Comparison of programming languages (object-oriented programming)

Programming language comparisons

* General comparison
* Assignment
* Basic syntax
* Basic instructions
* Comments
* Control flow
* Foreach loops
* While loops
* For loops
* Do-while
* Exception handling
* Enumerated types
* Generators
* Anonymous functions
* Conditional expressions
* Functional instructions
* Arrays
* Associative arrays
* Scope
* String operations
* String functions
* Higher-order functions
* Type systems
* List comprehension
* Object-oriented programming
* Object-oriented constructors
* Operators

* Evaluation strategy
* List of "Hello World" programs

* Languages with dependent types
* Comparison of type systems

* * *

Comparison of individual languages

* ALGOL 58's influence on ALGOL 60
* ALGOL 60: Comparisons with other languages
* Comparison of ALGOL 68 and C++
* ALGOL 68: Comparisons with other languages
* Compatibility of C and C++
* Comparison of Pascal and Borland Delphi
* Comparison of Object Pascal and C
* Comparison of Pascal and C
* Comparison of Java and C++
* Comparison of C# and Java
* Comparison of C# and Visual Basic .NET
* Comparison of Visual Basic and Visual Basic .NET

* v
* t
* e

This comparison of programming languages compares how object-oriented programming languages such as C++, Java, Smalltalk, Object Pascal, Perl, Python, and others manipulate data structures.

## Contents

* 1 Object construction and destruction
* 2 Class declaration
* 3 Class members
* 3.1 Constructors and destructors
* 3.2 Fields
* 3.3 Methods
* 3.4 Properties
* 3.4.1 Manually implemented
* 3.4.2 Automatically implemented
* 3.5 Overloaded operators
* 3.5.1 Standard operators
* 3.5.2 Indexers
* 3.5.3 Type casts
* 4 Member access
* 5 Member availability
* 6 Special variables
* 7 Special methods
* 8 Type manipulation
* 9 Namespace management
* 10 Contracts
* 11 See also
* 12 References and notes

## Object construction and destruction

| construction  | destruction

|
|

ABAP Objects  | `data variable type ref to class .
create object variable «exporting parameter = argument».`[1] | [2][3]
APL (Dyalog)  | `variable←⎕NEW class «parameters»` | `⎕EX 'variable'`
C++ | `class variable«(parameters)»;`[4] or
`class *variable = new class«(parameters)»;`[5] | `delete pointer;`
C# | `class variable = new class(parameters);` | `variable.Dispose();`[3]
Java | [3]
D | `destroy(variable);`
eC | `class «instance handle» { «properties/data members assignments, instance method overrides» }` | `delete instance handle;`
Objective-C (Cocoa)  | `class *variable = *'''variable '''= <nowiki>[[class alloc ] initWithFoo:parameter «bar:parameter ...»];` | `[variable release];`
Swift | `let variable = class(parameters)` |
Python | `variable = class(parameters)` | `del variable`[3] (Normally not needed)
Visual Basic .NET | `Dim variable As New class(parameters)` | `variable.Dispose()`[3]
Xojo | `Dim variable As New class(parameters)` | `variable = Nil`
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);`[3]
Perl 5  | `«my »$variable = class->new«(parameters)»;` | `undef($variable);`
Raku | `«my »$variable = class.new«(parameters)»;` | `$variable.undefine;`
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`[6] | [3]
F# | `let variable = «new »class(«parameters»)`
Smalltalk | The class is an Object.
Just send a message to a class, usually `#new` or `#new:`, and many others, for example:

Point x: 10 y: 20.
Array with: -1 with: 3 with: 2.

JavaScript | `var variable = new class«(parameters)»` or
`var variable = { «key1: value1«, key2: value2 ...»»}` | [3]
Object Pascal (Delphi)  | `ClassVar := ClassType.ConstructorName(parameters);` | `ClassVar.Free;`
Scala |

val obj = new Object // no parameters
val obj = new Object(arg0, arg1, arg2...)
val obj = Object(arg0, arg1, arg2...) // case class
val obj = new Object(arg0, arg1, param1 = value1, ...) // named parameters

| [3]
COBOL | `INVOKE class "NEW" RETURNING variable` or
`MOVE class::"NEW" TO variable` |
Cobra | `variable «as class» = class(parameters)` | `variable.dispose`
ISLISP | `(setq variable (create (class <some-class> [:field-1 value-1 [:field-2 value-2] ..])))` | [3]

## Class declaration

| class  | protocol | 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.` | N/A
APL (Dyalog)  | `:Class name «:parentclass» «,interfaces»`
`members`
`:EndClass` | `:Interface name`
`members`
`:EndInterface` | `:Namespace name`
`members`
`:EndNamespace`
C++  | `class name« : public parentclasses`[7]`» { members };` |  | `namespace name { members }`
C#  | `class name« : «parentclass»«, interfaces»» { members }` | `interface name« : parentinterfaces» { members }`
D  | `module name;
members`
eC  | `class name« : base class» { «default member values assignments» «members» }` | `` | `namespace name;`
Java  | `class name« extends parentclass»« implements interfaces» { members }` | `interface name« extends parentinterfaces» { members }` | `package name; members`
PHP  | `namespace name; members`
Objective-C  | `@interface name« : parentclass»`[8]`«< protocols >» { instance_fields } method_and_property_declarations @end
@implementation name method_implementations @end`[9] | `@protocol name«< parentprotocols >» members @end` | [10]
Swift  | `class name« : «parentclass»«, protocols»» { members }` | `protocol name« : parentprotocols» { members }` |
Python  | `class name«(parentclasses[7])»:
members` | [11] | `__all__ = [ member1,member2,... ]`
Visual Basic .NET  | `Class name« Inherits parentclass»« Implements interfaces»
members
End Class` | `Interface name« Inherits parentinterfaces»
members
End Interface` | `Namespace name
members
End Namespace`
Xojo  | `Class name« Inherits parentclass»« Implements interfaces»
members
End Class` | `Interface name« Inherits parentinterfaces»
members
End Interface` | `Module name
members
End Module`
Eiffel  | `class name« inherit parentclasses[7]»
members
end` | N/A
Perl  | `package name; «@ISA = qw(parentclasses[7]);» members 1;` |  | `package name; members`
Raku  | `class name «is parentclass «is parentclass ...[7]»» «does role «does role ...»» { members }` | `role name «does role «does role ...»» { members }` | `module name { members }`
Ruby  | `class name« < parentclass»
members
end` |  | `module name
members
end`
Windows PowerShell  | N/A
OCaml  | `class name «parameters» = object «(self)» «inherit parentclass «parameters» «inherit parentclass «parameters» ...[7]»» members end` |  | `module name
members`
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`
Smalltalk  | [12] |  | [13]
JavaScript (ES6)  | `class`` name «extends parentclass» { members } ` |  |
Object Pascal (Delphi)  |

