Inheritance/Multiple

Multiple inheritance allows to specify that one class is a subclass of several other classes. Some languages allow multiple inheritance for arbitrary classes, others restrict it to interfaces, some don't allow it at all.

Write two classes (or interfaces) Camera and MobilePhone, then write a class CameraPhone which is both a Camera and a MobilePhone.

There is no need to implement any functions for those classes.

Ada

Ada 2005 has added interfaces, allowing a limited form of multiple inheritance. <lang ada> package Multiple_Interfaces is

  type Camera is tagged null record;
  type Mobile_Phone is limited Interface;
  type Camera_Phone is new Camera and Mobile_Phone with null record;

end Multiple_Interfaces; </lang>

C++

<lang cpp> class Camera {

 // ...

};

class MobilePhone {

 // ...

};

class CameraPhone:

 public Camera,
 public MobilePhone

{

 // ...

}; </lang>

Common Lisp

<lang common-lisp>(defclass camera () ()) (defclass mobile-phone () ()) (defclass camera-phone (camera mobile-phone) ())</lang>

D

<lang d> interface Camera { /+ member function prototypes go here +/ } interface MobilePhone { /+ member function prototypes go here +/ } class CameraPhone: Camera,MobilePhone { /+ member function implementations for Camera, MobilePhone, and CameraPhone go here +/ } </lang>

E

E does not have multiple inheritance as a built-in feature. In fact, E only has inheritance at all as a light syntactic sugar over delegation (message forwarding). However, using that facility it is possible to implement multiple inheritance.

This is a quick simple implementation of multiple inheritance. It simply searches (depth-first and inefficiently) the inheritance tree for a method; it does not do anything about diamond inheritance. These shortcomings could be fixed if more powerful multiple inheritance were needed.

<lang e>def minherit(self, supers) {

   def forwarder match [verb, args] {
       escape __return {
           if (verb == "__respondsTo") {
               def [verb, arity] := args
               for super ? (super.__respondsTo(verb, arity)) in supers {
                   return true
               }
               return false
           } else if (verb == "__getAllegedType") {
               # XXX not a complete implementation
               return supers[0].__getAllegedType()
           } else {
               def arity := args.size()
               for super ? (super.__respondsTo(verb, arity)) in supers {
                   return E.call(super, verb, args)
               }
               throw(`No parent of $self responds to $verb/$arity`)
           }
       }
   }
   return forwarder

}</lang>

The task example:

<lang e>def makeCamera(self) {

   return def camera extends minherit(self, []) {
       to takesPictures() { return true }
   }

}

def makeMobilePhone(self) {

   return def mobilePhone extends minherit(self, []) {
       to makesCalls() { return true }
       to internalMemory() { return 64*1024 }
   }

}

def makeCameraPhone(self) {

   return def cameraPhone extends minherit(self, [
       makeCamera(self),
       makeMobilePhone(self),
   ]) {
       to internalMemory() {
           return super.internalMemory() + 32 * 1024**2
       }
   }

}</lang >

And testing that it works as intended:

<lang e> ? def p := makeCameraPhone(p) > [p.takesPictures(), p.makesCalls(), p.internalMemory()]

value: [true, true, 33619968]

</lang>

J

coclass 'Camera'

create=: verb define
NB. creation-specifics for a camera go here
)

destroy=: codestroy

NB. additional camera methods go here

coclass 'MobilePhone'

create=: verb define
NB. creation-specifics for a mobile phone go here
)

destroy=: codestroy

NB. additional phone methods go here

coclass 'CameraPhone'
coinsert 'Camera MobilePhone'

create=: verb define
  create_Camera_ f.y
  create_MobilePhone_ f.y
  NB. creation details specific to a camera phone go here
)

destroy=: verb define
  codestroy ''
) 

NB. additional camera-phone methods go here

The adverb Fix (f.) is needed as shown so the superclass constructors get executed in the object, not in the superclass.

Java

Java does not allow multiple inheritance, but you can "implement" multiple interfaces. All methods in interfaces are abstract (they don't have an implementation). When you implement an interface you need to implement the specified methods. <lang java>public interface Camera{

  //functions here with no definition...
  //ex:
  //public void takePicture();

}</lang> <lang java>public interface MobilePhone{

  //functions here with no definition...
  //ex:
  //public void makeCall();

}</lang> <lang java>public class CameraPhone implements Camera, MobilePhone{

  //functions here...

}</lang>

Logtalk

Logtalk supports multiple inheritance. There is no "class" keyword in Logtalk; an "object" keyword is used instead (Logtalk objects play the role of classes, meta-classes, instances, or prototypes depending on the relations with other objects).

:- object(camera,
    ...).
    ...
:- end_object.

:- object(mobile_phone,
    ...).
    ...
:- end_object.

:- object(camera_phone,
    specializes(camera, mobile_phone),
    ...).
    ...
:- end_object.

OCaml

<lang ocaml>class camera =

 object (self)
   (*functions go here...*)
 end</lang>

<lang ocaml>class mobile_phone =

 object (self)
   (*functions go here...*)
 end</lang>

<lang ocaml>class camera_phone =

 object (self)
   inherit camera
   inherit mobile_phone
   (*functions go here...*)
 end</lang>

Perl

<lang perl>package Camera;

functions go here...

1;</lang>

<lang perl>package MobilePhone;

functions go here...

1;</lang>

<lang perl>package CameraPhone; use Camera; use MobilePhone; @ISA = qw( Camera MobilePhone );

functions go here...

1;</lang>

or

<lang perl>package CameraPhone; use base 'Camera'; use base 'MobilePhone';

functions go here...

</lang>

Pop11

;;; load object support
lib objectclass;

define :class Camera;
   ;;; slots go here
enddefine;

define :class MobilePhone;
   ;;; slots go here
enddefine;

define :class CameraPhone is Camera, MobilePhone;
   ;;; extra slots go here
enddefine;

;;; methods go here

Python

<lang python>class Camera:

 pass #functions go here...</lang>

<lang python>class MobilePhone:

 pass #functions go here...</lang>

<lang python>class CameraPhone(Camera, MobilePhone):

 pass #functions go here...</lang>

Ruby

Ruby does not have multiple inheritance, but you can mix modules into classes: <lang ruby>module Camera

 # define methods here

end class MobilePhone

 # define methods here

end class CameraPhone < MobilePhone

 include Camera
 # define methods here

end</lang>

Slate

<lang slate> define: #Camera. define: #MobilePhone. define: #CameraPhone &parents: {Camera. MobilePhone}. </lang>

Tcl

Requires Tcl 8.6 or Tcl 8.5+TclOO <lang Tcl>oo::class create Camera oo::class create MobilePhone oo::class create CameraPhone {

   superclass Camera MobilePhone

}</lang>