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 lots of 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".
Contents
See an instructive example for C# language below.
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++, don't support properties, and require 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, do natively support properties. While C++ doesn't have first class properties, they can be emulated due to operator overloading. Also note that some C++ compilers support first class properties (the Microsoft C++ compiler as an example).
In most 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.
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:
C++, Microsoft & 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 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.
Ruby
Ruby also provides automatic getter/setter synthesizers defined as instance methods of Class.