Template:Short description In software engineering, the delegation pattern is an object-oriented design pattern that allows object composition to achieve the same code reuse as inheritance.
In delegation, an object handles a request by delegating to a second object (the delegate). The delegate is a helper object, but with the original context. With language-level support for delegation, this is done implicitly by having self in the delegate refer to the original (sending) object, not the delegate (receiving object). In the delegate pattern, this is instead accomplished by explicitly passing the original object to the delegate, as an argument to a method.<ref>Gamma et al. 1994</ref> "Delegation" is often used loosely to refer to the distinct concept of forwarding, where the sending object simply uses the corresponding member on the receiving object, evaluated in the context of the receiving object, not the original object.
This article uses "sending object/receiving object" for the two objects, rather than "receiving object/delegate", emphasizing which objects send and receive the delegation call, not the original call.
DefinitionEdit
In the Introduction to Gamma et al. 1994, delegation is defined as:
ExampleEdit
In the example below (using the Kotlin programming language), the class Window delegates the area() call to its internal Rectangle object (its delegate).
<syntaxhighlight lang="kotlin">class Rectangle(val width: Int, val height: Int) {
fun area() = width * height
}
class Window(val bounds: Rectangle) {
// Delegation fun area() = bounds.area()
}</syntaxhighlight>
Language supportEdit
Some languages have special support for delegation built in. For example, in the Kotlin programming language the by keyword<ref>{{#invoke:citation/CS1|citation
|CitationClass=web
}}</ref> delegates to another object's interface:
<syntaxhighlight lang="kotlin">interface ClosedShape {
fun area(): Int
}
class Rectangle(val width: Int, val height: Int) : ClosedShape {
override fun area() = width * height
}
// The ClosedShape implementation of Window delegates to that of the Rectangle that is bounds class Window(private val bounds: Rectangle) : ClosedShape by bounds</syntaxhighlight>
See alsoEdit
- Delegation (object-oriented programming)
- Forwarding (object-oriented programming)
- Aspect-oriented programming
- Delegation (computing)
- Design pattern
- Facade pattern
- Schizophrenia (object-oriented programming)
ReferencesEdit
External linksEdit
- What Is Delegation, WikiWikiWeb
- Delegation on Rosetta Code