Comparison of programming languages (object-oriented programming)

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Template:ProgLangCompare This Comparison of programming languages compares how object-oriented programming languages such as C++, PHP, Perl and others manipulate data structures.


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Object creation and destruction

creation destruction
ABAP Objects data variable type ref to class .
create object variable «exporting parameter = argument[1]». 		[2][3]
C++ (STL) class variable«(parameters)»; or
class *variable = new class«(parameters)»; 		delete pointer;
C# class variable = new class(parameters); variable.Dispose();[3]
Java variable.dispose();[3]
Objective-C class *variable = [[class alloc ] init]; or
class *variable = [[class alloc ] initWithFoo:parameter «bar:parameter ...»]; 		[variable release];
Python variable = class(parameters) del variable [3](Normally not needed)
Visual Basic .NET Dim variable As New class(parameters) variable.Dispose()[3]
Eiffel create variable or
create «{TYPE}» variable.make_foo «(parameters)» or
variable := create {TYPE} or
variable := create {TYPE}.make_foo «(parameters)» 		[3]
PHP $variable = new class(parameters); unset($variable);
Perl 5 «my »$variable = class->new(parameters); undef($variable);
Ruby variable = class.new«(parameters)» [3]
Windows PowerShell $variable = New-Object «-TypeName» class ««-ArgumentList» parameters» Remove-Variable «-Name» variable
OCaml let variable = new class «parameters» or
let variable = object members end[4] 		[3]
F# let variable = «new »class(«parameters»)

File declaration

class interface namespace
ABAP Objects class name definition «inheriting from parentclass». «interfaces: interfaces.» method_and_field_declarations endclass.
class name implementation. method_implementations endclass. 		interface name. members endinterface. Template:N/a
C++ (STL) class name« : public parentclasses[5]» { members }; namespace name { members }
C# class name« : parentclass«, interfaces»» { members } interface name { members }
Java class name« extends parentclass»« implements interfaces» { members } package name; members
Objective-C @interface name : parentclass «< protocols >» { instance_fields } method_and_property_declarations @end
@implementation name method_implementations @end[6] 		@protocol name members @end
Python class name(parentclasses[5]):
Template:Keypress members 		[7] __all__ = [ member1,member2,... ]
Visual Basic .NET Class name« Inherits parentclass»« Implements interfaces»
members
End Class 		Interface name
members
End Interface 		Namespace name
members
End Namespace
Eiffel class name« inherit parentclasses[5]»
members
end 		colspan=2 Template:N/a
PHP class name« extends parentclass»« implements interfaces» { members } interface name { members } namespace name; members
Perl package name; «@ISA = qw(parentclasses[5]);» members 1; package name; members
Ruby class name« < parentclass»
members
end
Windows PowerShell colspan=3 Template:N/a
OCaml class name «parameters» = object «(self)» «inherit parentclass «parameters» «inherit parentclass «parameters» ...[5]»» members end
F# type name«(parameters)» «as this» = class «inherit parentclass«(parameters)» «as base»» members «interface interface with implementation «interface interface with implementation ...»» end type name = interface members end namespace name
members

Class members

Constructors and destructors

constructor destructor finalizer[8]
ABAP Objects methods constructor «importing parameter = argument»
method constructor. instructions endmethod.[9] 		Template:N/a
C++ (STL) class(«parameters») «: initializers[10]» { instructions } ~class() { instructions }
C# class(«parameters») { instructions } void Dispose(){ instructions } ~class() { instructions }
Java void dispose() { instructions } void finalize() { instructions }
Eiffel [11] [12]
Objective-C - (id)init { instructions... return self; } or
- (id)initWithFoo:parameter «bar:parameter ...» { instructions... return self; } 		- (void)dealloc { instructions }
Python def __init__(self«, parameters»):
Template:Keypress instructions 		def __del__(self):
Template:Keypress instructions
Visual Basic .NET Sub New(«parameters»)
instructions
End Sub 		Sub Dispose()
instructions
End Sub 		Overrides Sub Finalize()
instructions
End Sub
PHP function __construct(«parameters») { instructions } function __destruct() { instructions }
Perl sub new { my ($class«, parameters») = @_; my $self = {}; instructions ... bless($self, $class); return $self; } sub DESTROY { my ($self) = @_; instructions }
Ruby def initialize«(parameters)»
instructions
end 		Template:N/a
Windows PowerShell colspan=3 Template:N/a
OCaml initializer instructions[13] Template:N/a
F# do instructions or
new(parameters) = expression [14] 		member this.Dispose() = instructions override this.Finalize() = instructions

