Delegates

Delegates
You are encouraged to solve this task according to the task description, using any language you may know.

A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

• Keep an optional delegate instance.
• Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".

Delegate:

• Implement "thing" and return the string "delegate implementation"

Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

Ada

All that is needed in order to implement this is a common base type. The delegator holds a pointer to an "untyped" object from the base class. Querying if the target implements the delegate interface is done using run-time type identification. <lang ada>with Ada.Text_IO; use Ada.Text_IO;

procedure Delegation is

  package Things is
     -- We need a common root for our stuff
     type Object is tagged null record;
     type Object_Ptr is access all Object'Class;
     
     -- Objects that have operation thing
     type Substantial is new Object with null record;
     function Thing (X : Substantial) return String;
     
     -- Delegator objects
     type Delegator is new Object with record
        Delegate : Object_Ptr;
     end record;
     function Operation (X : Delegator) return String;
     
     No_Thing  : aliased Object;      -- Does not have thing
     Has_Thing : aliased Substantial; -- Has one
  end Things;
     
  package body Things is
     function Thing (X : Substantial) return String is
     begin
        return "delegate implementation";
     end Thing;
  
     function Operation (X : Delegator) return String is
     begin
        if X.Delegate /= null and then X.Delegate.all in Substantial'Class then
           return Thing (Substantial'Class (X.Delegate.all));
        else
           return "default implementation";
        end if;
     end Operation;
  end Things;

  use Things;

  A : Delegator; -- Without a delegate

begin

  Put_Line (A.Operation);
  A.Delegate := No_Thing'Access; -- Set no thing
  Put_Line (A.Operation);
  A.Delegate := Has_Thing'Access; -- Set a thing
  Put_Line (A.Operation);

end Delegation;</lang> Sample output:

default implementation
default implementation
delegate implementation

Aikido

<lang aikido> class Delegator {

   public generic delegate = none

   public function operation {
       if (typeof(delegate) == "none") {
           return "default implementation"
       }
       return delegate()
   }

}

function thing {

   return "delegate implementation"

}

// default, no delegate var d = new Delegator() println (d.operation())

// delegate var d1 = new Delegator() d1.delegate = thing println (d1.operation())

</lang>

C

As best you can do, without support for classes. <lang c>#include <stdio.h>

1. include <stdlib.h>
2. include <string.h>

typedef const char * (*Responder)( int p1);

typedef struct sDelegate {

   Responder operation;

} *Delegate;

/* Delegate class constructor */ Delegate NewDelegate( Responder rspndr ) {

   Delegate dl = malloc(sizeof(struct sDelegate));
   dl->operation = rspndr;
   return dl;

}

/* Thing method of Delegate */ const char *DelegateThing(Delegate dl, int p1) {

   return  (dl->operation)? (*dl->operation)(p1) : NULL;

}

/** * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */ typedef struct sDelegator {

   int     param;
   char    *phrase;
   Delegate delegate;

} *Delegator;

const char * defaultResponse( int p1) {

   return "default implementation";

}

static struct sDelegate defaultDel = { &defaultResponse };

/* Delegator class constructor */ Delegator NewDelegator( int p, char *phrase) {

   Delegator d  = malloc(sizeof(struct sDelegator));
   d->param = p;
   d->phrase = phrase;
   d->delegate = &defaultDel;	/* default delegate */
   return d;

}

/* Operation method of Delegator */ const char *Delegator_Operation( Delegator theDelegator, int p1, Delegate delroy) {

   const char *rtn;
   if (delroy) {
       rtn = DelegateThing(delroy, p1);
       if (!rtn) {			/* delegate didn't handle 'thing' */
           rtn = DelegateThing(theDelegator->delegate, p1);
       }
   }
   else 		/* no delegate */
       rtn = DelegateThing(theDelegator->delegate, p1);

   printf("%s\n", theDelegator->phrase );
   return rtn;

}

/** * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */ const char *thing1( int p1) {

   printf("We're in thing1 with value %d\n" , p1);
   return "delegate implementation";

}

int main() {

   Delegate del1 = NewDelegate(&thing1);
   Delegate del2 = NewDelegate(NULL);
   Delegator theDelegator = NewDelegator( 14, "A stellar vista, Baby.");

   printf("Delegator returns %s\n\n", 
           Delegator_Operation( theDelegator, 3, NULL));
   printf("Delegator returns %s\n\n", 
           Delegator_Operation( theDelegator, 3, del1));
   printf("Delegator returns %s\n\n",
           Delegator_Operation( theDelegator, 3, del2));
   return 0;

}</lang>

C++

Delegates in the C# or D style are available in C++ through std::tr1::function class template. These delegates don't exactly match this problem statement though, as they only support a single method call (which is operator()), and so don't support querying for support of particular methods.

