit Bird()
override x.Sing() = "tra-la-la"
type Ostrich() =
inherit Bird()
override x.Move() = printfn "walking"
override x.Sing() = "hiss hiss!"

[edit] Genyris

In Genyris by default there are no constructors. In effect all classes are Abstract until they are used to tag (describe) an object. This in keeping with the language's roots in Description Logic. To prevent the class ever being associated with an instance it suffices to force the validator to fail.

class AbstractStack()
def .valid?(object) nil
tag AbstractStack some-object # always fails

However this is not much use if we want to use an abstract class to define an interface. Here is a quasi-abstract class which can be used to tag objects if they conform to the class's membership expectations. In this case it wants two methods, .enstack and .destack:

class StackInterface()
def .valid?(object)
bound? .enstack
is-instance? .enstack Closure
bound? .destack
is-instance? .destack Closure

So if ever we find an object which conforms to the validator it can be tagged. Here's a 'traditional' class definition using the Object class which does provide a constructor:

class XYZstack(Object)
def .init()
var .items ()
def .enstack(object)
setq .items (cons object .items)
def .destack()
var tmp (car .items)
setq .items (cdr .items)

Now we can tag an object that conforms to the Interface:

tag StackInterface (XYZstack(.new))

[edit] Go

Go's interface type is an abstract type. It defines a set of methods that a value must have.

interface {
Name() string
SetName(name string)
Method1(value float64) int

A variable of an interface type can hold a value of any type that implements the methods that are specified in the interface. You don't need to explicitly "declare" that the type "implements" the interface or anything like that -- the compatibility is purely structural based on the methods.

[edit] Groovy

Translation of: Java

As in Java, methods that are declared but not implemented are called "abstract" methods. An interface is a class-level typing construct that can only contain abstract method declarations (well, and constants, but pay no attention to those).

public interface Interface {
int method1(double value)
int method2(String name)
int add(int a, int b)

An abstract class may implement some of its methods and leave others unimplemented. The unimplemented methods and the class itself must be declared "abstract".

public abstract class Abstract1 {
abstract public int methodA(Date value)
abstract protected int methodB(String name)
int add(int a, int b) { a + b }

An abstract class may also be used to partially implement an interface. Here class "Abstract2" implements the "add" method from the inherited "Interface", but leaves the other two methods, "method1" and "method2", unimplemented. Abstract methods that an abstract class inherits from an interface or another abstract class do not have to be redeclared.

public abstract class Abstract2 implements Interface {
int add(int a, int b) { a + b }

Interfaces and abstract classes cannot be instantiated directly. There must be a "concrete subclass" that contains a complete implementation in order to instantiate an object.

public class Concrete1 implements Interface {
public int method1(double value) { value as int }
public int method2(String name) { (! name) ? 0 : name.toList().collect { it as char }.sum() }
public int add(int a, int b) { a + b }
public class Concrete2 extends Abstract1 {
public int methodA(Date value) { value.toCalendar()[Calendar.DAY_OF_YEAR] }
protected int methodB(String name) { (! name) ? 0 : name.toList().collect { it as char }.sum() }
public class Concrete3 extends Abstract2 {
public int method1(double value) { value as int }
public int method2(String name) { (! name) ? 0 : name.toList().collect { it as char }.sum() }

Notice that there are no extra descriptive keywords on the interface method declarations. Interface methods are assumed to be both abstract and public.

Obligatory test:

def c1 = new Concrete1()
assert c1 instanceof Interface
println (new Concrete1().method2("Superman"))
def c2 = new Concrete2()
assert c2 instanceof Abstract1
println (new Concrete2().methodB("Spiderman"))
def c3 = new Concrete3()
assert c3 instanceof Interface
assert c3 instanceof Abstract2
println (new Concrete3().method2("Hellboy"))

Obligatory test output:


Like Java, Groovy does not allow subclasses to inherit from multiple superclasses, even abstract superclasses, but it does let subclasses inherit from multiple interfaces.

[edit] Haskell

In Haskell an abstract type is a type class. A type class specifies an interface. One can then define "instances" to provide implementations of the type class for various types.

For example, the built-in type class Eq (the types that can be compared for equality) can be declared as follows:

class  Eq a  where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool

Default implementations of the functions can be provided:

class  Eq a  where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
x /= y = not (x == y)
x == y = not (x /= y)

Here default implementations of each of the operators is circularly defined in terms of the other, for convenience of the programmer; so the programmer only needs to implement one of them for it to work.