Fields

public private protected friend
ABAP Objects public section.[15] data field type type. private section.[15] data field type type. protected section.[15] data field type type. [16]
C++ (STL) public: type field; private: type field; protected: type field; [17]
C# public type field «= value»; private type field «= value»; protected type field «= value»; internal type field «= value»;
Java type field «= value»;
Eiffel feature
field: TYPE 		feature {NONE}
field: TYPE 		feature {current_class}
field: TYPE 		feature {FRIEND}
field: TYPE
Objective-C @public type field; @private type field; @protected type field; @package type field;
Python self.field = value
Just assign a value to it in a method 		self.__field = value
Just assign a value to it in a method 		colspan=2 Template:N/a
Visual Basic .NET Public field As type «= value» Private field As type «= value» Protected field As type «= value» Friend field As type «= value»
PHP public $field «= value»; private $field «= value»; protected $field «= value»;
Perl $self->{field} = value;
Just assign a value to it in a method 		colspan=3 Template:N/a
Ruby colspan=2 Template:N/a @field = value
Just assign a value to it in a method
Windows PowerShell Add-Member
«-MemberType »NoteProperty
«-Name »Bar «-Value »value
-InputObject variable 		colspan=3 Template:N/a
OCaml colspan=2 Template:N/a val «mutable» field = value rowspan=2 Template:N/a
F# Template:N/a let «mutable» field = value Template:N/a

Methods

basic/void method value-returning method
ABAP Objects methods name «importing parameter = argument» «exporting parameter = argument» «changing parameter = argument» «returning value(parameter)»
method name. instructions endmethod.[18] 		[19]
C++ (STL)[20] void foo(«parameters») { instructions } type foo(«parameters») { instructions ... return value; }
C#
Java
Eiffel foo ( «parameters» )
do
instructions
end 		foo ( «parameters» ): TYPE
do
instructions...
Result := value
end
Objective-C - (void)foo«:parameter «bar:parameter ...»» { instructions } - (type)foo«:parameter «bar:parameter ...»» { instructions... return value; }
Python def foo(self«, parameters»):
Template:Keypress instructions 		def foo(self«, parameters»):
Template:Keypress instructions
Template:Keypress return value
Visual Basic .NET Sub Foo(«parameters»)
instructions
End Sub 		Function Foo(«parameters») As type
instructions
...
Return value
End Function
PHP function foo(«parameters») { instructions } function foo(«parameters») { instructions ... return value; }
Perl sub foo { my ($self«, parameters») = @_; instructions } sub foo { my ($self«, parameters») = @_; instructions ... return value; }
Ruby def foo«(parameters)»
instructions
end 		def foo«(parameters)»
instructions
expression resulting in return value
end
or
def foo«(parameters)»
instructions
return value
end
Windows PowerShell Add-Member «-MemberType» ScriptMethod «-Name» foo «-Value» { «param(parameters)» instructions } -InputObject variable Add-Member «-MemberType» ScriptMethod «-Name» foo «-Value» { «param(parameters)» instructions ... return value } -InputObject variable
OCaml rowspan=2 Template:N/a method foo «parameters» = expression
F# member this.foo(«parameters») = expression

Properties

How to declare a property named "Bar"

Manually implemented

read-write read-only write-only
ABAP Objects colspan=3 Template:N/a
C++ (STL) colspan=3 Template:N/a
C# type Bar {
get { instructions ... return value; }
set { instructions } } 		type Bar { get { instructions ... return value; } } type Bar { set { instructions } }
Java colspan=3 Template:N/a
Objective-C @property (readwrite) type bar;
and then inside @implementation
- (type)bar { instructions }
- (void)setBar:(type)value { instructions } 		@property (readonly) type bar;
and then inside @implementation

