Delegation pattern

In software engineering, the delegation pattern is a design pattern in object-oriented programming that allows object composition to achieve the same code reuse as inheritance.

In the example below (using the Kotlin programming language), the class Window delegates the area() call to its internal Rectangle object (its delegate).

class Rectangle(val width: Int, val height: Int) {
    fun area() = width * height
}

class Window(val bounds: Rectangle) {
    // Delegation
    fun area() = bounds.area()
}

Some languages have special support for delegation built in.

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.[1] 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.

Definition

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

Delegation is a way to make composition as powerful for reuse as inheritance [Lie86, JZ91]. In delegation, two objects are involved in handling a request: a receiving object delegates operations to its delegate. This is analogous to subclasses deferring requests to parent classes. But with inheritance, an inherited operation can always refer to the receiving object through the this member variable in C++ and self in Smalltalk. To achieve the same effect with delegation, the receiver passes itself to the delegate to let the delegated operation refer to the receiver.[2]

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.

See also

References

  1. ↑ Gamma et al. 1994
  2. ↑ Gamma, Erich; Helm, Richard; Johnson, Ralph; Vlissides, John (1995). Design patterns : elements of reusable object-oriented software (14. print. ed.). Reading, Mass.: Addison-Wesley. p. 20. ISBN 0-201-63361-2.
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