Consider the following function which uses the operator == of the type class Eq from above. The arguments to == above were of the unknown type "a", which is of class Eq, so the type of the expression below now must include this restriction:

func :: (Eq a) => a -> Bool
func x = x == x

Suppose I make a new type

data Foo = Foo {x :: Integer, str :: String}

One could then provide an implementation ("instance") the type class Eq with this type

instance Eq Foo where
(Foo x1 str1) == (Foo x2 str2) =
(x1 == x2) && (str1 == str2)

And now I can, for example, use the function "func" on two arguments of type Foo.

[edit] Icon and Unicon

Unicon does not distinguish between abstract and concrete classes. An abstract class is a class with abstract methods. Icon is not object-oriented.

class abstraction()
abstract method compare(l,r) # generates runerr(700, "method compare()")

[edit] J

J does not support abstract types, as defined here. In J, types are typically treated as a necessary evil, which should be minimized, disguised, hidden, neglected or ignored wherever practical. (2=1+1 regardless of the type of 1 and the type of 2, but 2 and '2' are very different things.) And allowing user defined types would complicate this approach.

Note also: Types are sometimes thought of as being related to function domains. But, in the general case, domains of independently defined functions are independent of each other, but nevertheless, the intersections of these domains often enough are not empty.

[In fact, the real motivator for types is the need to allocate finite resources to represent numbers (or whatever else you choose to imagine is being represented). For example: 32 bit integers vs. 64 bit integers vs. 32 bit ieee-854 floating point and 64 bit ieee-854 floating point. Additionally, some operations are sensitive to other details related to these abstractions - the classic examples including overflow vs. carry (add with carry, addition overflow) which depend on the range of numbers involved (2s complement vs. unsigned vs. 1s complement). And then people get carried away trying to "generalize types" rather than "use types" which triggers a need for standardization which mostly means prohibiting some of the most annoying generalizations, which is then followed by other people objecting to those choices... and there is no stopping these trends, which leaves many people fascinated and perhaps horrified at the consequences.]

That said: it's useful to define a type, in the context of J, as "the set of values which may result from a specific parenthesized expression". And, if compilation to machine code is supported, it may also be useful to define constraint mechanisms to be used in expressions, so that machine code may be more easily generated.

(You can find a variety of languages with rather elaborate implementations of types, but as a general rule those elaborate type systems are either (a) inadequate to represent J arrays, or (b) adequate to represent J arrays but with painfully slow implementations for many typical use cases - especially involving large data sets. That said, you can also find cases where these languages perform well - especially if you tailor the problem to the language or vice versa.)

[edit] Java

Methods that don't have an implementation are called abstract methods in Java. A class that contains an abstract method or inherits one but did not override it must be an abstract class; but an abstract class does not need to contain any abstract methods. An abstract class cannot be instantiated. If a method is abstract, it must be public or protected

public abstract class Abs {
abstract public int method1(double value);
abstract protected int method2(String name);
int add(int a, int b){
return a+b;

Interfaces in Java may not implement any methods and all methods are implicitly public and abstract.

public interface Inter {
int method1(double value);
int method2(String name);
int add(int a, int b);

[edit] Julia

Abstract types cannot be instantiated, and serve only as nodes in the type graph, thereby describing sets of related concrete types: those concrete types which are their descendants.


abstract «name»
abstract «name» <: «supertype»


abstract Number
abstract Real <: Number
abstract FloatingPoint <: Real
abstract Integer <: Real
abstract Signed <: Integer
abstract Unsigned <: Integer

See more [1]

[edit] Lasso

Instead of abstract classes or interfaces, Lasso uses a trait system.

define abstract_trait => trait {
require get(index::integer)
provide first() => .get(1)
provide second() => .get(2)
provide third() => .get(3)
provide fourth() => .get(4)
define my_type => type {
parent array
trait { import abstract_trait }
public onCreate(...) => ..onCreate(:#rest)
local(test) = my_type('a','b','c','d','e')
#test->first + "\n"
#test->second + "\n"
#test->third + "\n"
#test->fourth + "\n"

[edit] Logtalk

In Logtalk, methods (predicates) must be declared but their definition is not mandatory. Being a logic-based language and making use of the closed-world assumption, invoking a method that is declared but not defined simply fails. If necessary, is trivial to define a method such that it throws an exception. Moreover, Logtalk doesn't define an "abstract" or "virtual" keyword. Instead it uses an operational definition where e.g. a class is considered abstract if it doesn't provide a method for creating new instances.