- (type)bar { instructions }

Eiffel feature -- Access
x: TYPE assign set_x
feature -- Settings
set_x (a_x: like x) do instructions ensure x_set: verification end
Python def setBar(self, value):
Template:Keypress instructions
def getBar(self):
Template:Keypress instructions
Template:Keypress return value
bar = property(getBar, setBar) 		def getBar(self):
Template:Keypress instructions
Template:Keypress return value
bar = property(getBar) 		def setBar(self, value):
Template:Keypress instructions
bar = property(fset = setBar)
Visual Basic .NET Property Bar() As type
Get
instructions
Return value
End Get
Set (ByVal Value As type)
instructions
End Set
End Property 		ReadOnly Property Bar() As type
Get
instructions
Return value
End Get
End Property 		WriteOnly Property Bar() As type
Set (ByVal Value As type)
instructions
End Set
End Property
PHP function __get($property) {
switch ($property) {
case 'Bar' : instructions ... return value;
} }
function __set($property, $value) {
switch ($property) {
case 'Bar' : instructions
} } 		function __get($property) {
switch ($property) {
case 'Bar' : instructions ... return value;
} } 		function __set($property, $value) {
switch ($property) {
case 'Bar' : instructions
} }
Perl colspan=3 Template:N/a
Ruby def bar
instructions
expression resulting in return value
end
def bar=(value)
instructions
end 		def bar
instructions
expression resulting in return value
end 		def bar=(value)
instructions
end
Windows PowerShell Add-Member
«-MemberType »ScriptProperty
«-Name »Bar «-Value »{ instructions ... return value }
«-SecondValue »{ instructions }
-InputObject variable 		Add-Member
«-MemberType »ScriptProperty
«-Name »Bar «-Value »{ instructions ... return value}
-InputObject variable 		Add-Member
«-MemberType »ScriptProperty
«-Name »Bar -SecondValue { instructions }
-InputObject variable
OCaml colspan=3 Template:N/a
F# member this.Bar with get() = expression and set(value) = expression member this.Bar = expression member this.Bar with set(value) = expression

Automatically implemented

read-write read-only write-only
ABAP Objects colspan=3 Template:N/a
C++ (STL) colspan=3 Template:N/a
C# type Bar { get; set; } type Bar { get; private set; } colspan=1 Template:N/a
Java colspan=3 Template:N/a
Objective-C @property (readwrite) type bar;
and then inside @implementation
@synthesize bar; 		@property (readonly) type bar;
and then inside @implementation
@synthesize bar;
Eiffel
Python
Visual Basic .NET Property Bar As type« = initial_value» (VB 10)
PHP
Perl colspan=3 Template:N/a
Ruby attr_accessor :bar attr_reader :bar attr_writer :bar
Windows PowerShell
OCaml colspan=3 Template:N/a
F#

Overloaded operators

Standard operators

unary binary
ABAP Objects colspan=4 Template:N/a
C++ (STL) type operatorsymbol() { instructions } type operatorsymbol(type operand2) { instructions }
C# static type operator symbol(type operand) { instructions } static type operator symbol(type operand1, type operand2) { instructions }
Java rowspan=2 colspan=2 Template:N/a
Objective-C
Eiffel [21] op_name alias "symbol": TYPE
do instructions end 		op_name alias "symbol" (operand: TYPE1): TYPE2
do instructions end
Python def __opname__(self):
Template:Keypress instructions
Template:Keypress return value 		def __opname__(self, operand2):
Template:Keypress instructions
Template:Keypress return value
Visual Basic .NET Shared Operator symbol(operand As type) As type
instructions
End Operator 		Shared Operator symbol(operand1 As type, operand2 As type) As type
instructions
End Operator
PHP [22]
Perl use overload "symbol" => sub { my ($self) = @_; instructions }; use overload "symbol" => sub { my ($self, $operand2, $operands_reversed) = @_; instructions };
Ruby def symbol
instructions
expression resulting in return value
end 		def symbol(operand2)
instructions
expression resulting in return value
end
Windows PowerShell rowspan=2 colspan=2 Template:N/a
OCaml
F# static member (symbol) operand = expression static member (symbol) (operand1, operand2) = expression

