Property (programming)

A property, in some object-orientedprogramming languages, is a special sort of class member, intermediate in functionality between a field (or data member) and a method. The syntax for reading and writing of properties is like for fields, but property reads and writes are (usually) translated to 'getter' and 'setter' method calls. The field-like syntax is easier to read and write than many method calls, yet the interposition of method calls "under the hood" allows for data validation, active updating (e.g., of GUI elements), or implementation of what may be called "read-only fields".

Support in languages

Programming languages that support properties include ActionScript 3, C#, D, Delphi/Free Pascal, eC, F#, Kotlin, JavaScript, Objective-C 2.0, Python, Scala, Swift, Lua, and Visual Basic.

Some object-oriented languages, such as Java and C++, do not support properties, requiring the programmer to define a pair of accessor and mutator methods instead.

Oberon-2 provides an alternative mechanism using object variable visibility flags.

Other languages designed for the Java Virtual Machine, such as Groovy, natively support properties.

While C++ does not have first class properties, they can be emulated with operator overloading.

Also note that some C++ compilers support first class properties as language extensions.

• In Microsoft Visual Studio,GCC, and llvm/clang, the __declspec(property) creates properties similar to C#.
• Borland C++ and Borland/CodeGear/Embarcadero C++Builder use the __property keyword.

In many object oriented languages properties are implemented as a pair of accessor/mutator methods, but accessed using the same syntax as for public fields. Omitting a method from the pair yields a read-only or an uncommon write-only property.

In some languages with no built-in support for properties, a similar construct can be implemented as a single method that either returns or changes the underlying data, depending on the context of its invocation. Such techniques are used e.g. in Perl.

Some languages (Ruby, Smalltalk) achieve property-like syntax using normal methods, sometimes with a limited amount of syntactic sugar.

Syntax variants

Some languages follow well-established syntax conventions for formally specifying and utilizing properties and methods.

Among these conventions:

• Dot notation
• Bracket notation

Dot notation

The following example demonstrates dot notation in JavaScript.

document.createElement('pre');

Bracket notation

The following example demonstrates bracket notation in JavaScript.

document['createElement']('pre');

Example syntax

C#

classPen{privateintcolor;// private field// public propertypublicintColor{get{returnthis.color;}set{if(value>0){this.color=value;}}}}

// accessing:Penpen=newPen();intcolor_tmp=0;// ...pen.Color=17;color_tmp=pen.Color;// ...pen.Color=~pen.Color;// bitwise complement ...// another silly example:pen.Color+=1;// a lot clearer than "pen.set_Color(pen.get_Color() + 1)"!

Recent C# versions also allow "auto-implemented properties" where the backing field for the property is generated by the compiler during compilation. This means that the property must have a setter. However, it can be private.

classShape{publicintHeight{get;set;}publicintWidth{get;privateset;}}

C++

C++ does not have first class properties, but there exist several ways to emulate properties to a limited degree. Two of which follow:

Using Standard C++

#include<iostream>template<typenameT>classproperty{Tvalue;public:T&operator=(constT&i){returnvalue=i;}// This template class member function template serves the purpose to make// typing more strict. Assignment to this is only possible with exact identical types.// The reason why it will cause an error is temporary variable created while implicit type conversion in reference initialization.template<typenameT2>T2&operator=(constT2&i){T2&guard=value;throwguard;// Never reached.}// Implicit conversion back to T. operatorTconst&()const{returnvalue;}};structFoo{// Properties using unnamed classes.class{intvalue;public:int&operator=(constint&i){returnvalue=i;}operatorint()const{returnvalue;}}alpha;class{floatvalue;public:float&operator=(constfloat&f){returnvalue=f;}operatorfloat()const{returnvalue;}}bravo;};structBar{// Using the property<>-template.property<bool>alpha;property<unsignedint>bravo;};intmain(){Foofoo;foo.alpha=5;foo.bravo=5.132f;Barbar;bar.alpha=true;bar.bravo=true;// This line will yield a compile time error// due to the guard template member function.::std::cout<<foo.alpha<<", "<<foo.bravo<<", "<<bar.alpha<<", "<<bar.bravo<<::std::endl;return0;}

Also see Stack Overflow for a more detailed example.

C++, Microsoft, GCC, LLVM/clang and C++Builder-specific

An example taken from the MSDN documentation page.

// declspec_property.cppstructS{inti;voidputprop(intj){i=j;}intgetprop(){returni;}__declspec(property(get=getprop,put=putprop))intthe_prop;};intmain(){Ss;s.the_prop=5;returns.the_prop;}

D

classPen{privateintm_color;// private field// public get propertypublicintcolor(){returnm_color;}// public set propertypublicvoidcolor(intvalue){m_color=value;}}

autopen=newPen;pen.color=~pen.color;// bitwise complement// the set property can also be used in expressions, just like regular assignmentinttheColor=(pen.color=0xFF0000);

In D version 2, each property accessor or mutator must be marked with @property:

classPen{privateintm_color;// private field// public get property@propertypublicintcolor(){returnm_color;}// public set property@propertypublicvoidcolor(intvalue){m_color=value;}}