Logtalk supports the definition of interfaces (protocols), which can contain public, protected, and private declarations of methods (predicates). In addition, an object can qualify an implements relation with an interface (protocol) using the keywords "public", "protected", and "private".

:- protocol(datep).
:- public(today/3).
:- public(leap_year/1).
:- public(name_of_day/3).
:- public(name_of_month/3).
:- public(days_in_month/3).
:- end_protocol.

[edit] Lua

Lua does not include built-in object oriented paradigms. These features can be added using simple code such as the following:

BaseClass = {}
function class ( baseClass )
local new_class = {}
local class_mt = { __index = new_class }
function new_class:new()
local newinst = {}
setmetatable( newinst, class_mt )
return newinst
if not baseClass then baseClass = BaseClass end
setmetatable( new_class, { __index = baseClass } )
return new_class
function abstractClass ( self )
local new_class = {}
local class_mt = { __index = new_class }
function new_class:new()
error("Abstract classes cannot be instantiated")
if not baseClass then baseClass = BaseClass end
setmetatable( new_class, { __index = baseClass } )
return new_class
BaseClass.class = class
BaseClass.abstractClass = abstractClass

The 'class' function produces a new class from an existing parent class (BaseClass is default). From this class other classes or instances can be created. If a class is created through the 'abstractClass' function, however, the resulting class will throw an error if one attempts to instantiate it. Example:

A = class()         -- New class A inherits BaseClass by default
AA = A:class() -- New class AA inherits from existing class A
B = abstractClass() -- New abstract class B
BB = B:class() -- BB is not abstract
A:new() -- Okay: New class instance
AA:new() -- Okay: New class instance
B:new() -- Error: B is abstract
BB:new() -- Okay: BB is not abstract

[edit] Mathematica

Mathematica is a symbolic language and as such does not support traditional object oriented design patterns. However, it is quite easy to define pseudo-interfaces that depend on an object implementing a set of functions:

(* Define an interface, Foo, which requires that the functions Foo, Bar, and Baz be defined *)
InterfaceFooQ[obj_] := ValueQ[Foo[obj]] && ValueQ[Bar[obj]] && ValueQ[Baz[obj]];
PrintFoo[obj_] := Print["Object ", obj, " does not implement interface Foo."];
PrintFoo[obj_?InterfaceFooQ] := Print[
"Foo: ", Foo[obj], "\n",
"Bar: ", Bar[obj], "\n",
"Baz: ", Baz[obj], "\n"];
(* Extend all integers with Interface Foo *)
Foo[x_Integer] := Mod[x, 2];
Bar[x_Integer] := Mod[x, 3];
Baz[x_Integer] := Mod[x, 5];
(* Extend a particular string with Interface Foo *)
Foo["Qux"] = "foo";
Bar["Qux"] = "bar";
Baz["Qux"] = "baz";
(* Print a non-interface object *)
PrintFoo[{"Some", "List"}];
(* And for an integer *)
(* And for the specific string *)
(* And finally a non-specific string *)
Object {Some,List} does not implement interface Foo.

Foo: 0
Bar: 2
Baz: 3

Foo: foo
Bar: bar
Baz: baz

Object foobarbaz does not implement interface Foo.

Note that, in this implementation, the line between interface and abstract type is blurred. It could be argued that PrintFoo[] is a concrete member of the abstract type InterfaceFoo, or that it's a separate function that accepts anything implementing the interface InterfaceFoo.

[edit] Nemerle

using System.Console;
namespace RosettaCode
abstract class Fruit
abstract public Eat() : void;
abstract public Peel() : void;
virtual public Cut() : void // an abstract class con contain a mixture of abstract and implemented methods
{ // the virtual keyword allows the method to be overridden by derivative classes
WriteLine("Being cut.");
interface IJuiceable
Juice() : void; // interfaces contain only the signatures of methods
class Orange : Fruit, IJuiceable
public override Eat() : void // implementations of abstract methods need to be marked override
WriteLine("Being eaten.");
public override Peel() : void
WriteLine("Being peeled.");
public Juice() : void
WriteLine("Being juiced.");