Indexers

read-write read-only write-only
ABAP Objects colspan=3 Template:N/a
C++ (STL) type& operator[](type index) { instructions } type operator[](type index) { instructions }
C# type this[type index] {
get{ instructions }
set{ instructions } } 		type this[type index] { get{ instructions } } type this[type index] { set{ instructions } }
Java rowspan=2 colspan=4 Template:N/a
Objective-C
Eiffel [21] bracket_name alias "[]" (index: TYPE): TYPE assign set_item
do instructions end
set_item (value: TYPE; index: TYPE):
do instructions end 		bracket_name alias "[]" (index: TYPE): TYPE
do instructions end
Python def __getitem__(self, index):
Template:Keypress instructions
Template:Keypress return value
def __setitem__(self, index, value):
Template:Keypress instructions 		def __getitem__(self, index):
Template:Keypress instructions
Template:Keypress return value 		def __setitem__(self, index, value):
Template:Keypress instructions
Visual Basic .NET Default Property Item(Index As type) As type
Get
instructions
End Get
Set(ByVal Value As type)
instructions
End Set
End Property 		Default ReadOnly Property Item(Index As type) As type
Get
instructions
End Get
End Property 		Default WriteOnly Property Item(Index As type) As type
Set(ByVal Value As type)
instructions
End Set
End Property
PHP [23]
Perl colspan=3 Template:N/a
Ruby def [](index)
instructions
expression resulting in return value
end
def []=(index, value)
instructions
end 		def [](index)
instructions
expression resulting in return value
end 		def []=(index, value)
instructions
end
Windows PowerShell rowspan=2 colspan=3 Template:N/a
OCaml
F# member this.Item with get(index) = expression and set index value = expression member this.Item with get(index) = expression member this.Item with set index value = expression

Type casts

downcast upcast
ABAP Objects colspan=2 Template:N/a
C++ (STL) operator returntype() { instructions }
C# static explicit operator returntype(type operand) { instructions } static implicit operator returntype(type operand) { instructions }
Java rowspan=4 colspan=2 Template:N/a
Objective-C
Eiffel [21]
Python
Visual Basic .NET Shared Narrowing Operator CType(operand As type) As returntype
instructions
End Operator 		Shared Widening Operator CType(operand As type) As returntype
instructions
End Operator
PHP rowspan=5 colspan=2 Template:N/a
Perl
Ruby
Windows PowerShell
OCaml
F#

Member access

How to access members of an object x

object member class member namespace member
method field property
ABAP Objects x->method(«parameters[24]»). x->field Template:N/a x=>field or x=>method(«parameters[24]»). Template:N/a
C++ (STL) x.method(parameters) or
ptr->method(parameters) 		x.field or
ptr->field 		cls::member ns::member
Objective-C [x method«:parameter «bar:parameter ...»»] x->field x.property (2.0 only) or
[x property] 		[cls method«:parameter «bar:parameter ...»»]
C# x.method(parameters) x.field x.property cls.member ns.member
Java Template:N/a
Python x.property
Visual Basic .NET
Windows PowerShell [cls]::member
F# Template:N/a cls.member
Eiffel x.method«(parameters)» x.field {cls}.member rowspan=2 Template:N/a
Ruby Template:N/a x.property cls.member
PHP x->method(parameters) x->field x->property cls::member ns\member
Perl x->method«(parameters)» x->{field} cls->method«(parameters)» ns::member
OCaml x#method «parameters» colspan=2 Template:N/a

