Property (programming)

A property, in some object-oriented programming 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,[citation needed] 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.[citation needed]

Oberon-2 provides an alternative mechanism using object variable visibility flags.[citation needed]

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.[citation needed]

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. [citation needed]

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.

Bracket notation

The following example demonstrates bracket notation in JavaScript.

Example syntax

C#

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.

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++

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

An example adapted from the MSDN .

D

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

Delphi/Free Pascal

eC

F#

JavaScript

ActionScript 3.0

Objective-C 2.0

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

PHP

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 ). Python 2.6 added a new syntax involving decorators for defining properties.

Ruby

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