`ClassName = Class «(ClassParent, Interfaces)»
private
// Private members(include Methods and Fields)
public
// Public members
protected
// Protected members
published
// Published members
end;`

|  | `package name; members`
Scala  |

class ConcreteClass(constructor params)
extends ParentClass
with Trait1 with Trait2 with Trait2 {
// members

trait TraitName
extends OtherTrait1
with OtherTrait2 with OtherTrait3 {
// members

package name

COBOL  | `CLASS-ID. name« INHERITS« FROM» parentclasses».`

`FACTORY« IMPLEMENTS interfaces».`
`class-members`
`END FACTORY.`
`OBJECT« IMPLEMENTS interfaces».`
`instance-members`
`END OBJECT.`

`END CLASS name.`

| `INTERFACE-ID. name« INHERITS« FROM» interfaces».`

`members`

`END INTERFACE name.`

| N/A
Cobra  | `class name «inherits parentclass» «implements interfaces»
members` | `interface name «inherits parentinterfaces»
members` | `namespace name
members`
ISLISP  | `(defclass name (base-class) ((x :initform 0 :accessor get-x :initarg x)) (:abstractp nil))` |  |

## Class members

### Constructors and destructors

| constructor  | destructor  | finalizer[14]

|
|
|

ABAP Objects  | `methods constructor «importing parameter = argument»
method constructor. instructions endmethod.`[15] | N/A
APL (Dyalog)  | `∇ name`
`:Implements Constructor «:Base «expr»»`
`instructions`
`∇` |  | `∇ name`
`:Implements Destructor`
`instructions`
`∇`
C++  | `class(«parameters») «: initializers`[16]`» { instructions }` | `~class() { instructions }` |
C#  | `class(«parameters») { instructions }` | `void Dispose(){ instructions }` | `~class() { instructions }`
D  | `this(«parameters») { instructions }` |  | `~this() { instructions }`
eC  | `class() { instructions }` | `~class() { instructions }` |
Java  | `class(«parameters») { instructions }` |  | `void finalize() { instructions }`
Eiffel  | [17] |  | [18]
Objective-C (Cocoa)  | `- (id)init { instructions... return self; } or
- (id)initWithFoo:parameter «bar:parameter ...» { instructions... return self; }` | `- (void)dealloc { instructions }` | `- (void)finalize { instructions }`
Swift  | `init(«parameters») { instructions }` | `deinit { instructions }` |
Python  | `def __init__(self«, parameters»):
instructions` |  | `def __del__(self):
instructions`
Visual Basic .NET  | `Sub New(«parameters»)
instructions
End Sub` | `Sub Dispose()
instructions
End Sub` | `Overrides Sub Finalize()
instructions
End Sub`
Xojo  | `Sub Constructor(«parameters»)
instructions
End Sub` | `Sub Destructor()
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 }` |
Raku  | `submethod BUILD { instructions } or
«multi » method new(««$self: »parameters») { self.bless(*, field1 => value1, ...); ... instructions }` | `submethod DESTROY { instructions }` |
Ruby  | `def initialize«(parameters)»
instructions
end` | N/A
Windows PowerShell  | N/A
OCaml  | `initializer instructions`[19] | N/A
F#  | `do instructions or
new(parameters) =  expression`[20] | `member this.Dispose() = instructions` | `override this.Finalize() = instructions`
JavaScript  | `function name(«parameters») { instructions }`[21] | N/A
JavaScript (ES6)  | `constructor(``«parameters») { instructions }`
COBOL  | N/A[22] | N/A
Cobra  | `cue init(parameters)
base.init
instructions` | `def dispose
instructions` |
ISLISP  | `(defmethod initialize-object ((instance <class-name>) initvalues)` |