Member availability

Has member? Handler for missing member
Method Field Method Field
ABAP Objects rowspan=2 colspan=4 Template:N/a
C++ (STL)
Objective-C [x respondsToSelector:@selector(method)] Template:N/a forwardInvocation: Template:N/a
C# (using reflection)
Java
Eiffel colspan=4 Template:N/a
Python hasattr(x, "method") and callable(x.method) hasattr(x, "field") __getattr__()
Visual Basic .NET (using reflection)
Windows PowerShell (using reflection)
F# (using reflection)
Eiffel colspan=4 Template:N/a
Ruby x.respond_to?(:method) Template:N/a method_missing() Template:N/a
PHP method_exists(x, "method") property_exists(x, "field") __call() __get() / __set()
Perl x->can("method") defined x->{field} AUTOLOAD
OCaml colspan=4 Template:N/a

Special variables

current object current object's parent object null reference
Smalltalk self super nil
ABAP Objects me super initial
C++ (STL) *this [25] NULL
C# this base[26] null
Java super[26]
Objective-C self super nil
Python self[27] super(current_class_name, self)[5]
super() (3.x only) 		None
Visual Basic .NET Me MyBase Nothing
Eiffel Current Precursor «{superclass}» «(args)»[26][28] Void
PHP $this parent[26] NULL
Perl $self[27] $self->SUPER[26] undef
Ruby self super«(args)»[29] nil
Windows PowerShell $this $NULL
OCaml self[30] super[31] Template:N/a[32]
F# this base[26] null

Special methods

String representation Object copy Value equality Object comparison Hash code Object ID
Human-readable Source-compatible
ABAP Objects colspan=6 Template:N/a
C++ (STL) x == y[33]
C# x.ToString() x.Clone() x.Equals(y) x.CompareTo(y) x.GetHashCode()
Java x.toString() x.clone()[34] x.equals(y) x.compareTo(y) x.hashCode() System.identityHashCode(x)
Objective-C [x description] [x copy][35] [x isEqual:y] [x compare:y][36] [x hash]
Python str(x)[37] repr(x)[38] copy.copy(x)[39] x == y[40] cmp(x ,y)[41] hash(x)[42] id(x)
Visual Basic .NET x.ToString() x.Clone() x.Equals(y) x.CompareTo(y) x.GetHashCode()
Eiffel x.out x.twin x.is_equal(y) When x is COMPARABLE, one can simply do x < y When x is HASHABLE, one can simply do x.hash_code When x is IDENTIFIED, one can simply do x.object_id
PHP sprintf("%s", x)[43] clone x[44] x == y spl_object_hash(x)
Perl "$x"[45]
Ruby x.to_s x.inspect x.dup or
x.clone 		x == y or
x.eql?(y) 		x <=> y x.hash x.object_id
Windows PowerShell x.ToString() x.Clone() x.Equals(y) x.CompareTo(y) x.GetHashCode()
OCaml Oo.copy x x = y Hashtbl.hash x Oo.id x
F# x.ToString() sprintf "%A" x x.Clone() x.Equals(y) x.CompareTo(y) x.GetHashCode()

Type manipulation

Get object type Is instance of (includes subtypes) Upcasting Downcasting
ABAP Objects colspan=2 Template:N/a = ?=
C++ (STL) typeid(x) dynamic_cast<type *>(&x) != NULL rowspan=6 Template:N/a[46] (type) x or dynamic_cast<type*>(ptr)
C# x.GetType() x is type (type) x or x as type
Java x.getClass() x instanceof class (type) x
Objective-C [x class] [x isKindOfClass:[class class]]
Visual Basic .NET x.GetType() TypeOf x Is type CType(x, type) or TryCast(x, type)
Eiffel x.generating_type attached {TYPE} x attached {TYPE} x as down_x
Python type(x) isinstance(x, type) colspan=2 rowspan=4 Template:N/a[47]
PHP get_class(x) x instanceof class
Perl ref(x) x->isa("class")
Ruby x.class or
x.type 		x.instance_of?(type) or
x.kind_of?(type)
Windows PowerShell x.GetType() x -is [type] Template:N/a[46] [type]x or x -as [type]
OCaml colspan=2 Template:N/a[48] (x :> type) Template:N/a
F# x.GetType() x :? type (x :?> type)

