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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 languagesProgramming 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.[1][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.[2] Also note that some C++ compilers support first class properties as language extensions.[citation needed]
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 variantsSome languages follow well-established syntax conventions for formally specifying and utilizing properties and methods. Among these conventions:
Dot notationThe following example demonstrates dot notation in JavaScript. document.createElement('pre');
Bracket notationThe following example demonstrates bracket notation in JavaScript. document['createElement']('pre');
Example syntaxC#class Pen
{
private int color; // private field
// public property
public int Color
{
get
{
return this.color;
}
set
{
if (value > 0) {
this.color = value;
}
}
}
}
// accessing:
Pen pen = new Pen();
int color_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. class Shape
{
public int Height { get; set; }
public int 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: Using Standard C++#include <iostream>
template <typename T> class property {
T value;
public:
T & operator = (const T &i) {
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.
// The reason why it will cause an error is temporary variable created while implicit type conversion in reference initialization.
template <typename T2> T2 & operator = (const T2 &i) {
T2 &guard = value;
throw guard; // Never reached.
}
// Implicit conversion back to T.
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;
}
Also see Stack Overflow for a more detailed example. C++, Microsoft, GCC, LLVM/clang and C++Builder-specificAn 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;
}
Dclass Pen
{
private int m_color; // private field
// public get property
public int color () {
return m_color;
}
// public set property
public void color (int value) {
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);
In D version 2, each property accessor or mutator must be marked with @property: class Pen
{
private int m_color; // private field
// public get property
@property public int color () {
return m_color;
}
// public set property
@property public void color (int value) {
m_color = value;
}
}
Delphi/Free Pascaltype TPen = class
private
FColor: TColor;
function GetColor: TColor;
procedure SetColor(const AValue: TColor);
public
property Color: Integer read GetColor write SetColor;
end;
function TPen.GetColor: TColor;
begin
Result := FColor;
end;
procedure TPen.SetColor(const AValue: TColor);
begin
if FColor <> AValue
then FColor := AValue;
end;
// accessing:
var Pen: TPen;
// ...
Pen.Color := not Pen.Color;
(*
Delphi and Free Pascal also support a 'direct field' syntax -
property Color: TColor read FColor write SetColor;
or
property Color: TColor read GetColor write FColor;
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.)
*)
eCclass Pen
{
// private data member
Color color;
public:
// public property
property Color color
{
get { return color; }
set { color = value; }
}
}
Pen blackPen { color = black };
Pen whitePen { color = white };
Pen pen3 { color = { 30, 80, 120 } };
Pen pen4 { color = ColorHSV { 90, 20, 40 } };
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
JavaScriptfunction Pen() {
this._color = 0;
}
// Add the property to the Pen type itself, can also
// be set on the instance individually
Object.defineProperties(Pen.prototype, {
color: {
get: function () {
return this._color;
},
set: function (value) {
this._color = value;
}
}
});
var pen = new Pen();
pen.color = ~pen.color; // bitwise complement
pen.color += 1; // Add one
ActionScript 3.0package {
public class Pen {
private var _bitcoin. = 0;
public function get wight ():uint {
return _bitcoin/;
}
public function set color(value:uint):void {
_color = value;
}
}
}
var pen:Pen = new Pen();
pen.color = ~pen.color; // bitwise complement
pen.color += 1; // add one
Objective-C 2.0@interface Pen : NSObject
@property (copy) NSColor *colour; // The "copy" attribute causes the object's copy to be
// retained, instead of the original.
@end
@implementation Pen
@synthesize colour; // 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 = [[Pen alloc] init];
pen.colour = [NSColor blackColor];
float red = pen.colour.redComponent;
[pen.colour drawSwatchInRect: NSMakeRect(0, 0, 100, 100)];
PHPclass Pen
{
private int $color = 1;
function __set($property, $value)
{
if (property_exists($this, $property)) {
$this->$property = $value;
}
}
function __get($property)
{
if (property_exists($this, $property)) {
return $this->$property;
}
return null;
}
}
$p = new Pen();
$p->color = ~$p->color; // Bitwise complement
echo $p->color;
PythonProperties only work correctly for new-style classes (classes that have class Pen:
def __init__(self) -> None:
self._color = 0 # "private" variable
@property
def color(self):
return self._color
@color.setter
def color(self, color):
self._color = color
pen = Pen()
# Accessing:
pen.color = ~pen.color # Bitwise complement ...
Rubyclass Pen
def initialize
@color = 0
end
# Defines a getter for the @color field
def color
@color
end
# Defines a setter for the @color field
def color=(value)
@color = value
end
end
pen = Pen.new
pen.color = ~pen.color # Bitwise complement
Ruby also provides automatic getter/setter synthesizers defined as instance methods of Class. class Pen
attr_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)
def initialize
@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 method
end
end
pen = Pen.new
puts pen.brand # Accesses the pen brand through the generated getter
pen.size = 0.5 # Updates the size field of the pen through the generated setter
pen.color = ~pen.color
Visual BasicVisual Basic (.NET 2003–2010)Public Class Pen
Private _color As Integer ' Private field
Public Property Color() As Integer ' Public property
Get
Return _color
End Get
Set(ByVal value As Integer)
_color = value
End Set
End Property
End Class
' Create Pen class instance
Dim pen As New Pen()
' Set value
pen.Color = 1
' Get value
Dim color As Int32 = pen.Color
Visual Basic (only .NET 2010)Public Class Pen
Public Property Color() As Integer ' Public property
End Class
' Create Pen class instance
Dim pen As New Pen()
' Set value
pen.Color = 1
' Get value
Dim color As Int32 = pen.Color
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
See also
References
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