CSC/ECE 517 Fall 2010/ch6 6a PC

Delegation-based programming languages

" I do not care how it is done, or who is it assigned to. If something is understood to be placed in the hands of another party, it is delegation " - 'Anonymous'

Introduction

Dictionary defines delegation as the assignment of authority and responsibility to another person to carry out specific activities.In programming context it is nothing different. It is basically entrusting an action to another agent. This term was initially introduced by Henry Lieberman in his 1986 paper "Using Prototypical Objects to Implement Shared Behavior in Object-Oriented Systems". [1] He defines delegation in object oriented languages as a programming language feature making use of the method lookup rules for dispatching so-called self-calls. Thus delegation is like inheritance done manually through object composition.

The above diagram depicts that a message receiver is asking another object (message holder) to do something on its behalf which may delegate it to someone and so on and so forth

Delegation

Delegation, is also referred to as aggregation,consultation, or forwarding. In delegation one class may contain an instance of another class, and delegate some responsibility to that class. This is also referred to as the has-a relationship. Aggregation should not be confused with composition. Both aggregation and composition are used to describe one object containing another object, but composition implies ownership [2]. Aggregation is more general and doesn't imply any responsibilities for memory management. A class which contains other classes is called a composite class, while a class being contained is called a composited or composed class [3].Delegation is a very powerful reuse technique. The provides run time flexibility. It is also called dynamic inheritance.[4]

Delegation can be implemented in many programming languages like Ada, Aikido, C, C#, C++, Common Lisp, D, E, Go, J, Java, JavaScript, Logtalk, Objective-C,Oz, Perl,Perl 6, PHP, PicoLisp, Pop11, Python, Ruby, TCL, Vorpal [5]

Example in a Java like language:

class Delegate
{
    void doSomething()
    {
        // "this" is also known as "current", "me" and "self" in other languages
        this.callMe()
    }
    void callMe() 
    {
        print("I am Delegate")
    }
}

class Delegator
{
    private Delegate d;  // delegationlink
    public Delegator(Delegate d)
    {
        this.d = d;
    }
    void doSomething() {
        d.doSomething() // call doSomething() on the Delegate instance
    }
    void callMe() {
        print("I am Delegator")
    }
}
d = new Delegate()
b = new Delegator(d) // establish delegation between two objects

Here calling b.doSomething() will result in "I am Delegate" being printed, since class Delegator "delegates" the method callMe() to a given object of class Delegate.

Delegation vs Inheritance

Inheritance:

• Inheritance is restricted to compile time
• Since it targets type rather than instances, it cannot be removed/changed at runtime
• Inheritance can be statically type-checked

Delegation:

• Delegation takes place at run-time
• Since it takes place at run-time, it can even be removed at run-time
• Cannot guarantee static type-safety (By using interfaces, delegation can be made more flexible and typesafe)

Hints to use Inheritance

• Inheritance is used to create sub-categories of objects
• Mostly identified in 'is-a' relationships between objects
• Inheritance is used when two objects are of the same type but one of the objects may need to have its own customized functionality. The child object just inherits from the parent and writes its own implementation of the feature it needs extra.
• Inheritance can be implemented on objects of the same type only.

Here we have an abstract base class. We extend this to to provide something that we can represent any product by. We then provide a few specialisations for typical products like book,cd.This is a usual case when inheritance is used.

Hints to use Delegation

• Delegation is used when two objects aren't of the same type but one has methods and attributes that the other wants to use internally instead of writing its own implementation
• Identified as 'as-a' relationships where the type of included object may change during runtime.
• Delegation is used when methods implementation needs to be handled by some other class within the parent tree. The class that will handle the method isn't known at compile time.
• Code execution within objects needs to be determined dynamically.

In the above diagram, class C inherits from A. Also, class C has a method called as Call which simple calls the method Call from class B. Hence, class C is delegating its functionality to be implemented by Class B. For this, class C has an object reference to class B.

Situations where you can use delegation

Consider following situation:

• Objective: We want a robot to have a heat sensor capability.
• Initial design: So we initially decide to build a Robot class like the one shown below:

• Problem with the design: Now all Robots should have a heat sensor. What if some robot did not have a heat sensor capability?
• Solution: So we make a Robot base class and a VolcanoRobot class which inherits from Robot and performs the heat-sensor operations

• Problem: But with this design whenever we want to modify anything related to the heat sensor, we will have to change the robot class. Also, with our design we have exposed heat sensor methods to Robot class.
• Solution: Hence, in such situations delegation is best.

• New design: Delegate the heat sensor functionality to Heat Sensor class. VolcanoRobot still has the 3 methods that are related to the sensor, but those are wrapper methods, they do nothing but to call the sensor corresponding ones, and that’s exactly what delegation is, just delegate functionality to the contained parts(delegates).

Delegation and composition go hand in hand to provide a flexible neat solution and also it serves the principle “separate changeable code from static one” . The price that one needs to pay for this is that we need wrapper methods, and extra time needed in processing because of the call of these wrapper methods.