<lang Cpp>

1. include <tr1/memory>
2. include <string>
3. include <iostream>
4. include <tr1/functional>

using namespace std; using namespace std::tr1; using std::tr1::function;

// interface for all delegates class IDelegate { public:

   virtual ~IDelegate() {}

};

//interface for delegates supporting thing class IThing { public:

   virtual ~IThing() {}
   virtual std::string Thing() = 0;

};

// Does not handle Thing class DelegateA : virtual public IDelegate { };

// Handles Thing class DelegateB : public IThing, public IDelegate {

   std::string Thing()
   {
       return "delegate implementation";
   }

};

class Delegator { public:

   std::string Operation()
   {
       if(Delegate) //have delegate
          if (IThing * pThing = dynamic_cast<IThing*>(Delegate.get()))
           //delegate provides IThing interface
           return pThing->Thing();
       
       return "default implementation";
   }

   shared_ptr<IDelegate> Delegate;

};

int main() {

   shared_ptr<DelegateA> delegateA(new DelegateA());
   shared_ptr<DelegateB> delegateB(new DelegateB());
   Delegator delegator;

   // No delegate
   std::cout << delegator.Operation() << std::endl;

   // Delegate doesn't handle "Thing"
   delegator.Delegate = delegateA;
   std::cout << delegator.Operation() << std::endl;

   // Delegate handles "Thing"
   delegator.Delegate = delegateB;
   std::cout << delegator.Operation() << std::endl;

/* Prints:

 default implementation
 default implementation
 delegate implementation
*/

} </lang>

Common Lisp

Translation of: Python

In CLOS, methods exist apart from classes, and are specialized based on the types of their arguments. This example defines two classes (delegator and delegate), and a thing generic method which is specialized in three ways: (1) for 'any' argument, providing a default method; (2) for delegators, where thing is recursively applied to the delegator's delegate (if there is one); and (3) for delegates.

<lang lisp>(defgeneric thing (object)

 (:documentation "Thing the object."))

(defmethod thing (object)

 "default implementation")

(defclass delegator ()

 ((delegate
   :initarg :delegate
   :reader delegator-delegate)))

(defmethod thing ((delegator delegator))

 "If delegator has a delegate, invoke thing on the delegate,

otherwise return \"no delegate\"."

 (if (slot-boundp delegator 'delegate)
   (thing (delegator-delegate delegator))
   "no delegate"))

(defclass delegate () ())

(defmethod thing ((delegate delegate))

 "delegate implementation")

(let ((d1 (make-instance 'delegator))

     (d2 (make-instance 'delegator :delegate nil))
     (d3 (make-instance 'delegator :delegate (make-instance 'delegate))))
 (assert (string= "no delegate" (thing d1)))
 (assert (string= "default implementation" (thing d2)))
 (assert (string= "delegate implementation" (thing d3))))</lang>

D

D has built-in delegates, so we can skip createing additional Delegate object and pass real delegate directly to Delegator.

This example uses tango for output.

Library: tango

<lang D>import tango.io.Stdout;

class Delegator {

   private char[] delegate() hasDelegate;

public:

   char[] operation() {
       if (hasDelegate is null)
           return "default implementation";
       return hasDelegate();
   }

   typeof(this) setDg(char[] delegate() dg)
   {
       hasDelegate = dg;
       return this;
   }

}

int main(char[][] args) {

   auto dr = new Delegator();
   auto thing = delegate char[]() { return "delegate implementation"; };

   Stdout ( dr.operation ).newline;
   Stdout ( dr.operation ).newline;
   Stdout ( dr.setDg(thing).operation ).newline;
   return 0;

}</lang>

Dart

I didn't find a way to check for existing methods, so the version with Object doesn't work yet. The code is adapted from the Java version, but using var instead of an Interface <lang dart>class Delegator {

 var delegate;

 String operation() {
   if (delegate == null)
     return "default implementation";
   else
     return delegate.thing();
 }

}

class Delegate {

 String thing() => "delegate implementation";

}

main() {

 // Without a delegate:
 Delegator a = new Delegator();
 Expect.equals("default implementation",a.operation());

 // any object doesn't work unless we can check for existing methods
 // a.delegate=new Object();
 // Expect.equals("default implementation",a.operation());

 // With a delegate:
 Delegate d = new Delegate();
 a.delegate = d;
 Expect.equals("delegate implementation",a.operation());