### Fields

| public  | private  | protected  | friend

|
|
|
|

ABAP Objects  | `public section.[23] data field type type.` | `private section.[23] data field type type.` | `protected section.[23] data field type type.` | [24]
APL (Dyalog)  | `:Field Public field «← value»` | `:Field «Private» field «← value»` |  |
C++  | `public: type field;` | `private: type field;` | `protected: type field;` | [25]
C#  | `public type field «= value»;` | `private type field «= value»;` | `protected type field «= value»;` | `internal type field «= value»;`
D  |  | `package type field «= value»;`
Java  | `protected type field «= value»;` | `type field «= value»;`
eC  | `public type field;` | `private type field;`
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;`
Swift  | N/A
Smalltalk  | N/A  | [26] | N/A
Python  | `self.field = value`[27] | N/A[28] | 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»`
Xojo  | `Public field As type «= value»` | `Private field As type «= value»` | `Protected field As type «= value»` | N/A
PHP  | `public $field «= value»;` | `private $field «= value»;` | `protected $field «= value»;` |
Perl  | `$self->{field} = value;`[27] | N/A
Raku  | `has« type »$.field« is rw»` | `has« type »$!field` | N/A
Ruby  | N/A  | `@field = value`[27] |
Windows PowerShell  | `Add-Member
«-MemberType »NoteProperty
«-Name »Bar «-Value »value
-InputObject variable` | N/A
OCaml  | N/A  | `val «mutable» field = value` | N/A
F#  | N/A  | `let «mutable» field = value` | N/A
JavaScript  | `this.field = value
this["field"] = value`[27] |  |  |
COBOL  | N/A  | level-number field clauses.[29] | N/A  | N/A
Cobra  | `var field «as type» «= value»` | `var __field «as type» «= value»` | `var _field «as type» «= value»` |
ISLISP  | `(field :initform value :accessor accessor-name :initarg keyword)` |  |  |

### 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.`[30] | [31]
APL (Dyalog)  | `∇ «left argument» name «right arguments»`
`instructions`
`∇` | `∇ result ← «left argument» name «right arguments»`
`instructions`
`∇`
C++[32]

`type foo(«parameters»);`

The implementation of methods is usually provided in a separate source file, with the following syntax

`type class::foo(«parameters») { instructions }`[33]
| `void foo(«parameters») { instructions }` | `type foo(«parameters») { instructions ... return value; }`

Java
eC  | `void ««type of 'this'»::»foo(«parameters») { instructions }` | `type ««type of this»::»foo(«parameters») { instructions ... return value; }`
Eiffel  | `foo ( «parameters» )

instructions
end` | `foo ( «parameters» ): TYPE

instructions...
Result := value
end`
Objective-C  | `- (void)foo«:parameter «bar:parameter ...»» { instructions }` | `- (type)foo«:parameter «bar:parameter ...»» { instructions... return value; }`
Swift  | `func foo(«parameters») { instructions }` | `func foo(«parameters») -> type { instructions... return value }`
Python  | `def foo(self«, parameters»):
instructions` | `def foo(self«, parameters»):
instructions
return value`
Visual Basic .NET  | `Sub Foo(«parameters»)
instructions
End Sub` | `Function Foo(«parameters») As type
instructions
...
Return value
End Function`
Xojo  | `Sub Foo(«parameters»)
instructions
End Sub` | `Function Foo(«parameters») As type
instructions
...
Return value
End Function`
PHP  | `function foo(«parameters»)«: void» { instructions }` | `function foo(«parameters»)«: type» { instructions ... return value; }`
Perl  | `sub foo { my ($self«, parameters») = @_; instructions }` | `sub foo { my ($self«, parameters») = @_; instructions ... return value; }`
Raku  | `«has »«multi »method foo(««$self:  »parameters») { instructions }` | `«has «type »»«multi »method foo(««$self:  »parameters») { instructions ... return value; }`
Ruby  | `def foo«(parameters)»
instructions
end` | `def foo«(parameters)»
instructions
expression resulting in return value
end

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  | N/A  | `method foo «parameters» = expression`
F#  | `member this.foo(«parameters») = expression`
JavaScript  | `this.method = function(«parameters») {instructions}
name«.prototype.method = function(«parameters») {instructions}`[34] | ` this.method = function(«parameters») {instructions... return value;}
name«.prototype.method = function(«parameters») {instructions... return value;}`[34]
Javascript (ES6)  | `foo(«parameters») {instructions}` | `foo(«parameters») {instructions... return value;}`
COBOL  | `METHOD-ID. foo.
«DATA DIVISION.
LINKAGE SECTION.
parameter declarations»
PROCEDURE DIVISION« USING parameters».`

`instructions`

`END METHOD foo.`

| `METHOD-ID. foo.
DATA DIVISION.
LINKAGE SECTION.
«parameter declarations»
result-var declaration
PROCEDURE DIVISION« USING parameters» RETURNING result-var.`

`instructions`

`END METHOD foo.`

Cobra  | `def foo(parameters)
instructions` | `def foo(parameters) as type
instructions
return value` |
ISLISP  | `(defgeneric method (arg1 arg2))
(defmethod method ((arg1 <class1> arg2 <class2>) ...)` |

### Properties

How to declare a property named "Bar"

#### Manually implemented

| read-write  | read-only  | write-only

|
|
|

ABAP Objects  | N/A
APL (Dyalog)  | `:Property Bar`
`∇ result ← Get`
`instructions`
`∇`
`∇ Set arguments`
`instructions`
`∇`
`:EndProperty Bar` | `:Property Bar`
`∇ result ← Get`
`instructions`
`∇`
`:EndProperty Bar` | `:Property Bar`
`∇ Set arguments`
`instructions`
`∇`
`:EndProperty Bar`
C++  | N/A
C#  | `type Bar {
get { instructions ... return value; }
set { instructions } }` | `type Bar { get { instructions ... return value; } }` | `type Bar { set { instructions } }`
D  | `@property type bar() { instructions ... return value; }
@property type bar(type value) { instructions ... return value; }` | `@property type bar() { instructions ... return value; }` | `@property type bar(type value) { instructions ... return value; }`
eC  | `property type Bar {
get { instructions ... return value; }
set { instructions } }` | `property type Bar { get { instructions ... return value; } }` | `property type Bar { set { instructions } }`
Java  | N/A
Objective-C 2.0 (Cocoa)  | `@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 }` | N/A
Swift  | `var bar : type { get { instructions } set«(newBar)» { instructions } }` | `var bar : type { instructions }` | N/A
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):
instructions
def getBar(self):
instructions
return value
bar = property(getBar, setBar)`[35] | `def getBar(self):
instructions
return value
bar = property(getBar)` | `def setBar(self, value):
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`
Xojo  | `ComputedProperty Bar() As type
Get
instructions
Return value
End Get
Set (ByVal Value As type)
instructions
End Set
End ComputedProperty` | `ComputedProperty Bar() As type
Get
instructions
Return value
End Get
End ComputedProperty` | `ComputedProperty Bar() As type
Set (value As type)
instructions
End Set
End ComputedProperty`
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  | `sub Bar {
my $self = shift;
if (my $Bar = shift) {
# setter
$self->{Bar} = $Bar;
return $self;
} else {
# getter
return $self->{Bar};

