Property (programming)
A property, in some object-oriented programming languages, is a special sort of class member, intermediate between a field (or data member) and a method. Properties are read and written like fields, but property reads and writes are (usually) translated to get and set method calls. The field-like syntax is said to be easier to read and write than lots of method calls, yet the interposition of method calls allows for data validation, active updating (as of GUI visuals), and/or read-only 'fields'. That is, properties are intermediate between member code (methods) and member data (instance variables) of the class, and properties provide a higher level of encapsulation than public fields.
Support in languages
Programming languages that support properties include Delphi/Free Pascal, Visual Basic, C#, D, eC, F#, Objective-C 2.0, ActionScript 3, JavaScript, Python and Vala. Some object-oriented languages, such as Java, 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 write-only property, the latter being rather uncommon.
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.
Example syntax
Delphi/Free Pascal
type TPen = class
private
m_Color: Integer;
function Get_Color: Integer;
procedure Set_Color(RHS: Integer);
public
property Color: Integer read Get_Color write Set_Color;
end;
function TPen.Get_Color: Integer;
begin
Result := m_Color
end;
procedure TPen.Set_Color(RHS: Integer);
begin
m_Color := RHS
end;
// accessing:
var pen: TPen;
// ...
pen.Color := not pen.Color;
(*
Delphi also supports a 'direct field' syntax -
property Color: Integer read m_Color write Set_Color;
or
property Color: Integer read Get_Color write m_Color;
where the compiler generates the exact same code as for reading and writing
a 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 member
access with a method call.)
*)
Visual Basic 6
' in a class named clsPen
Private m_Color As Long
Public Property Get Color() As Long
Color = m_Color
End Property
Public Property Let Color(ByVal RHS As Long)
m_Color = RHS
End Property
' accessing:
Dim pen As New clsPen
' ...
pen.Color = Not pen.Color
Visual Basic (.NET to 2010)
Public Class Pen
Private m_Color As Integer ' Private field
Public Property Color As Integer ' Public property
Get
Return m_Color
End Get
Set(ByVal Value As Integer)
m_Color = Value
End Set
End Property
End Class
' accessing:
Dim pen As New Pen()
' ...
pen.Color = Not pen.Color
C#
class Pen
{
private int m_Color; // private field
public int Color // public property
{
get
{
return m_Color;
}
set
{
m_Color = value;
}
}
}
// accessing:
Pen pen = new Pen();
// ...
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.
class Shape {
public Int32 Height { get; set; }
public Int32 Width { get; private set; }
}
C++
C++ does not have first class properties, but there exist several ways to emulate properties to a limited degree. Two of which follow:
#include <iostream>
template <typename T> class property {
T value;
public:
T & operator = (const T &i) {
::std::cout << i << ::std::endl;
return value = 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.
template <typename T2> T2 & operator = (const T2 &i) {
::std::cout << "T2: " << i << ::std::endl;
T2 &guard = value;
throw guard; // Never reached.
}
operator T const & () const {
return value;
}
};
struct Foo {
// Properties using unnamed classes.
class {
int value;
public:
int & operator = (const int &i) { return value = i; }
operator int () const { return value; }
} alpha;
class {
float value;
public:
float & operator = (const float &f) { return value = f; }
operator float () const { return value; }
} bravo;
};
struct Bar {
// Using the property<>-template.
property <bool> alpha;
property <unsigned int> bravo;
};
int main () {
Foo foo;
foo.alpha = 5;
foo.bravo = 5.132f;
Bar bar;
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;
return 0;
}
C++, Microsoft & C++Builder specific
An example taken from the MSDN documentation page.
// declspec_property.cpp
struct S
{
int i;
void putprop(int j)
{
i = j;
}
int getprop()
{
return i;
}
__declspec(property(get = getprop, put = putprop)) int the_prop;
};
int main()
{
S s;
s.the_prop = 5;
return s.the_prop;
}
Dev D
class Pen
{
private int m_color; // private field
// public get property
public int color () {
return m_color;
}
// public set property
public int color (int value) {
return m_color = value;
}
}
auto pen = new Pen;
pen.color = ~pen.color; // bitwise complement
// the set property can also be used in expressions, just like regular assignment
int theColor = (pen.color = 0xFF0000);
JavaScript
function Pen() {
var m_color; // Private
this.__defineGetter__("color", function(){
return m_color;
});
this.__defineSetter__("color", function(value) {
m_color = value;
});
}
var pen = new Pen();
pen.Color = ~pen.Color; // bitwise complement
pen.Color += 1; // Add one
Python
Properties only work correctly for new-style classes (classes which have object as an ancestor), 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; this only works on properties that are readable.
class Pen(object):
def __init__(self):
self.__color = 0 # "private" variable
self.__writeonly = "You can't read this!"
@property
def color(self):
return self.__color
@color.setter
def color(self, color):
self.__color = color
def _set_writeonly(self, new_value):
self.__writeonly = new_value
writeonly = property(fset = _set_writeonly) # write-only access is provided (reading throws an exception)''
pen = Pen()
# accessing:
pen.color = ~pen.color # bitwise complement ...
print pen.writeonly # raise "AttributeError: unreadable attribute"
pen.writeonly = "Something Else" # <code>__writeonly</code> is now "Something Else"
print pen._Pen__writeonly
PHP
class Pen {
private $_color;
function __set($property, $value) {
if ($property == 'Color') {
return $this->_color = $value;
}
}
function __get($property) {
if ($property == 'Color') {
return $this->_color;
}
}
}
$p = new Pen();
$p->Color = ~$p->Color; // bitwise complement
echo $p->Color;
F#
type Pen() = class
let mutable _color = 0
member this.Color
with get() = _color
and set value = _color <- value
end
let pen = new Pen()
pen.Color <- ~~~pen.Color
Objective-C 2.0
@interface Pen : NSObject {
NSColor *color;
}
@property(copy) NSColor *color; // color values always copied.
@end
@implementation Pen
@synthesize color; // synthesize accessor methods.
@end
// Example Usage
Pen *pen = [[Pen alloc] init];
pen.color = [NSColor blackColor];
float red = pen.color.redComponent;
[pen.color drawSwatchInRect:NSMakeRect(0, 0, 100, 100)];
Note that the modern Objective-C runtime can synthesize instance variables for properties; hence, explicit declaration of instance variables is not necessary, but still possible.