}</lang>

Delphi

Translation of the Java example found at Wikipedia. <lang Delphi>unit Printer;

interface

type

 // the "delegate"
 TRealPrinter = class
 public
   procedure Print;
 end;

 // the "delegator"
 TPrinter = class
 private
   FPrinter: TRealPrinter;
 public
   constructor Create;
   destructor Destroy; override;
   procedure Print;
 end;

implementation

{ TRealPrinter }

procedure TRealPrinter.Print; begin

  Writeln('Something...');

end;

{ TPrinter }

constructor TPrinter.Create; begin

 inherited Create;
 FPrinter:= TRealPrinter.Create;

end;

destructor TPrinter.Destroy; begin

 FPrinter.Free;
 inherited;

end;

procedure TPrinter.Print; begin

 FPrinter.Print;

end;

end.</lang> <lang Delphi>program Delegate;

{$APPTYPE CONSOLE}

uses

 SysUtils,
 Printer in 'Printer.pas';

var

 PrinterObj: TPrinter;

begin

 PrinterObj:= TPrinter.Create;
 try
   PrinterObj.Print;
   Readln;
 finally
   PrinterObj.Free;
 end;

end.</lang>

E

<lang e>def makeDelegator {

   /** construct without an explicit delegate */
   to run() {
       return makeDelegator(null)
   }

   /** construct with a delegate */
   to run(delegateO) { # suffix because "delegate" is a reserved keyword
       def delegator {
           to operation() {
               return if (delegateO.__respondsTo("thing", 0)) {
                          delegateO.thing()
                      } else {
                          "default implementation"
                      }
           }
       }
       return delegator
   }

}

? def delegator := makeDelegator() > delegator.operation()

1. value: "default implementation"

? def delegator := makeDelegator(def doesNotImplement {}) > delegator.operation()

1. value: "default implementation"

? def delegator := makeDelegator(def doesImplement { > to thing() { return "delegate implementation" } > }) > delegator.operation()

1. value: "default implementation"</lang>

F#

<lang fsharp>type Delegator() =

 let defaultOperation() = "default implementation"
 let mutable del = null

 // write-only property "Delegate"
 member x.Delegate with set(d:obj) = del <- d
 
 member x.operation() =
   if del = null then
     defaultOperation()
   else
     match del.GetType().GetMethod("thing", [||]) with
     | null -> defaultOperation()
     | thing -> thing.Invoke(del, [||]) :?> string 
     

type Delegate() =

 member x.thing() = "delegate implementation"

let d = new Delegator() assert (d.operation() = "default implementation")

d.Delegate <- "A delegate may be any object" assert (d.operation() = "default implementation")

d.Delegate <- new Delegate() assert (d.operation() = "delegate implementation")</lang>

Go

<lang go>package main import "fmt"

type Delegator struct {

   delegate interface{} // the delegate may be any type

}

// interface that represents anything that supports thing() type Thingable interface {

   thing() string

}

func (self Delegator) operation() string {

   if v, ok := self.delegate.(Thingable); ok {
       return v.thing()
   }
   return "default implementation"

}

type Delegate int // any dummy type

func (Delegate) thing() string {

   return "delegate implementation"

}

func main() {

   // Without a delegate:
   a := Delegator{}
   fmt.Println(a.operation()) // prints "default implementation"

   // With a delegate that does not implement "thing"
   a.delegate = "A delegate may be any object"
   fmt.Println(a.operation()) // prints "default implementation"

   // With a delegate:
   var d Delegate
   a.delegate = d
   fmt.Println(a.operation()) // prints "delegate implementation"

}</lang>

J

Life becomes slightly cleaner if we delegate to ourselves in the absence of some other delegate.

<lang J>coclass 'delegator'

 operation=:3 :'thing__delegate ::thing y'
 thing=: 'default implementation'"_
 setDelegate=:3 :'delegate=:y'  NB. result is the reference to our new delegate
 delegate=:<'delegator'

coclass 'delegatee1'

coclass 'delegatee2'

 thing=: 'delegate implementation'"_

NB. set context in case this script was used interactively, instead of being loaded cocurrent 'base'</lang>

Example use:

<lang J> obj=:conew'delegator'

  operation__obj

default implementation

  setDelegate__obj conew'delegatee1'

┌─┐ │4│ └─┘

  operation__obj

default implementation

  setDelegate__obj conew'delegatee2'

┌─┐ │5│ └─┘

  operation__obj

delegate implementation</lang>

Java

This implementation uses an interface called Thingable to specify the type of delegates that respond to thing(). The downside is that any delegate you want to use has to explicitly declare to implement the interface. The upside is that the type system guarantees that when the delegate is non-null, it must implement the "thing" method.