[edit] NetRexx

/* NetRexx */
options replace format comments java crossref symbols binary
-- -----------------------------------------------------------------------------
class RCAbstractType public final
method main(args = String[]) public constant
say ' Testing' RCAbstractType.class.getSimpleName
say ' Creating an object of type:' Concrete.class.getSimpleName
conk = Concrete()
say 'getClassName:'.right(20) conk.getClassName
say 'getIfaceName:'.right(20) conk.getIfaceName
say 'mustImplement:'.right(20) conk.mustImplement
say 'canOverride1:'.right(20) conk.canOverride1
say 'canOverride2:'.right(20) conk.canOverride2
say 'callOverridden2:'.right(20) conk.callOverridden2
-- -----------------------------------------------------------------------------
class RCAbstractType.Iface interface
ifaceName = RCAbstractType.Iface.class.getSimpleName
method getIfaceName() public returns String
method canOverride1() public returns String
method canOverride2() public returns String
-- -----------------------------------------------------------------------------
class RCAbstractType.Abstraction abstract implements RCAbstractType.Iface
properties inheritable
className = String
method Abstraction() public
method mustImplement() public abstract returns String
method getClassName() public returns String
return className
method setClassName(nm = String) public
className = nm
method getIfaceName() public returns String
return RCAbstractType.Iface.ifaceName
method canOverride1() public returns String
return 'In' RCAbstractType.Abstraction.class.getSimpleName'.canOverride1'
method canOverride2() public returns String
return 'In' RCAbstractType.Abstraction.class.getSimpleName'.canOverride2'
-- -----------------------------------------------------------------------------
class RCAbstractType.Concrete extends RCAbstractType.Abstraction
method Concrete() public
method mustImplement() public returns String
return 'In' RCAbstractType.Concrete.class.getSimpleName'.mustImplement'
method canOverride2() public returns String
return 'In' RCAbstractType.Concrete.class.getSimpleName'.canOverride2'
method callOverridden2() public returns String
return super.canOverride2
  Testing RCAbstractType
  Creating an object of type: Concrete
       getClassName: Concrete
       getIfaceName: Iface
      mustImplement: In Concrete.mustImplement
       canOverride1: In Abstraction.canOverride1
       canOverride2: In Concrete.canOverride2
    callOverridden2: In Abstraction.canOverride2

[edit] NewLISP

; file:   abstract.lsp
; url: http://rosettacode.org/wiki/Abstract_type
; author: oofoe 2012-01-28
; Abstract Shape Class
(new Class 'Shape) ; Derive new class.
(define (Shape:Shape ; Shape constructor.
(pen "X")) ; Default value.
(list (context) ; Assemble data packet.
(list 'pen pen)
(list 'size (args))))
(define (Shape:line x) ; Print out row with 'pen' character.
(dotimes (i x)
(print (lookup 'pen (self))))
(define (Shape:draw)) ; Placeholder, does nothing.
; Derived Objects
(new Shape 'Box)
(define (Box:draw) ; Override base draw method.
(let ((s (lookup 'size (self))))
(dotimes (i (s 0)) (:line (self) (s 0)))))
(new Shape 'Rectangle)
(define (Rectangle:draw)
(let ((size (lookup 'size (self))))
(dotimes (i (size 1)) (:line (self) (size 0)))))
; Demonstration
(:draw (Shape)) ; Nothing happens.
(println "A box:")
(:draw (Box "O" 5)) ; Create Box object and call draw method.
(println "\nA rectangle:")
(:draw (Rectangle "R" 32 4))

Sample output:

A box:

A rectangle:

[edit] Nim

In Nim type classes can be seen as an abstract type. Type classes specify interfaces, which can be instantiated by concrete types.

Comparable = generic x, y
(x < y) is bool
Container[T] = generic c
c.len is ordinal
items(c) is iterator
for value in c:
type(value) is T

[edit] Nit

# Task: abstract type
# Methods without implementation are annotated `abstract`.
# Abstract classes and interfaces can contain abstract methods and concrete (i.e. non-abstract) methods.
# Abstract classes can also have attributes.
module abstract_type
interface Inter
fun method1: Int is abstract
fun method2: Int do return 1
abstract class Abs
fun method1: Int is abstract
fun method2: Int do return 1
var attr: Int

[edit] Oberon-2

Works with: oo2c Version 2
Animal = POINTER TO AnimalDesc;
(* Cat inherits from Animal *)
Cat = POINTER TO CatDesc;
CatDesc = RECORD (AnimalDesc) END;

[edit] Objeck

class ClassA {
method : virtual : public : MethodA() ~ Int;
method : public : MethodA() ~ Int {
return 0;

[edit] OCaml

[edit] Virtual

The equivalent of what is called abstract type in the other OO examples of this page is just called virtual in Objective Caml to define virtual methods and virtual classes:

class virtual foo =
method virtual bar : int

[edit] Abstract Type

In OCaml what we call an abstract type is not OO related, it is only a type defined without definition, for example:

type t

it is used for example to hide an implementation from the interface of a module or for type algebra.

