software engineering Software engineering is a systematic engineering approach to software development. A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate computer software. The term '' ...

, the delegation pattern is an

object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of pro ...

design pattern A design pattern is the re-usable form of a solution to a design problem. The idea was introduced by the architect Christopher Alexander and has been adapted for various other disciplines, particularly software engineering. The " Gang of Four" b ...

that allows

object composition In computer science, object composition and object aggregation are closely related ways to combine objects or data types into more complex ones. In conversation the distinction between composition and aggregation is often ignored. Common kind ...

to achieve the same

code reuse In software development (and computer programming in general), code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software, following the reusability principles. Code reuse may be achiev ...

inheritance Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...

. In delegation, an

object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...

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 


The self is an individual as the object of that individual’s own reflective consciousness. Since the ''self'' is a reference by a subject to the same subject, this reference is necessarily  subjective. The sense of having a self—or ''selfhood ...

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. Note that "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. Note that 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.

Definition

In the Introduction to Gamma et al. 1994, Grady Booch defined delegation as:

Example

In the example below (using the Kotlin programming language), the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

Window delegates the area() call to its internal Rectangle object (its delegate). class Rectangle(val width: Int, val height: Int) class Window(val bounds: Rectangle)

Language support

Some languages have special support for delegation built in. For example, in the Kotlin programming language, we could write: interface ClosedShape class Rectangle(val width: Int, val height: Int) : ClosedShape class Window(private val bounds: ClosedShape) : ClosedShape by bounds

References

External links

What Is Delegation
WikiWikiWeb
Delegation
on

Rosetta Code Rosetta Code is a wiki-based programming website with implementations of common algorithms and solutions to various programming problems in many different programming languages. It is named for the Rosetta Stone, which has the same text inscribe ...

{{DEFAULTSORT:Delegation Pattern Articles with example C++ code Articles with example Java code Software design patterns

Definition

Example

Language support

See also

References

External links