Delegation Design Pattern

It is a design pattern in object oriented languages where an object expresses certain behavior to the outside but in reality delegates responsibility for implementing that behaviour to an associated object.

Consider the following examples in java:

class Subordinate  // the "delegate"
{  
    void performTask() 
    { 
      System.out.print("Subordinate is performing"); 
    }
}
class Manager // the "delegator"
{ 
    Subordinate slave = new Subordinate();  // create the delegate 
    void performTask() 
    { 
      slave.performTask(); // delegation
    } 
}
public class Main {
    // to the outside world it looks like Manager is actually doing work.
    public static void main(String[] args) {
        Manager boss = new Manager();
        boss.performTask();
    }
}

have to write more stuff here. Examples from various programming lang.

Delegation as a Language Feature

Prototype-based programming is a classless type of Object oriented programming technique which performs the work of inheritance in class-based languages. Delegation is the language feature that supports prototype-based programming. It is the mechanism by which code is selected for execution in a dynamic way. Delegation is the process by which a function or method, referred to in the context of one object, is found in another object. When a method is referred to by code defined for an object but is not itself found to be defined in the object the language runtime searches other objects, called delegates. Depending on the object model of the individual language the delegates may defined when the object is created or may be specified by the method-call syntax. The analogous process in a class-based language is dispatching to an inherited method, and an object's delegates perform the role of superclasses in a class-based language.[6]

Delegation in Ruby

Delegation pattern in Ruby is implemented in 3 ways:

1)Forwardable lib:Forwardable lib is a library that supports delegation, it has 2 modules Forwardable and SingleForwardable:

Forwardable module

The Forwardable module provides delegation of specified methods to a designated object, using the methods def_delegator and def_delegators.

• def_delegator(obj, method, alias = method) : Defines a method method which delegates to obj. If alias is provided, it is used as the name for the delegate method.

• def_delegators(obj, *methods): Shortcut for defining multiple delegator methods, but with no provision for using a different name.

2)SingleForwardable module: The SingleForwardable module provides delegation of specified methods to a designated object, using the methods def_delegators. This module is similar to Forwardable, but it works on objects themselves, instead of their defining classes.

3)Delegate lib:Delegate Lib is another lib that provides delegation in Ruby

Example of delegation in Ruby

Using Forwardable module provided in Ruby, this is how we would implement Robot example seen above:

require 'forwardable'
class Robot
  # Extending provides class methods
  extend Forwardable
  # Use of  def_delegators
  def_delegators :@arm,:package,:stack
  # Use of  def_delegator
  def_delegator :@heat_sensor, :measure ,:measure_heat
  def initialize
    @heat_sensor = HeatSensor.new
    @arm = RobotArm.new
  end
end
class HeatSensor
  #Celsius or Fahrenheit scale
  def measure(scale="c")
    t = rand(100)
    t = scale=="c" ? t : t * (9/5)
    puts "Heat is #{t}° #{scale.upcase}"
  end
end
class RobotArm
  def stack(boxes_number=1)
    puts "Stacking #{boxes_number} box(es)"
  end
  def package
    puts "Packaging"
  end
end

Using Single Forwardable module:

require "forwardable"
require "date"
Date = Date.today # output will be <Date: 4909665/2,0,2299161> # Prepare object for delegation
date.extend SingleForwardable #=> #<Date: 4909665/2,0,2299161> # Add delegation for Time.now
date.def_delegator :Time, "now","with_time"
puts date.with_time #=>Thu Jan 01 23:03:04 +0200 2009

Using delegate lib:

require "delegate"
require "date"
# Notice the class definition
class CurrentDate < DelegateClass(Date)
  def initialize
    @date = Date.today
    # Pass the object to be delegated to the superclass. 
    super(@date)
  end
 
  def to_s
    @date.strftime "%Y/%m/%d"
  end
 
  def with_time
    Time.now
  end
end
 
cdate = CurrentDate.new ## Notice how delegation works. Instead of doing cdate.date.day and defining attr_accessor for the date just do c.day
puts cdate.day #=>1
puts cdate.month #=>1
puts cdate.year #=>2009 # Testing added methods like to_s
puts cdate #=> 2009/01/01
puts cdate.with_time #=> Thu Jan 01 23:22:20 +0200 2009

Delegation in Java

Java supports delegation in the same way as other languages do. By using an instance of the class we would have otherwise inherited, and then forwarding messages to the instance we can do delegation in Java.[7] e.g. We can associate a class with a thread in two ways:

• By inheriting directly from class Thread.
• By implementing the Runnable interface and delegating to a Thread object.

Examples of Delegation in Java

Given below are the two examples of delegation, first being a simple one while the second being a complex example.

