Delegation (object-oriented programming)

This sense of delegation as programming language feature making use of the method lookup rules for dispatching so-called self-calls was defined by Lieberman in his 1986 paper "Using Prototypical Objects to Implement Shared Behavior in Object-Oriented Systems".

Delegation is dependent upon dynamic binding, as it requires that a given method call can invoke different segments of code at runtime. It is used throughout macOS (and its predecessor NeXTStep) as a means of customizing the behavior of program components.[3] It enables implementations such as making use of a single OS-provided class to manage windows, because the class takes a delegate that is program-specific and can override default behavior as needed. For instance, when the user clicks the close box, the window manager sends the delegate a windowShouldClose: call, and the delegate can delay the closing of the window, if there is unsaved data represented by the window's contents.

Delegation can be characterized (and distinguished from forwarding) as late binding of self:[4]

... messages sent to the self (or this) variable in the parent will "come back" to the object that originally received the message.

That is, the self in a method definition in the receiving object is not statically bound to that object at definition time (such as compile time or when the function is attached to an object), but rather at evaluation time, it is bound to the original object.

It has been argued that delegation may in some cases be preferred to inheritance to make program code more readable and understandable.[5] Despite explicit delegation being fairly widespread, relatively few major programming languages implement delegation as an alternative model to inheritance. The precise relationship between delegation and inheritance is complicated; some authors consider them equivalent, or one a special case of the other.[6]

In languages that support delegation via method lookup rules, method dispatching is defined the way it is defined for virtual methods in inheritance: It is always the most specific method that is chosen during method lookup. Hence it is the original receiver entity that is the start of method lookup even though it has passed on control to some other object (through a delegation link, not an object reference).

Delegation has the advantage that it can take place at run time and affect only a subset of entities of some type and can even be removed at run time. Inheritance, by contrast, typically targets the type rather than the instances, and is restricted to compile time. On the other hand, inheritance can be statically type-checked, while delegation generally cannot without generics (although a restricted version of delegation can be statically typesafe[7]). Delegation can be termed "run-time inheritance for specific objects."

Here is a pseudocode example in a C#/Java like language:

class A {
    void foo() {
        // "this" also known under the names "current", "me" and "self" in other languages
        this.bar();
    }

    void bar() {
        print("a.bar");
    }
}

class B {
    private delegate A a; // delegation link

    public B(A a) {
        this.a = a;
    }

    void foo() {
        a.foo(); // call foo() on the a-instance
    }

    void bar() {
        print("b.bar");
    }
}

a = new A();
b = new B(a); // establish delegation between two objects

Calling b.foo() will result in b.bar being printed, since this refers to the original receiver object, b, within the context of a. The resulting ambiguity of this is referred to as object schizophrenia.

Translating the implicit this into an explicit parameter, the call (in B, with a a delegate) a.foo() translates to A.foo(b), using the type of a for method resolution, but the delegating object b for the this argument.

Using inheritance, the analogous code (using capital letters to emphasize that resolution is based on classes, not objects) is:

class A {
    void foo() {
        this.bar();
    }

    void bar() {
        print("A.bar");
    }
}

class B extends A {
    public B() {}

    void foo() {
        super.foo(); // call foo() of the superclass (A)
    }

    void bar() {
        print("B.bar");
    }
}

b = new B();

Calling b.foo() will result in B.bar. In this case, this is unambiguous: there is a single object, b, and this.bar() resolves to the method on the subclass.

Programming languages in general do not support this unusual form of delegation as a language concept, but there are a few exceptions.

class C extends A {
    delegationlink D d;
}

• Delegation pattern
• Adapter pattern
• Hooking
• Continuation
• Implementation inheritance
• Inheritance semantics
• Schizophrenia (object-oriented programming)
• Virtual inheritance
• Wrapper function

Distinguish:

• Delegate (CLI)
• Delegation (programming)
• Object aggregation
• Forwarding (object-oriented programming)

• A new way to implement Delegate in C++
• Fast delegates in C++
• PerfectJPattern Open Source Project, Provides a reusable implementation of Delegates in Java