}` | `sub Bar {
my $self = shift;
if (my $Bar = shift) {
# read-only
die "Bar is read-only\n";
} else {
# getter
return $self->{Bar};

}` | `sub Bar {
my $self = shift;
if (my $Bar = shift) {
# setter
$self->{Bar} = $Bar;
return $self;
} else {
# write-only
die "Bar is write-only\n";

Raku  | 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  | N/A
F#  | `member this.Bar with get() = expression and set(value) = expression` | `member this.Bar = expression` | `member this.Bar with set(value) = expression`
JavaScript (ES6)  | `get bar(«parameters») { instructions ... return value}set bar(«parameters») { instructions } ` | `get`` bar(«parameters») { instructions ... return value}` | `set bar(«parameters») { instructions }`
COBOL  | `METHOD-ID. GET PROPERTY bar.
DATA DIVISION.
LINKAGE SECTION.
return-var declaration
PROCEDURE DIVISION RETURNING return-var.`

`instructions`

`END METHOD.
METHOD-ID. SET PROPERTY bar.
DATA DIVISION.
LINKAGE SECTION.
value-var declaration
PROCEDURE DIVISION USING value-var.`

`instructions`

`END METHOD.`

| `METHOD-ID. GET PROPERTY bar.
DATA DIVISION.
LINKAGE SECTION.
return-var declaration
PROCEDURE DIVISION RETURNING return-var.`

`instructions`

`END METHOD.`

| `METHOD-ID. SET PROPERTY bar.
DATA DIVISION.
LINKAGE SECTION.
value-var declaration
PROCEDURE DIVISION USING value-var.`

`instructions`

`END METHOD.`

Cobra  | `pro bar «as type»
get
instructions
return value
set
instructions` | `get bar «as type»
instructions
return value` | `set bar «as type»
instructions`
ISLISP  | N/A

#### Automatically implemented

| read-write  | read-only  | write-only

|
|
|

ABAP Objects  | N/A
C++  | N/A
C#  | `type Bar { get; set; }` | `type Bar { get; private set; }` | `type Bar { private get; set; }`
D  | N/A
Java  | N/A
Objective-C 2.0 (Cocoa)  | `@property (readwrite) type bar;
`and then inside `@implementation
@synthesize bar;` | `@property (readonly) type bar;
`and then inside `@implementation
@synthesize bar;` | N/A
Swift  | `var bar : type` | `let bar : type` | N/A
Eiffel  |  |  |
Python  | `@property
def bar(self):
instructions
@bar.setter
def bar(self, value):
instructions` | `@property
def bar(self):
instructions` | `bar = property()
@bar.setter
def bar(self, value):
instructions`
Visual Basic .NET  | `Property Bar As type« = initial_value» (VB 10)` |  |
PHP  |  |  |
Perl[36] | `use base qw(Class::Accessor);
__PACKAGE__->mk_accessors('Bar');` | `use base qw(Class::Accessor);
__PACKAGE__->mk_ro_accessors('Bar');` | `use base qw(Class::Accessor);
__PACKAGE__->mk_wo_accessors('Bar');`
Raku  | N/A
Ruby  | `attr_accessor :bar` | `attr_reader :bar` | `attr_writer :bar`
Windows PowerShell  |  |  |
OCaml  | N/A
F#  | `member val Bar = value with get, set` |  |
COBOL  | `level-number bar clauses PROPERTY.` | `level-number bar clauses PROPERTY «WITH» NO SET.` | `level-number bar clauses PROPERTY «WITH» NO GET.`
Cobra  | `pro bar from var «as type»` | `get bar from var «as type»` | `set bar from var «as type»`