Namespace management

Import namespace Import item
qualified unqualified
ABAP Objects
C++ (STL) using namespace ns; using ns::item ;
C# using ns;
Java import ns.*; import ns.item
Objective-C
Visual Basic .NET Imports ns
Eiffel
Python import ns from ns import * from ns import item
PHP use ns; use ns/item;
Perl use ns;
Ruby
Windows PowerShell
OCaml open ns
F#

Contracts

Precondition Postcondition Check Invariant Loop
ABAP Objects colspan=5 rowspan=6 Template:N/a
C++ (STL)
C#
Java
Objective-C
Visual Basic .NET
Eiffel f
require tag: expression
do end 		f
do
ensure tag: expression
end 		f
do
check tag: expression end
end 		class X
invariant tag: expression
end 		from instructions
invariant
tag: expression
until
expr
loop
instructions
variant
tag: expression
end
Python colspan=5 rowspan=7 Template:N/a
PHP
Perl
Ruby
Windows PowerShell
OCaml
F#

See also

References and notes

  1. ↑ parameter = argument may be repeated if the constructor has several parameters
  2. ↑ SAP reserved to himself the use of destruction
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 This language uses garbage collection to release unused memory.
  4. ↑ OCaml objects can be created directly without going through a class.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 This language supports multiple inheritance. A class can have more than one parent class
  6. ↑ Usually the @interface portion is placed into a header file, and the @interface portion is placed into a separate source code file.
  7. ↑ In Python interfaces are classes whose methods have pass as their bodies
  8. ↑ A finalizer is called by the garbage collector when an object is about to be garbage-collected. There is no guarantee on when it will be called or if it will be called at all.
  9. ↑ In ABAP, the constructor is to be defined like a method (see comments about method) with the following restrictions: the method name must be "constructor", and only "importing" parameters can be defined
  10. ↑ An optional comma-separated list of initializers for member objects and parent classes goes here. The syntax for initializing member objects is "member_name(parameters)" This works even for primitive members, in which case one parameter is specified and that value is copied into the member. The syntax for initializing parent classes is "class_name(parameters)". If an initializer is not specified for a member or parent class, then the default constructor is used.
  11. ↑ Any Eiffel procedure can be used as a creation procedure, aka constructors. See Eiffel paragraph at Constructor (computer science).
  12. ↑ Implementing {DISPOSABLE}.dispose ensures that dispose will be called when object is garbage collected.
  13. ↑ This "initializer" construct is rarely used. Fields in OCaml are usually initialized directly in their declaration. Only when additional imperative operations are needed is "initializer" used. The "parameters to the constructor" in other languages are instead specified as the parameters to the class in OCaml. See the class declaration syntax for more details.
  14. ↑ This syntax is usually used to overload constructors
  15. ↑ 15.0 15.1 15.2 Scope identifier must appear once in the file declaration, all variable declarations after this scope identifier have his scope, until another scope identifier or the end of class declaration is reached
  16. ↑ In ABAP, you don't declare specific fields or methods to be accessible by outside things. Rather, you declare outside classes to be friends to have access to the class's fields or methods.
  17. ↑ In C++, you don't declare specific fields to be accessible by outside things. Rather, you declare outside functions and classes to be friends to have access to the class's fields. See friend function and friend class for more details.
  18. ↑ The declaration and implementation of methods in ABAP are separate. methods statement is to be used inside the class definition. method (without "s") is to be used inside the class implementation. parameter = argument can be repeated if there are several parameters.
  19. ↑ In ABAP, the return parameter name is explicitly defined in the method signature within the class definition
  20. ↑ The declaration and implementation of methods in C++ are usually separate. Methods are declared inside the class definition (which is usually included in a header file) using the syntax
    type foo(«parameters»);
    The implementation of methods is usually provided in a separate source file, with the following syntax
    type class::foo(«parameters») { instructions }
