Propriété (informatique)

Pour les articles homonymes, voir Propriété (homonymie).

En programmation orientée objet ou en RDF, une propriété est un élément de description d'un objet. Un type de classe regroupe, entre autres, l'ensemble des propriétés communes à un type d'objet.

Une propriété, dans certains langages de programmation orientés objet, est une sorte de membre de classe. Il est un champ du point de vue de l'interface d'une classe mais il est développé comme une méthode qui va lire et modifier le contenu de l'instance. Son usage courant est de pouvoir rajouter des instructions au moment de la modification ou de la lecture d'une instance d'une classe sans pour autant modifier l'interface de cette classe.

Exemple de syntaxe

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++ n'a pas de propriétés de classe, mais il existe plusieurs manières d'imiter les propriétés jusqu'à une certaine limite. Voici deux exemple :

#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

Un exemple tiré de la 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;
}

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);

Dans D version 2, chaque accesseur de propriété doit être marqué avec @property:

class Pen
{
    private int m_color; // private field
 
    // public get property
    @property public int color () {
        return m_color;
    }
 
    // public set property
    @property public int color (int value) {
        return m_color = value;
    }
}

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.)
*)

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

JavaScript

function 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

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)];

Notez que le moderne Objective-C runtime peut convertir les variables d'instance en propriétés, d'où la déclaration explicite des variables d'instance qui n'est pas nécessaire, mais toujours possible.

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;

Python

Les propriétés ne fonctionne correctement que pour les nouveaux types de classe qui sont uniquement disponibles en Python 2.2 ou plus récents. Python 2.6 ajoute une nouvelle syntaxe pour définir les propriétés.

class Pen(object):
    def __init__(self):
        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 ...

Ruby

class Pen
    def initialize
        @color = 0
    end
    # there is actually a shortcut for these: attr_accessor :color will
    # synthetise both methods automatically, it was expanded for
    # compatibility with properties actually worth writing out
    def color
        @color
    end
    def color = value
        @color = value
    end
end
 
pen = Pen.new
pen.color = ~pen.color

Visual Basic

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

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

Voir aussi

• Interface (informatique)