Simple Java Example

In this example, the class C has method which does not perform itself and rather delegates to class A, the methods f() and g(). It seems that Class C is doing the work but in reality class A is doing it.[8]

 class A {
   void f() { system.out.println("A: doing f()"); }
   void g() { system.out.println("A: doing g()"); }
 }

 class C {
   // delegation
   A a = new A();

   void f() { a.f(); }
   void g() { a.g(); }

   // normal attributes
   X x = new X();
   void y() { /* do stuff */ }
 }

 void main() {
   C c = new C();

   c.f();
   c.g();
 }

Complex Java Example

By using interfaces, delegation can be made more flexible[ class-C need not refer to class-A or class-B, delegation is abstracted] and typesafe. Here, class C can delegate to either class A or class B. The implements clauses, improves type safety, since this ensures that each of the class must implements the methods in the interface. The main tradeoff being more code.

interface I {
   void f();
   void g();
 }

 class A implements I {
   void f() { system.out.println("A: doing f()"); }
   void g() { system.out.println("A: doing g()"); }
 }

 class B implements I {
   void f() { system.out.println("B: doing f()"); }
   void g() { system.out.println("B: doing g()"); }
 }

 class C implements I {
   // delegation
   I i = new A();

   void f() { i.f(); }
   void g() { i.g(); }

   // normal attributes
   void toA() { i = new A(); }
   void toB() { i = new B(); }
 }

 void main() {
   C c = new C();

   c.f();
   c.g();
 }

Table of Comparison

Programming language	Support for Delegation	Syntax for implementing Delegation
C++	No in built support.Delegation has to be done manually.Time consuming	Create a delegation link in delegator. class MyClass { FuBar fb; //delegation link void Fu() { fb.Fu(); } // void Bar() { bar.Bar(); } }
Perl 6	Yes via a pragma for delegating method calls to attributes of an object	use delegation attr1 => [qw( method1 method2 method3 )], attr2 => [qw( method4 method5 )], attr3 => __ALL__, attr4 => __ALL__, # etc.  ; calls like: $obj->method3(@args); $obj->method5(@other_args); would act as if they were: $obj->{attr1}->method3(@args); $obj->{attr2}->method5(@other_args);
Python	Yes. Using Automatic Delegation (via __getattr__ and __setattr__) it can provide delegation with uses similar to inheritance without any limits and better control.	def __getattr__(self, attr): return getattr(self.file, attr) def __setattr__(self, attr, value): return setattr(self.file, attr, value)
PHP	Yes	lass Delegator { function __construct() { $this->delegate = NULL ; } function operation() { if(method_exists($this->delegate, "thing")) return $this->delegate->thing() ; return 'default implementation' ; } } class Delegate { function thing() { return 'Delegate Implementation' ; } } $a = new Delegator() ; print "{$a->operation()}\n" ; $a->delegate = 'A delegate may be any object' ; print "{$a->operation()}\n" ; $a->delegate = new Delegate() ; print "{$a->operation()}\n" ;
Tcl	Yes.Delegation-based model for Tk widgets.Included in Tcllib	//defining a delegate method Object instproc delegate {method obj} { my set delegate($method) $obj } Object instproc unknown {m args} { if {[my exists delegate($m)]} { eval [my set delegate($m)] $m $args } elseif {[my exists delegate(*)]} { eval [my set delegate(*)] $m $args } }
Java	Yes. Two ways.	1st way: Using an interface called Thingable to specify the type of delegates that respond to thing() 2nd way: Use of traditional wrapper methods.
Objective-C	Yes	Delegating class has an outlet or property, usually one that is named delegate; if it is an outlet, it includes methods for setting and accessing the value of the outlet
Ruby	Yes	Wrapper method, delegate lib, forwardable module.

Conclusion

Thus delegation is a powerful design/reuse technique, and can be viewed as a relationship between objects where one object forwards certain method calls to another object, called its delegate. Delegation is simply passing a duty off to someone/something else.

Delegation pattern is better than inheritance in some ways and it is supported by many languages in various ways as in the given table. Delegation is useful to express relationship among classes, reducing coupling between classes, run-time flexibility – the delegate can be changed at run-time. It has certain disadvantages too, such as, it is not directly supported by most popular object-oriented languages, also it doesn’t facilitate dynamic polymorphism.

Hence the choice of using delegation depends on various factors such as programming language-support for multiple inheritance or design constraints requiring polymorphism.

References and Notes

• [1] Using prototypical objects to implement shared behavior in object-oriented systems
• [2] Aggregation
• [3] Object Composition
• [4] Issues Involved in Supporting Behavioral Evolution
• [5] Delegates
• [6] Darwin Project
• [7] Delegation as a Language Feature
• [8] Delegation in Ruby
• [9] Delegation Design Pattern
• [10] Delegation in Java
• [11] Examples of delegation in Java
• [12] Tcl example
• [13] Python delegation example
• [14] Perl delegation
• [15] PHP delegation
• [16] []

Further Reading

• Delegation Pattern in detail
• Delegation in Ruby