Definitions

color property

Property (programming)

In some object-oriented programming languages, a property is a special sort of class member, intermediate between a field (or data member) and a method. You read and write a property just as you read and write a field, but this is (usually) translated to get and set method calls. The field-like syntax is 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, Objective C 2.0, Python and Vala. Some object-oriented languages, such as C++ and Java, don't support properties, and require the programmer to define a pair of accessor and mutator methods instead.

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

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)"!

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);

Python

Properties only work correctly for new-style classes (classes which has object as an ancestor), and are only available in Python 2.2 and newer (see the relevant secion of the tutorial Unifying types and classes in Python 2.2).

class Pen(object):

   def __init__(self):
       self.__color = 0 # "private" variable
       self.__writeonly = "You can't read this!"
   def _set_color(self, color):
       self.__color = color
   def _get_color(self):
       return self.__color
   color = property(_get_color, _set_color) # read/write access translates to get/set methods
   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()

  1. accessing:

pen.color = ~pen.color # bitwise complement ...

print pen.writeonly # raise "AttributeError: unreadable attribute" pen.writeonly = "Something Else" # __writeonly is now "Something Else" print pen._Pen__writeonly

PHP

class Pen {
   private $_color;

   function __set($property, $value) {
       switch ($property) {
           case 'Color': $this->_color = $value; break;
} }

   function __get($property) {
       switch ($property) {
           case 'Color': return $this->_color; break;
} } } $p = new Pen(); $p->Color = !$p->Color; echo $p->Color;

eC

class Pen
{
   Color color;
   public property Color color
   {
get { return color; } set { color = value; } }
}
// Example Usage
Pen pen { red }; Pen pen { color = red };
pen.color = ~pen.color;
pen.color += 10;
pen.color.r = 255;
pen.color = 0xFF0000;
pen.color = { 255, 0, 0 }; pen.color = ColorHSV { 0, 100, 100 }; pen.color = ColorLab { 53, 79, 66 }; pen.color = ColorCMYK { 0, 100, 100, 0 };

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 new]; pen.color = [NSColor blackColor]; float red = pen.color.redComponent; [pen.color drawSwatchInRect:NSMakeRect(0, 0, 100, 100)];

See also

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