Delphi/Free Pascal

typeTPen=classprivateFColor:TColor;functionGetColor:TColor;procedureSetColor(constAValue:TColor);publicpropertyColor:IntegerreadGetColorwriteSetColor;end;functionTPen.GetColor:TColor;beginResult:=FColor;end;procedureTPen.SetColor(constAValue:TColor);beginifFColor<>AValuethenFColor:=AValue;end;

// accessing:varPen:TPen;// ...Pen.Color:=notPen.Color;(*Delphi and Free Pascal also support a 'direct field' syntax -property Color: TColor read FColor write SetColor;orproperty Color: TColor read GetColor write FColor;where the compiler generates the exact same code as for reading and writinga field. This offers the efficiency of a field, with the safety of a property.(You can't get a pointer to the property, and you can always replace the memberaccess with a method call.)*)

eC

classPen{// private data memberColorcolor;public:// public propertypropertyColorcolor{get{returncolor;}set{color=value;}}}PenblackPen{color=black};PenwhitePen{color=white};Penpen3{color={30,80,120}};Penpen4{color=ColorHSV{90,20,40}};

F#

typePen()=classletmutable_color=0memberthis.Colorwithget()=_colorandsetvalue=_color<-valueend

letpen=newPen()pen.Color<-~~~pen.Color

JavaScript

functionPen(){this._color=0;}// Add the property to the Pen type itself, can also// be set on the instance individuallyObject.defineProperties(Pen.prototype,{color:{get:function(){returnthis._color;},set:function(value){this._color=value;}}});

varpen=newPen();pen.color=~pen.color;// bitwise complementpen.color+=1;// Add one

ActionScript 3.0

package{publicclassPen{privatevar_color:uint=0;publicfunctiongetcolor():uint{return_color;}publicfunctionsetcolor(value:uint):void{_color=value;}}}

varpen:Pen=newPen();pen.color=~pen.color;// bitwise complementpen.color+=1;// add one

Objective-C 2.0

@interfacePen : NSObject@property(copy)NSColor*colour;// The "copy" attribute causes the object's copy to be// retained, instead of the original.@end@implementationPen@synthesizecolour;// Compiler directive to synthesise accessor methods.// It can be left behind in Xcode 4.5 and later.@end

The above example could be used in an arbitrary method like this:

Pen*pen=[[Penalloc]init];pen.colour=[NSColorblackColor];floatred=pen.colour.redComponent;[pen.colourdrawSwatchInRect:NSMakeRect(0,0,100,100)];

PHP

classPen{privateint$color=1;function__set($property,$value){if(property_exists($this,$property)){$this->$property=$value;}}function__get($property){if(property_exists($this,$property)){return$this->$property;}returnnull;}}

$p=newPen();$p->color=~$p->color;// Bitwise complementecho$p->color;

Python

Properties only work correctly for new-style classes (classes that have object as a superclass), and are only available in Python 2.2 and newer (see the relevant section of the tutorial Unifying types and classes in Python 2.2). Python 2.6 added a new syntax involving decorators for defining properties.

classPen:def__init__(self)->None:self._color=0# "private" variable@propertydefcolor(self):returnself._color@color.setterdefcolor(self,color):self._color=color

pen=Pen()# Accessing:pen.color=~pen.color# Bitwise complement ...

Ruby

classPendefinitialize@color=0end# Defines a getter for the @color fielddefcolor@colorend# Defines a setter for the @color fielddefcolor=(value)@color=valueendendpen=Pen.newpen.color=~pen.color# Bitwise complement

Ruby also provides automatic getter/setter synthesizers defined as instance methods of Class.

classPenattr_reader:brand# Generates a getter for @brand (Read-Only)attr_writer:size# Generates a setter for @size (Write-Only)attr_accessor:color# Generates both a getter and setter for @color (Read/Write)definitialize@color=0# Within the object, we can access the instance variable directly@brand="Penbrand"@size=0.7# But we could also use the setter method defined by the attr_accessor Class instance methodendendpen=Pen.newputspen.brand# Accesses the pen brand through the generated getterpen.size=0.5# Updates the size field of the pen through the generated setterpen.color=~pen.color

Visual Basic

Visual Basic (.NET 2003–2010)

PublicClassPenPrivate_colorAsInteger' Private fieldPublicPropertyColor()AsInteger' Public propertyGetReturn_colorEndGetSet(ByValvalueAsInteger)_color=valueEndSetEndPropertyEndClass

' Create Pen class instanceDimpenAsNewPen()' Set valuepen.Color=1' Get valueDimcolorAsInt32=pen.Color

Visual Basic (only .NET 2010)

PublicClassPenPublicPropertyColor()AsInteger' Public propertyEndClass

' Create Pen class instanceDimpenAsNewPen()' Set valuepen.Color=1' Get valueDimcolorAsInt32=pen.Color

Visual Basic 6

' in a class named clsPenPrivatem_ColorAsLongPublicPropertyGetColor()AsLongColor=m_ColorEndPropertyPublicPropertyLetColor(ByValRHSAsLong)m_Color=RHSEndProperty

' accessing:DimpenAsNewclsPen' ...pen.Color=Notpen.Color

See also

• Attribute (computing)
• Bound property
• Field (computer science)
• Indexer (programming)
• Method (computer programming)
• Mutator method
• Uniform access principle