Example of abstracting a type in an interface:

module Foo : sig
type t
end = struct
type t = int * int

Pure abstract types in the implementation:

type u
type v
type 'a t
type ut = u t
type vt = v t

[edit] Oforth

Oforth properties implement abstract types.

Classes have only one parent, but can have multiple properties.

Unlike interfaces, properties can include method implementations and attributes (see lang/Comparable.of for instance).

Property new: Spherical(r)
Spherical method: radius { @r }
Spherical method: setRadius(r) { r := r }
Spherical method: perimeter { @r 2 * Pi * }
Spherical method: surface { @r sq Pi * 4 * }
Object Class new: Ballon(color)
Ballon is: Spherical
Ballon method: initialize(color, r) { color := color self setRadius(r) }
Object Class new: Planete(name)
Planete is: Spherical
Planete method: initialize(n, r) { n := name self setRadius(r) }

Usage :

func: testProperty
| b p |
Ballon new($red, 0.1) ->b
System.Out "Ballon radius is : " << b radius << cr
System.Out "Ballon perimeter is : " << b perimeter << cr
System.Out "Ballon surface is : " << b surface << cr
Planete new("Earth", 6371000.0) ->p
System.Out "Earth radius is : " << p radius << cr
System.Out "Earth perimeter is : " << p perimeter << cr
System.Out "Earth surface is : " << p surface << cr
Ballon radius    is : 0.1
Ballon perimeter is : 0.628318530717959
Ballon surface   is : 0.125663706143592
Earth radius    is : 6371000
Earth perimeter is : 40030173.5920411
Earth surface   is : 510064471909788

[edit] ooRexx

[edit] Interface

-- Example showing a class that defines an interface in ooRexx
-- shape is the interface class that defines the methods a shape instance
-- is expected to implement as abstract methods. Instances of the shape
-- class need not directly subclass the interface, but can use multiple
-- inheritance to mark itself as implementing the interface.
say r
-- check for instance of
if r~isa(.shape) then say "a" r~name "is a shape"
say "r~area:" r~area
say c
-- check for instance of shape works even if inherited
if c~isa(.shape) then say "a" c~name "is a shape"
say "c~area:" c~area
-- a mixin is still a class and can be instantiated. The abstract methods
-- will give an error if invoked
say g
say g~name
say "g~area:" g~area -- invoking abstract method results in a runtime error.
-- the "MIXINCLASS" tag makes this avaiable for multiple inhertance
 ::class shape MIXINCLASS Object
 ::method area abstract
 ::method name abstract
-- directly subclassing the the interface
 ::class rectangle subclass shape
 ::method init
expose length width
use strict arg length=0, width=0
 ::method area
expose length width
return length*width
 ::method name
return "Rectangle"
-- inherits the shape methods
 ::class circle subclass object inherit shape
 ::method init
expose radius
use strict arg radius=0
 ::method area
expose radius
numeric digits 20
return radius*radius*3.14159265358979323846
 ::method name
return "Circle"

[edit] Abstract Type

  -- Example showing an abstract type in ooRexx                         
-- shape is the abstract class that defines the abstract method area
-- which is then implemented by its two subclasses, rectangle and circle
-- name is the method inherited by the subclasses.
-- author: Rony G. Flatscher, 2012-05-26
-- changed/edited: Walter Pachl, 2012-05-28 28
-- highlighting: to come
say r
say r~name
say "r~area:" r~area
say c
say c~name
say "c~area:" c~area
say g
say g~name
say "g~area:" g~area -- invoking abstract method results in a runtime error.
 ::class shape
 ::method area abstract
 ::method name
return "self~class~id:" self~class~id
 ::class rectangle subclass shape
 ::method init
expose length width
use strict arg length=0, width=0
 ::method area
expose length width
return length*width
 ::class circle subclass shape
 ::method init
expose radius
use strict arg radius=0
 ::method area
expose radius
numeric digits 20
return radius*radius*3.14159265358979323846

[edit] OxygenBasic


macro ContainerClass(name,body)
  type name##Type body
  class name
  string buffer
  method constructor(sys n=1)
    buffer=nuls n*sizeof name##Type
  end method
  method destructor()
  end method
  method GetMembers(sys i,n) as name##Type
    sys le=len buffer
    sys en=(n+i)*sizeof name##type
    if le<en
      buffer+=nuls en-le 'auto expand
    end if
    return i+strptr buffer
  end method
end macro