### Overloaded operators

#### Standard operators

| unary  | binary  | function call

|
|
|

ABAP Objects  | N/A
C++  | `type operator symbol () { instructions }` | `type operator symbol (type operand2) { instructions }` | `type operator () («parameters») { instructions }`
C#  | `static type operator symbol(type operand) { instructions }` | `static type operator symbol(type operand1, type operand2) { instructions }` | N/A
D  | `type opUnary(string s)() if (s == "symbol") { instructions }` | `type opBinary(string s)(type operand2) if (s == "symbol") { instructions }
type opBinaryRight(string s)(type operand1) if (s == "symbol") switch (s) { instructions }` | `type opCall(«parameters») { instructions }`
Java  | N/A
Objective-C
Swift  | `func symbol(operand1 : type) -> returntype { instructions }` (outside class)  | `func symbol(operand1 : type1, operand2 : type2) -> returntype { instructions }` (outside class)  |
Eiffel[37] | `op_name alias "symbol": TYPE
do instructions end` | `op_name alias "symbol" (operand: TYPE1): TYPE2
do instructions end` |
Python  | `def __opname__(self):
instructions
return value` | `def __opname__(self, operand2):
instructions
return value` | `def __call__(self«, parameters»):
instructions
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` | N/A
Xojo  | `Function Operator_name(operand As type) As type
instructions
End Function` | N/A
PHP  | [38] | `function __invoke(«parameters») { instructions } (PHP 5.3+)`
Perl  | `use overload "symbol" => sub { my ($self) = @_; instructions };` | `use overload "symbol" => sub { my ($self, $operand2, $operands_reversed) = @_; instructions };` |
Raku  | `«our «type »»«multi »method prefix:<symbol> («$operand: ») { instructions ... return value; } or
«our «type »»«multi »method postfix:<symbol> («$operand: ») { instructions ... return value; } or
«our «type »»«multi »method circumfix:<symbol1 symbol2> («$operand: ») { instructions ... return value; }` | `«our «type »»«multi »method infix:<symbol> («$operand1: » type operand2) { instructions ... return value; }` | `«our «type »»«multi »method postcircumfix:<( )> («$self: » «parameters») { instructions }`
Ruby  | `def symbol
instructions
expression resulting in return value
end` | `def symbol(operand2)
instructions
expression resulting in return value
end` | N/A
Windows PowerShell  | N/A
OCaml
F#  | `static member (symbol) operand = expression` | `static member (symbol) (operand1, operand2) = expression` | N/A
COBOL  | N/A
ISLISP  | N/A

#### Indexers

| read-write  | read-only  | write-only

|
|
|

ABAP Objects  | N/A
APL (Dyalog)  | `:Property Numbered Default name`
`∇ result ← Get`
`instructions`
`∇`
`∇ Set arguments`
`instructions`
`∇`
`:EndProperty Bar` | `:Property Numbered Default Bar`
`∇ result ← Get`
`instructions`
`∇`
`:EndProperty Bar` | `:Property Numbered Default Bar`
`∇ Set arguments`
`instructions`
`∇`
`:EndProperty Bar`
C++  | `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 } }`
D  | `type opIndex(type index) { instructions }
type opIndexAssign(type value, type index) { instructions }` | `type opIndex(type index) { instructions }` | `type opIndexAssign(type value, type index) { instructions }`
Java  | N/A
Objective-C (recent Clang compiler)  | N/A  | `- (id)objectAtIndexedSubscript:(NSUInteger)index { instructions return value; } or
- (id)objectForKeyedSubscript:(id)index { instructions return value; }` | `- (void)setObject:(id)value atIndexedSubscript:(NSUInteger)index { instructions } or
- (void)setObject:(id)value forKeyedSubscript:(id)index { instructions }`
Swift  | `subscript (index : type) -> returntype { get { instructions } set«(newIndex)» { instructions } }` | `subscript (index : type) -> returntype { instructions }` |
Eiffel[37] | `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):
instructions
return value
def __setitem__(self, index, value):
instructions` | `def __getitem__(self, index):
instructions
return value` | `def __setitem__(self, index, value):
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  | [39]
Perl  | [40]
Raku  | `«our «type »»«multi »method postcircumfix:<[ ]> is rw («$self: » type $index) { instructions ... return value; } or
«our «type »»«multi »method postcircumfix:<{ }> is rw («$self: » type $key) { instructions ... return value; }` | `«our «type »»«multi »method postcircumfix:<[ ]>(«$self: » type $index) { instructions ... return value; } or
«our «type »»«multi »method postcircumfix:<{ }> («$self: » type $key) { instructions ... return value; }` | 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  | 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`
COBOL  | N/A
Cobra  | `pro[index «as type»] as type
get
instructions
return value
set
instructions` | `get[index «as type»] as type
instructions
return value` | `set[index «as type»] as type
instructions`

#### Type casts

| downcast  | upcast

|
|

ABAP Objects  | N/A
C++  |  | `operator returntype() { instructions }`
C#  | `static explicit operator returntype(type operand) { instructions }` | `static implicit operator returntype(type operand) { instructions }`
D  |  | `T opCast(T)() if (is(T == type)) { instructions }`
eC  |  | `property T { get { return «conversion code»; } }`
Java  | N/A
Objective-C
Eiffel[37]
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  | N/A
Perl
Raku  |  | `multi method type«($self:)» is export { instructions }`
Ruby  | N/A
Windows PowerShell
OCaml
F#  |  |
COBOL  | N/A

## Member access

How to access members of an object x

| object member  | class member  | namespace member

|
|
|

| method  | field  | property
ABAP Objects  | `x->method(«parameters»).`[41] | `x->field` | N/A  | `x=>field or x=>method(«parameters[41]»).` | N/A
C++  | `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 ...»»]` |
Smalltalk  | `x method«:parameter «bar:parameter ...»»` | N/A  |  | `cls method«:parameter «bar:parameter ...»»` |
Swift  | `x.method(parameters)` |  | `x.property` | `cls.member` |
APL (Dyalog)  | `left argument» x.method «right argument(s)»` | `x.field` | `x.property` | `cls.member` | `ns.member`
C#  | `x.method(parameters)`
Java  | N/A
D  | `x.property`
Python
Visual Basic .NET
Xojo
Windows PowerShell  | `[cls]::member`
F#  | N/A  | `cls.member`
eC  | `x.method«(parameters)»` | `x.field` | `x.property` | `cls::member` | `ns::member`
Eiffel  | `x.method«(parameters)»` | `x.field` |  | `{cls}.member` | N/A
Ruby  | 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`
Raku  | `x.method«(parameters)» or
x!method«(parameters)»` | `x.field or
x!field` |  | `cls.method«(parameters)» or
cls!method«(parameters)»` | `ns::member`
OCaml  | `x#method «parameters»` | N/A  |  |
JavaScript  | `x.method(parameters)
x["method"](parameters)` | `x.field
x["field"]` | `x.property
x["property"]` | `cls.member
cls["member"]` | N/A
COBOL  | `INVOKE x "method" «USING parameters» «RETURNING result» or
x::"method"«(«parameters»)»` | N/A  | `property OF x` | `INVOKE cls "method" «USING parameters» «RETURNING result» or
cls::"method"«(«parameters»)» or
property OF cls` | N/A
Cobra  | `x.method«(parameters)»` | `x.field` | `x.property` | `cls.member` | `ns.member`