    Although the body of a method could be included with the declaration inside the class definition, as shown in the table here, this is generally a bad idea. Because the class definition will need to be included with every source file which uses the fields or methods of the class, having code in the class definition will cause the method code to be compiled with every source file, increasing the size of the code. There are some circumstances, however, where it is useful to include the body of a method with the declaration. One reason is that the compiler will try to inline methods that are included in the class declaration; so if you have a very short one-liner method, it may make it faster to allow the compiler to inline it, by including the body along with the declaration. Also, if you have a template class or method, then all the code must be included with the declaration, because only with the code can the template be instantiated.
  21. ↑ 21.0 21.1 21.2 Although Eiffel does not support overloading of operators, it can define operators
  22. ↑ PHP does not support operator overloading natively, but support can be added using the "operator" PECL package.
  23. ↑ Your class needs to implement the ArrayAccess interface.
  24. ↑ 24.0 24.1 In ABAP, arguments must be passed using this syntax:
    x->method(«exporting parameter = argument» «importing parameter = argument» «changing parameter = argument» «returning value(parameter)»
    parameter = argument can be repeated if there are several parameters
  25. ↑ C++ doesn't have a "super" keyword, because multiple inheritance is possible, and so it may be ambiguous which base class is desired. Instead, you can use the BaseClassName::member syntax to access an overridden member in the specified base class. Microsoft Visual C++ provides a non-standard keyword "__super" for this purpose; but this is not supported in other compilers.[1]
  26. ↑ 26.0 26.1 26.2 26.3 26.4 26.5 The keyword here is not a value in itself and it can only be used to access a method of the superclass.
  27. ↑ 27.0 27.1 In this language, instance methods are passed the current object as the first parameter, which is conventionally named "self", but this is not required to be the case.
  28. ↑ "Precursor" in Eiffel is actually a call to the method of the same name in the superclass. So Precursor(args) is equivalent to "super.currentMethodName(args)" in Java. There is no way of calling a method of different name in the superclass.
  29. ↑ "super" in Ruby, unlike in other languages, is actually a call to the method of the same name in the superclass. So super(args) in Ruby is equivalent to "super.currentMethodName(args)" in Java. There is no way of calling a method of different name in the superclass.
  30. ↑ In OCaml, an object declaration can optionally start with a parameter which will be associated with the current object. This parameter is conventionally named "self", but this is not required to be the case. It is good practice to put a parameter there so that one can call one's own methods.
  31. ↑ In OCaml, an inheritance declaration ("inherit") can optionally be associated with a value, with the syntax "inherit parent_class «parameters» as super". Here "super" is the name we gave to the variable associated with this parent object. It can be named something else.
  32. ↑ However, if you really wanted the ability to have an "optional" value in OCaml, you would wrap the value inside an option type, whose values are None and Some x, which could be used to represent "null reference" and "non-null reference to an object" as in other languages.
  33. ↑ assuming that "x" and "y" are the objects (and not a pointer). Can be customized by overloading the object's == operator
  34. ↑ Only accessible from within the class itself, since the clone() method inherited from Object is protected, unless the class overrides the method and makes it public. If you use the clone() inherited from Object, your class will need to implement the Cloneable interface to allow cloning.
  35. ↑ Implemented by the object's copyWithZone: method
  36. ↑ compare: is the conventional name for the comparison method in Foundation classes. However, no formal protocol exists
  37. ↑ Can be customized by the object's __str__() method
  38. ↑ Can be customized by the object's __repr__() method
  39. ↑ Can be customized by the object's __copy__() method
  40. ↑ Can be customized by the object's __eq__() method
  41. ↑ Only in Python 2.x and before (removed in Python 3.0). Can be customized by the object's __cmp__() method
  42. ↑ Can be customized by the object's __hash__() method. Not all types are hashable (mutable types are usually not hashable)
  43. ↑ Can be customized by the object's __toString() method
  44. ↑ Can be customized by the object's __clone() method
  45. ↑ Can be customized by overloading the object's string-convertion operator
  46. ↑ 46.0 46.1 Upcasting is implicit in this language. A subtype instance can be used where a supertype is needed.
  47. ↑ Casting between types is not necessary.
  48. ↑ This language doesn't give run-time type information. It is unnecessary because it is statically typed and downcasting is not possible.

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