ContainerClass Vector3dArray, {double x,y,z}
end class


new Vector3dArray v(100)


let pv=v.GetMembers(50,3) 'offset, quantity
pv<=1,2,3, 10,20,30, 100,200,300


print pv[3].y

del v

[edit] Oz

Translation of: Python

There are no abstract types as part of the language, but we can do as in Python and raise exceptions:

class BaseQueue
meth init
raise notImplemented(self init) end
meth enqueue(Item)
raise notImplemented(self enqueue) end
meth dequeue(?Item)
raise notImplemented(self dequeue) end
meth printContents
{ForAll @contents Show}
Queue = {New BaseQueue init} %% throws

[edit] PARI/GP

GP is not object-oriented and cannot sensibly use abstract types. PARI can use the same solution as C.

[edit] Pascal and Object Pascal

In ObjectPascal mode FreePascal has classes and abstract methods.

See Delphi

[edit] Perl

package AbstractFoo;
use strict;
sub frob { die "abstract" }
sub baz { die "abstract" }
sub frob_the_baz {
my $self = shift;

Since Perl 5.12, the Yadda Yadda operator (...) dies with an Unimplemented error,

package AbstractFoo;
use strict;
sub frob { ... }
sub baz { ... }
sub frob_the_baz {
my $self = shift;

Perl 6 inspired roles are provided by the Moose library

package AbstractFoo;
use Moose::Role;
requires qw/frob baz/;
sub frob_the_baz {
my $self = shift;

Roles are also provided in a more lightweight form with Role::Tiny library

package AbstractFoo;
use Role::Tiny;
requires qw/frob baz/;
sub frob_the_baz {
my $self = shift;

[edit] Perl 6

Perl 6 supports roles, which are a bit like interfaces, but unlike interfaces in Java they can also contain some implementation.

use v6;
role A {
# must be filled in by the class it is composed into
method abstract() { ... };
# can be overridden in the class, but that's not mandatory
method concrete() { say '# 42' };
class SomeClass does A {
method abstract() {
say "# made concrete in class"
my $obj = SomeClass.new;
# output:
# made concrete in class
# 42

[edit] PHP

The following is for PHP 5.

Methods that don't have an implementation are called abstract methods in PHP. A class that contains an abstract method or inherits one but did not override it must be an abstract class; but an abstract class does not need to contain any abstract methods. An abstract class cannot be instantiated. If a method is abstract, it must be public or protected

abstract class Abs {
abstract public function method1($value);
abstract protected function method2($name);
function add($a, $b){
return a + b;

Interfaces in PHP may not implement any methods and all methods are public and implicitly abstract.

interface Inter {
public function method1($value);
public function method2($name);
public function add($a, $b);

[edit] PicoLisp

# In PicoLisp there is no formal difference between abstract and concrete classes.
# There is just a naming convention where abstract classes start with a
# lower-case character after the '+' (the naming convention for classes).
# This tells the programmer that this class has not enough methods
# defined to survive on its own.
(class +abstractClass)
(dm someMethod> ()
(bar) )

[edit] Python

class BaseQueue(object):
"""Abstract/Virtual Class

def __init__(self):
self.contents = list()
raise NotImplementedError
def Enqueue(self, item):
raise NotImplementedError
def Dequeue(self):
raise NotImplementedError
def Print_Contents(self):
for i in self.contents:
print i,

Python allows multiple inheritance and it's more common to implement "mix-in" classes rather than abstract interfaces. (Mix-in classes can implement functionality as well define interfaces).

In this example we're simply following the Python convention of raising the built-in "NotImplementedError" for each function which must be implemented by our subclasses. This is a "purely virtual" class because all of its methods raise the exception. (It is sufficient for __init__ to do so for any partial virtual abstractions since that still ensures that the exception will be raised if anyone attempts to instantiate the base/abstract class directly rather than one of its concrete (fully implemented) descendents).

The method signatures and the instantiation of a "contents" list shown here can be viewed as documentary hints to anyone inheriting from this class. They won't actually do anything in the derived classes (since these methods must be over-ridden therein).

In this case we've implemented one method (Print_Contents). This would be inherited by any derived classes. It could be over-ridden, of course. If it's not over-ridden it establishes a requirement that all derived classes provide some "contents" attribute which must allow for iteration and printing as shown. Without this method the class would be "purely virtual" or "purely abstract." With its inclusion the class becomes "partially implemented."

Note: This "BaseQueue" example should not be confused with Python's standard library Queue class. That is used as the principle "producer/consumer" communications mechanism among threads (and newer multiprocessing processes).