## Member availability

| Has member?  | Handler for missing member

|
|

| Method  | Field  | Method  | Field
APL (Dyalog)  | `3=x.⎕NC'method'` | `2=x.⎕NC'method'` | N/A
ABAP Objects  | N/A
C++
Objective-C (Cocoa)  | `[x respondsToSelector:@selector(method)]` | N/A  | `forwardInvocation:` | N/A
Smalltalk  | `x respondsTo: selector` | N/A  | `doesNotUnderstand:` | N/A
C#  | (using reflection)

Java
D  |  |  | `opDispatch()`
Eiffel  | N/A
Python  | `hasattr(x, "method") and callable(x.method)` | `hasattr(x, "field")` | `__getattr__()`
Visual Basic .NET  | (using reflection)
Xojo  | (using Introspection)
Windows PowerShell  | (using reflection)
F#  | (using reflection)
Ruby  | `x.respond_to?(:method)` | N/A  | `method_missing()` | N/A
PHP  | `method_exists(x, "method")` | `property_exists(x, "field")` | `__call()` | `__get() / __set()`
Perl  | `x->can("method")` | `exists x->{field}` | AUTOLOAD  |
Raku  | `x.can("method")` | `x.field.defined` | AUTOLOAD  |
OCaml  | N/A
JavaScript  | `typeof x.method === "function"` | `field in x` |  |
COBOL  | N/A

## Special variables

| current object  | current object's parent object  | null reference | Current Context of Execution

|
|
|
|

Smalltalk  | `self` | `super` | `nil` | `thisContext`
ABAP Objects  | `me` | `super` | `initial` |
APL (Dyalog)  | `⎕THIS` | `⎕BASE` | `⎕NULL` |
C++  | `*this` | [42] | `NULL, nullptr` |
C#  | `this` | `base`[43] | `null` |
Java  | `super`[43] |
D  |
JavaScript  | `super`[43] (ECMAScript 6)  | `null, undefined`[44] |
eC  | `this` |  | `null` |
Objective-C  | `self` | `super`[43] | `nil` |
Swift  | `self` | `super`[43] | `nil`[45] |
Python  | `self`[46] | `super(current_class_name, self)`[7]
`super()` (3.x only)  | `None` |
Visual Basic .NET  | `Me` | `MyBase` | `Nothing` |
Xojo  | `Me / Self` | `Parent` | `Nil` |
Eiffel  | `Current` | `Precursor «{superclass}» «(args)»`[43][47] | `Void` |
PHP  | `$this` | `parent`[43] | `null` |
Perl  | `$self`[46] | `$self->SUPER`[43] | `undef` |
Raku  | `self` | `SUPER` | `Nil` |
Ruby  | `self` | `super«(args)»`[48] | `nil` | `binding`
Windows PowerShell  | `$this` |  | `$NULL` |
OCaml  | `self`[49] | `super`[50] | N/A[51] |
F#  | `this` | `base`[43] | `null` |
COBOL  | `SELF` | `SUPER` | `NULL` |
Cobra  | `this` | `base` | `nil` |

## Special methods

| String representation  | Object copy  | Value equality  | Object comparison  | Hash code  | Object ID

|
|
|
|
|
|

Human-readable  | Source-compatible
ABAP Objects  | N/A
APL (Dyalog)  | `⍕x` | `⎕SRC x` | `⎕NS x` | `x = y` | N/A
C++  |  |  |  | `x == y`[52] |  |  | pointer to object can be converted into an integer ID
C#  | `x.ToString()` |  | `x.Clone()` | `x.Equals(y)` | `x.CompareTo(y)` | `x.GetHashCode()` | `System.Runtime.CompilerServices.RuntimeHelpers.GetHashCode(x)`
Java  | `x.toString()` |  | `x.clone()`[53] | `x.equals(y)` | `x.compareTo(y)`[54] | `x.hashCode()` | `System.identityHashCode(x)`
JavaScript  | `x.toString()` |  |  |  |  |  |
D  | `x.toString() or
std.conv.to!string(x)` | `x.stringof` |  | `x == y or
x.opEquals(y)` | `x.opCmp(y)` | `x.toHash()` |
eC  | `x.OnGetString(tempString, null, null) or
PrintString(x)` |  | `y.OnCopy(x)` |  | `x.OnCompare(y)` |  | object handle can be converted into an integer ID
Objective-C (Cocoa)  | `x.description` | `x.debugDescription` | `[x copy]`[55] | `[x isEqual:y]` | `[x compare:y]`[56] | `x.hash` | pointer to object can be converted into an integer ID
Swift  | `x.description`[57] | `x.debugDescription`[58] |  | `x == y`[59] | `x < y`[60] | `x.hashValue`[61] | `reflect(x).objectIdentifier!.uintValue()`
Smalltalk  | `x displayString` | `x printString` | `x copy` | `x = y` |  | `x hash` | `x identityHash`
Python  | `str(x)`[62] | `repr(x)`[63] | `copy.copy(x)`[64] | `x == y`[65] | `cmp(x, y)`[66] | `hash(x)`[67] | `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 use `x.hash_code` | When x is `IDENTIFIED`, one can use `x.object_id`
PHP  | `$x->__toString()` |  | `clone x`[68] | `x == y` | `` |  | `spl_object_hash(x)`
Perl  | `"$x"`[69] | `Data::Dumper->Dump([$x],['x'])`[70] | `Storable::dclone($x)`[71] |  |  |  | `Scalar::Util::refaddr( $x )`[72]
Raku  | `~x`[69] | `x.perl` | `x.clone` | `x eqv y` | `x cmp y` |  | `x.WHICH`
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#  | `string x or x.ToString() or sprintf "%O" x` | `sprintf "%A" x` | `x.Clone()` | `x = y or x.Equals(y)` | `compare x y or x.CompareTo(y)` | `hash x or x.GetHashCode()` |
COBOL  | N/A

