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, 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   creates properties similar to C#.
 * Borland C++ and Borland/CodeGear/Embarcadero C++Builder use the   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.

Bracket notation
The following example demonstrates bracket notation in JavaScript.

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++
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.

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

Objective-C 2.0
The above example could be used in an arbitrary method like this:

Python
Properties only work correctly for new-style classes (classes that have  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.

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