Starting from Python 2.6, abstract classes can be created using the standard abc module:

from abc import ABCMeta, abstractmethod
class BaseQueue():
"""Abstract Class

__metaclass__ = ABCMeta
def __init__(self):
self.contents = list()
def Enqueue(self, item):
def Dequeue(self):
def Print_Contents(self):
for i in self.contents:
print i,

[edit] Racket

#lang racket
(define animal-interface (interface () say))
(define cat% (class* object% (animal-interface) (super-new))) ;; error
(define cat% (class* object% (animal-interface)
(define/public (say)
(display "meeeeew!"))))
(define tom (new cat%))
(send tom say)

[edit] REBOL

rebol [
Title: "Abstract Type"
Author: oofoe
Date: 2009-12-05
URL: http://rosettacode.org/wiki/Abstract_type

; The "shape" class is an abstract class -- it defines the "pen"
; property and "line" method, but "size" and "draw" are undefined and
; unimplemented.
shape: make object! [
pen: "X"
size: none
line: func [count][loop count [prin self/pen] prin crlf]
draw: does [none]
; The "box" class inherits from "shape" and provides the missing
; information for drawing boxes.
box: make shape [
size: 10
draw: does [loop self/size [line self/size]]
; "rectangle" also inherits from "shape", but handles the
; implementation very differently.
rectangle: make shape [
size: 20x10
draw: does [loop self/size/y [line self/size/x]]
; Unlike some languages discussed, REBOL has absolutely no qualms
; about instantiating an "abstract" class -- that's how I created the
; derived classes of "rectangle" and "box", after all.
s: make shape [] s/draw ; Nothing happens.
print "A box:"
b: make box [pen: "O" size: 5] b/draw
print [crlf "A rectangle:"]
r: make rectangle [size: 32x5] r/draw

[edit] Ruby

The Python and Tcl provisos apply to Ruby too. Nevertheless, a
Library: RubyGems
package called abstraction exists where:
require 'abstraction'
class AbstractQueue
def enqueue(object)
raise NotImplementedError
def dequeue
raise NotImplementedError
class ConcreteQueue < AbstractQueue
def enqueue(object)
puts "enqueue #{object.inspect}"


irb(main):032:0> a = AbstractQueue.new
AbstractClassError: AbstractQueue is an abstract class and cannot be instantiated
        from /usr/lib/ruby/gems/1.8/gems/abstraction-0.0.3/lib/abstraction.rb:10:in `new'
        from (irb):32
        from :0
irb(main):033:0> c = ConcreteQueue.new
=> #<ConcreteQueue:0x7fdea114>
irb(main):034:0> c.enqueue('foo')
enqueue "foo"
=> nil
irb(main):040:0> c.dequeue
NotImplementedError: NotImplementedError
        from (irb):37:in `dequeue'
        from (irb):40
        from :0

[edit] Scala

Scala has abstract classes, which are classes that cannot be instantiated. They can contain implementation as well as just interface. Non-abstract classes, on the other hand, cannot contain interfaces without implementation.

Scala also has traits, which may contain implementations or not as needed, without any abstract requirement. On the other hand, traits must be mixed in a class, instead of being directly instantiated. That doesn't matter all that much, as they can be mixed with AnyRef, which is the base parent class of all user-defined classes.

Any element of a trait or class can be made abstract, including types, with a very different meaning that described in this page. Here are some examples:

abstract class X {
type A
var B: A
val C: A
def D(a: A): A
trait Y {
val x: X

When integrating with Java, traits without implementation appear as interfaces.

[edit] Seed7

The object orientation of Seed7 is based on inteface types. An abstract type consists of an interface type and interface functions, that use the interface. Interface (DYNAMIC) functions describe what can be done with objects of an interface type. Seed7 functions are freestanding and don't have an implicit this (or self) parameter. This concept allows multiple dispatch. Instead of an implicit this parameter an interface function has a parameter of an interface type. A function is automaticall attached to the interface, when it has an parameter of the interface type.

const type: myInterf is sub object interface;
const func integer: method1 (in myInterf: interf, in float: aFloat) is DYNAMIC;
const func integer: method2 (in myInterf: interf, in string: name) is DYNAMIC;
const func integer: add (in myInterf: interf, in integer: a, in integer: b) is DYNAMIC;

[edit] Sidef

Translation of: Perl 6
class A {
# must be filled in by the class which will inherit it
method abstract() { die 'Unimplemented' };
# can be overridden in the class, but that's not mandatory
method concrete() { say '# 42' };
class SomeClass << A {
method abstract() {
say "# made concrete in class"
var obj = SomeClass.new;
obj.abstract(); # made concrete in class
obj.concrete(); # 42

[edit] Standard ML

Standard ML does not have any built-in support for object-oriented programming. Instead it supports abstraction through a module system. Every module has a signature describing the types and values that can be accessed from outside a module.