## Type manipulation

| Get object type  | Is instance of (includes subtypes)  | Upcasting  | Downcasting

|
|
|
|

Runtime check  | No check
ABAP Objects  | N/A[73] | =  | ?=
C++  | `typeid(x)` | `dynamic cast<type *>(&x) != nullptr` | N/A[74] | `dynamic cast<type*>(ptr)` | `(type*) ptr or
static_cast<type*>(ptr)`
C#  | `x.GetType()` | `x is type` | `(type) x or x as type` |
D  | `typeid(x)` |  | `cast(type) x` |
Delphi  |  | `x is type` | `x as type` |
eC  | `x._class` | `eClass_IsDerived(x._class, type)` |  | `(type) x`
Java  | `x.getClass()` | `x instanceof class` | `(type) x` |
Objective-C (Cocoa)  | `[x class]`[75] | `[x isKindOfClass:[class class]]` |  | `(type*) x`
Swift  | `x.dynamicType` | `x is type` | `x as! type`
`x as? type`
JavaScript  | `x.constructor (If not rewritten.)` | `x instanceof class` | N/A[76]
Visual Basic .NET  | `x.GetType()` | `TypeOf x Is type` | N/A[74] | `CType(x, type) or TryCast(x, type)` |
Xojo  | `Introspection.GetType(x)` | `x IsA type` | N/A  | `CType(x, type)` | N/A
Eiffel  | `x.generating_type` | `attached {TYPE} x` | `attached {TYPE} x as down_x` |
Python  | `type(x)` | `isinstance(x, type)` | N/A[76]
PHP  | `get_class(x)` | `x instanceof class`
Perl  | `ref(x)` | `x->isa("class")`
Raku  | `x.WHAT` | `x.isa(class)` | N/A[74] | `type(x) or
x.type` |
Ruby  | `x.class` | `x.instance_of?(type) or
x.kind_of?(type)` | N/A[76]
Smalltalk  | `x class` | `x isKindOf: class`
Windows PowerShell  | `x.GetType()` | `x -is [type]` | N/A[74] | `[type]x or x -as [type]` |
OCaml  | N/A[77] | `(x :> type)` | N/A
F#  | `x.GetType()` | `x :? type` | `(x :?> type)` |
COBOL  | N/A  | `x AS type`[74] | N/A

## Namespace management

| Import namespace  | Import item

|
|

qualified  | unqualified
ABAP Objects  |  |  |
C++  |  | `using namespace ns;` | `using ns::item ;`
C#  |  | `using ns;` | `using item = ns.item;`
D  |  | `import ns;` | `import ns : item;`
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;` |  | `use ns qw(item);`
Raku  |  |
Ruby  |  |  |
Windows PowerShell  |  |  |
OCaml  |  | `open ns` |
F#  |  |
COBOL  | N/A

## Contracts

| Precondition  | Postcondition  | Check  | Invariant  | Loop

|
|
|
|
|

ABAP Objects  | N/A
C++
C#  | `Spec#:
type foo( «parameters» )
requires expression

body
}` | `Spec#:
type foo( «parameters» )
ensures expression

body

Java  | N/A
Objective-C
Visual Basic .NET
D  | `f
in { asserts }
body{ instructions }` | `f
out (result) { asserts }
body{ instructions }` | `assert(expression)` | `invariant() { expression }` |
Eiffel  | `f
require tag: expression
do end` | `f

ensure tag: expression
end` | `f

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  | N/A
PHP
Perl
Raku  | `PRE { condition }` | `POST { condition }` |  |  |
Ruby  | N/A
Windows PowerShell
OCaml

COBOL

## See also

* Object-oriented programming

## 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. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 This language uses garbage collection to release unused memory.
4. ↑ This syntax creates an object value with automatic storage duration
5. ↑ This syntax creates an object with dynamic storage duration and returns a pointer to it
6. ↑ OCaml objects can be created directly without going through a class.
7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 This language supports multiple inheritance. A class can have more than one parent class
8. ↑ Not providing a parent class makes the class a root class. In practice, this is almost never done. One should generally use the conventional base class of the framework one is using, which is `NSObject` for Cocoa and GNUstep, or `Object` otherwise.
9. ↑ Usually the `@interface` portion is placed into a header file, and the `@interface` portion is placed into a separate source code file.
10. ↑ Prefixes to class and protocol names conventionally used as a kind of namespace
11. ↑ In Python interfaces are classes which methods have pass as their bodies
12. ↑ The class is an Object.
Just send a message to the superclass (st-80) or the destination namespace (Visualworks).
13. ↑ The namespace is an Object.
Just send a message to the parent namespace.
14. ↑ 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.
15. ↑ 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
16. ↑ 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.
17. ↑ Any Eiffel procedure can be used as a creation procedure, aka constructors. See Eiffel paragraph at Constructor (computer science).
18. ↑ Implementing {DISPOSABLE}.dispose ensures that dispose will be called when object is garbage collected.
19. ↑ 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.
20. ↑ This syntax is usually used to overload constructors
21. ↑ In JavaScript, constructor is an object.
22. ↑ Constructors can be emulated with a factory method returning a class instance.
23. ↑ 23.0 23.1 23.2 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
24. ↑ In ABAP, specific fields or methods are not declared as accessible by outside things. Rather, outside classes are declared as friends to have access to the class's fields or methods.
25. ↑ In C++, specific fields are not declared as accessible by outside things. Rather, outside functions and classes are declared as friends to have access to the class's fields. See friend function and friend class for more details.
26. ↑ Just send a message to the class

class addInstVarName: field.
class removeInstVarName: field.

