Delegation

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Business

Delegation is handing a task over to a subordinate.

It is the assignment of authority and responsibility to another person to carry out specific activities. It allows a subordinate to make decisions, i.e. it is a shift of decision-making authority from one organizational level to another lower one.

Delegation, if properly done, is not abdication.

Ultimate responsibility CANNOT be delegated.

Factors affecting delegation

size of organization
importance of the duty or decision
task complexity
organizational culture
qualities of subordinates

Making delegation effective

clarify the assignment
specify the subordinate's range of discretion (authority should be commensurate with responsibility)
allow the subordinate to participate

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Object-oriented programming

In object-oriented programming there are two notions of delegation. In older times delegation was refering to passing on the execution to some other object. In recent times however, it means the method lookup rules when dispatching so-called self-calls as defined by Lieberman in his 1986 paper "Using Prototypical Objects to Implement Shared Behavior in Object-Oriented Systems" featuring the elephant Clyde. Despite the mature age of the Liebermann paper, delegation has been observed to be used in a "wrong" fashion as late as mid 1990s. Below is an example of the old definition.

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Old definition example

It is a very simple yet very powerful concept: Handing a task over to another part of the program. In OO it is used to describe the situation wherein an object passes a task to another object. Generally today, however, this mechanism is widely known as aggregatin.

In a C++ like language say there is a class A defined as...


class A 
{
  public:
    foo()
    {
      print("Object A doing the job");
    }
}

Now if there is another class B defined as...


class B
{
  public:
    A a;
    foo()
    {
       a.foo();
    }
}

... any object of class B will delegate the execution of its function foo to an object of class A.

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Modern definition

The short definition is that delegation defines method dispatching like how it is defined for virtual methods in inheritance: It is always the most specific method which is chosen during method-lookup---Hence it is the receiver entity which is the start of method lookup even though it has passed on control to some other object (throug 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 on the other hand 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 cannot (G. Kniesel has shown that a restricted version of delegation can be statically typesafe). Delegation can be termed "run-time inheritance for specific objects".

Example in a c++/java/c# like language


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

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

class B
{
   delegationlink A a
   foo() {
       a.foo() // call foo() on the a-instance
   }

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

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

calling b.foo() will result in b.bar to be printed on the screen. Using the old definition the result would have been a.bar

It should be noted that delegation is generally not generally available in programming languages. A few exceptions exists though. The language "Self" and Kniesels "Darwin".

Dual inheritance

If the language supports both delegation and inheritance one can do dual inheritance by utilizing both mechanisms at the same time as in


class C extends A
{
   delegationlink D d
}

This calls for additional rules for method lookup, as there now potentially two methods which can be denoted as the most specific (due to the two lookup paths).

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Literature

Henry Lieberman: Using prototypical objects to implement shared behavior in object-oriented systems. In: Conference proceedings on Object-oriented programming systems, languages and applications. Portland 1986, p. 214-223 ISSN 0362-1340 (Online at http://web.media.mit.edu/~lieber/Lieberary/OOP/Delegation/Delegation.html)


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