The act of giving a signature to a module is called ascription. There are two type of ascription: Transparent (written :</tt) and opaque (written <tt>:>). If a structure is ascribed transparently, none of the types are abstract. If it is ascribed opaquely, all types are abstract by default, but can be specified explicitly in the signature, in which case they are not abstract.

Here is an example signature for a queue data structure:

signature QUEUE = sig
type 'a queue
val empty : 'a queue
val enqueue : 'a -> 'a queue -> 'a queue
val dequeue : 'a queue -> ('a * 'a queue) option

Because we did not specify an implementation for 'a queue, the type will be abstract if we use opaque ascription. Instead we could create a version of the signature which specifies the type, in which case it will never be abstract:

signature LIST_QUEUE = sig
type 'a queue = 'a list
val empty : 'a queue
val enqueue : 'a -> 'a queue -> 'a queue
val dequeue : 'a queue -> ('a * 'a queue) option

Then say we have a structure ListQueue which implements queues as lists. If we write ListQueue :> QUEUE then the queue type will be abstract, but if we write ListQueue : QUEUE or ListQueue : LIST_QUEUE it won't.

[edit] Tcl

Works with: Tcl version 8.6
Library: TclOO

While in general Tcl does not use abstract classes at all (and has no need at all for interfaces due to supporting multiple inheritance and mixins), an equivalent effect can be had by concealing the construction methods on the class instance; instances are only created by subclassing the class first (or by mixing it in). In this example, the methods are also error-returning stubs...

oo::class create AbstractQueue {
method enqueue item {
error "not implemented"
method dequeue {} {
error "not implemented"
self unexport create new

[edit] Visual Basic

[edit] Abstract Classes

Visual Basic doesn't support abstract classes or implementation inheritance.

[edit] Interfaces

In Visual Basic, every class is also an interface that other classes can implement. It has this feature because it is based on COM.

[edit] Visual Basic .NET

[edit] Abstract Classes

  • Overridable means subclasses may change the method's implementation. By default, methods in VB cannot be overridden.
  • MustOverride means the subclasses must provide an implementation
  • By convention all abstract classes have one or more Protected constructors.
MustInherit Class Base
Protected Sub New()
End Sub
Public Sub StandardMethod()
End Sub
Public Overridable Sub Method_Can_Be_Replaced()
End Sub
Public MustOverride Sub Method_Must_Be_Replaced()
End Class

[edit] Interfaces

Interfaces may contain Functions, Subroutines, Properties, and Events.

Interface IBase
Sub Method_Must_Be_Implemented()
End Interface

[edit] zkl

In zkl, nothing is ever abstract, objects are always runnable. However, it is easy to define "meta" objects, objects that define an interface/api for a "class" of objects. For example, it is desirable for "stream" objects (such as File, List, etc) to share semantics so that code doesn't need to know what the source object really is.

class Stream{	// Mostly virtural base class
var [proxy protected]
isBroken = fcn { _broken.isSet() },
isClosed = fcn { return(_closed.isSet() or _broken.isSet()); };
fcn init{
var [protected]
_closed = Atomic.Bool(True),
_broken = Atomic.Bool(False),
whyBroken = Void;
fcn clear { _closed.clear(); _broken.clear(); return(self.topdog); }
fcn open { return(topdog.init(vm.pasteArgs())); }
fcn toStream { return(self); }
fcn close { _closed.set(); return(self.topdog); }
fcn flush { return(self.topdog); }
fcn read { throw(Exception.TheEnd); } // destructive or advance
fcn readln { throw(Exception.TheEnd); }
fcn write(x) { return(self.topdog); }
fcn writeln(x) { return(self.topdog); }
fcn walker { return((0).walker(*,wap((self.topdog.read.fpM(""))))); }
  • The topdog property is the "youngest" child in the inheritance tree (root if you view the tree upside down), it allows a "parent" (or super) to access or pass control to, the actual instance.
  • If you wish to "force" method implementation, you can have a meta method throw an NotImplementedError. This is run time thing, not compile time.

And now for a "real" object:

class DevNull(Stream){
var [const] fileName = "DevNull"; // compatibility with File
fcn init { Stream.init() }
fcn write(x) { return(0); }
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