27. ↑ 27.0 27.1 27.2 27.3 Just assign a value to it in a method
28. ↑ Python doesn't have private fields - all fields are publicly accessible at all times. A community convention exists to prefix implementation details with one underscore, but this is unenforced by the language.
29. ↑ All class data is 'private' because the COBOL standard does not specify any way to access it.
30. ↑ 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.
31. ↑ In ABAP, the return parameter name is explicitly defined in the method signature within the class definition
32. ↑ In C++, declaring and implementing methods is usually separate. Methods are declared in the class definition (which is usually included in a header file) using the syntax
33. ↑ Although the body of a method can be included with the declaration inside the class definition, as shown in the table here, this is generally bad practice. Because the class definition must be included with every source file which uses the fields or methods of the class, having code in the class definition causes the method code to be compiled with every source file, increasing the size of the code. Yet, in some circumstances, 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 a very short one-line method occurs, it may make it faster to allow a compiler to inline it, by including the body along with the declaration. Also, if a template class or method occurs, then all the code must be included with the declaration, because only with the code can the template be instantiated.
34. ↑ 34.0 34.1 Just assign a function to it in a method
35. ↑ Alternative implementation:

def bar():
doc = "The bar property."
def fget(self):
return self._bar
def fset(self, value):
self._bar = value
return locals()
bar = property(**bar())

36. ↑ these examples need the Class::Accessor module installed
37. ↑ 37.0 37.1 37.2 Although Eiffel does not support overloading of operators, it can define operators
38. ↑ PHP does not support operator overloading natively, but support can be added using the "operator" PECL package.
39. ↑ The class must implement the ArrayAccess interface.
40. ↑ The class must overload '@{}' (array dereference) or subclass one of Tie::Array or Tie::StdArray to hook array operations
41. ↑ 41.0 41.1 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
42. ↑ C++ doesn't have a "super" keyword, because multiple inheritance is possible, and so it may be ambiguous which base class is referenced. Instead, the `BaseClassName::member` syntax can be used 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 unsupported in other compilers.[1]
43. ↑ 43.0 43.1 43.2 43.3 43.4 43.5 43.6 43.7 43.8 The keyword here is not a value, and it can only be used to access a method of the superclass.
44. ↑ But be afraid, they have not the same value.
45. ↑ only for Optional types
46. ↑ 46.0 46.1 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.
47. ↑ "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.
48. ↑ "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.
49. ↑ 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.
50. ↑ 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 given to the variable associated with this parent object. It can be named differently.
51. ↑ However, if the ability to have an "optional" value in OCaml is needed, then wrap the value inside an `option` type, which 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.
52. ↑ assuming that "x" and "y" are the objects (and not pointers). Can be customized by overloading the object's `==` operator
53. ↑ Only accessible from within the class, since the `clone()` method inherited from `Object` is protected, unless the class overrides the method and makes it public. If using the `clone()` inherited from `Object`, the class must implement the `Cloneable` interface to allow cloning.
54. ↑ The class should implement the interface `Comparable` for this method to be standardized.
55. ↑ Implemented by the object's `copyWithZone:` method
56. ↑ `compare:` is the conventional name for the comparison method in Foundation classes. However, no formal protocol exists
57. ↑ Only if object conforms to the `Printable` protocol
58. ↑ Only if object conforms to the `DebugPrintable` protocol
59. ↑ Only if object conforms to the `Equatable` protocol
60. ↑ Only if object conforms to the `Comparable` protocol
61. ↑ Only if object conforms to the `hashValue` protocol
62. ↑ Can be customized by the object's `__str__()` method
63. ↑ Can be customized by the object's `__repr__()` method
64. ↑ Can be customized by the object's `__copy__()` method
65. ↑ Can be customized by the object's `__eq__()` method
66. ↑ Only in Python 2.x and before (removed in Python 3.0). Can be customized by the object's `__cmp__()` method
67. ↑ Can be customized by the object's `__hash__()` method. Not all types are hashable (mutable types are usually not hashable)
68. ↑ Can be customized by the object's `__clone()` method
69. ↑ 69.0 69.1 Can be customized by overloading the object's string conversion operator
70. ↑ This example requires useing Data::Dumper
71. ↑ This example requires useing Storable
72. ↑ This example requires useing Scalar::Util
73. ↑ Run-time type information in ABAP can be gathered by using different description Classes like CL_ABAP_CLASSDESCR.
74. ↑ 74.0 74.1 74.2 74.3 74.4 Upcasting is implicit in this language. A subtype instance can be used where a supertype is needed.
75. ↑ Only for non-class objects. If `x` is a class object, `[x class]` returns only `x`. The runtime method `object_getClass(x)` will return the class of `x` for all objects.
76. ↑ 76.0 76.1 76.2 This language is dynamically typed. Casting between types is unneeded.
77. ↑ This language doesn't give run-time type information. It is unneeded because it is statically typed and downcasting is impossible.

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