<lang java>interface Thingable {

   String thing();

}

class Delegator {

   public Thingable delegate;

   public String operation() {
       if (delegate == null)
           return "default implementation";
       else
           return delegate.thing();
   }

}

class Delegate implements Thingable {

   public String thing() {
       return "delegate implementation";
   }

}

// Example usage // Memory management ignored for simplification public class DelegateExample {

   public static void main(String[] args) {
       // Without a delegate:
       Delegator a = new Delegator();
       assert a.operation().equals("default implementation");

       // With a delegate:
       Delegate d = new Delegate();
       a.delegate = d;
       assert a.operation().equals("delegate implementation");

       // Same as the above, but with an anonymous class:
       a.delegate = new Thingable() {
               public String thing() {
                   return "anonymous delegate implementation";
               }
           };
       assert a.operation().equals("anonymous delegate implementation");
   }

}</lang>

JavaScript

Translation of: Python

<lang javascript>function Delegator() {

 this.delegate = null ;
 this.operation = function(){
   if(this.delegate && typeof(this.delegate.thing) == 'function')
     return this.delegate.thing() ;
   return 'default implementation' ;
 }

}

function Delegate() {

 this.thing = function(){
   return 'Delegate Implementation' ;
 }

}

function testDelegator(){

 var a = new Delegator() ;
 document.write(a.operation() + "\n") ;
 
 a.delegate = 'A delegate may be any object' ; 
 document.write(a.operation() + "\n") ;
 
 a.delegate = new Delegate() ;
 document.write(a.operation() + "\n") ;

}</lang>

Logtalk

We use prototypes instead of classes for simplicity. <lang logtalk>% define a category for holding the interface % and implementation for delegator objects

- category(delegator).

   :- public(delegate/1).
   :- public(set_delegate/1).

   :- private(delegate_/1).
   :- dynamic(delegate_/1).

   delegate(Delegate) :-
       ::delegate_(Delegate).

   set_delegate(Delegate) :-
       ::retractall(delegate_(Delegate)),
       ::assertz(delegate_(Delegate)).

- end_category.

% define a simpler delegator object, with a % method, operation/1, for testing delegation

- object(a_delegator,

   imports(delegator)).

   :- public(operation/1).

   operation(String) :-
       (   ::delegate(Delegate), Delegate::current_predicate(thing/1) ->
           % a delegate is defined that understands the method thing/1
           Delegate::thing(String)
       ;   % otherwise just use the default implementation
           String = 'default implementation'
       ).

- end_object.

% define an interface for delegate objects

- protocol(delegate).

   :- public(thing/1).

- end_protocol.

% define a simple delegate

- object(a_delegate,

   implements(delegate)).

   thing('delegate implementation').

- end_object.

% define a simple object that doesn't implement the "delegate" interface

- object(an_object).

- end_object.

% test the delegation solution when this file is compiled and loaded

- initialization((

   % without a delegate:
   a_delegator::operation(String1),
   String1 == 'default implementation',
   % with a delegate that does not implement thing/1:
   a_delegator::set_delegate(an_object),
   a_delegator::operation(String2),
   String2 == 'default implementation',
   % with a delegate that implements thing/1:
   a_delegator::set_delegate(a_delegate),
   a_delegator::operation(String3),
   String3 == 'delegate implementation'

)).</lang>

Objective-C

Works with: Cocoa

Works with: GNUstep

<lang objc>#import <Foundation/Foundation.h>

@interface Delegator : NSObject {

   id delegate;

}

- (id)delegate; - (void)setDelegate:(id)obj; - (NSString *)operation;

@end

@implementation Delegator

- (id)delegate {

   return delegate;

}

- (void)setDelegate:(id)obj {

   delegate = obj; // Weak reference

}

- (NSString *)operation {

   if ([delegate respondsToSelector:@selector(thing)])
       return [delegate thing];

   return @"default implementation";

}

@end

// Any object may implement these @interface NSObject (DelegatorDelegating)

- (NSString *)thing;

@end

@interface Delegate : NSObject

// Don't need to declare -thing because any NSObject has this method

@end

@implementation Delegate

- (NSString *)thing {

   return @"delegate implementation";

}

@end

// Example usage // Memory management ignored for simplification int main() {

   // Without a delegate:
   Delegator *a = [[Delegator alloc] init];
   NSLog(@"%d\n", [[a operation] isEqualToString:@"default implementation"]);

   // With a delegate that does not implement thing:
   [a setDelegate:@"A delegate may be any object"];
   NSLog(@"%d\n", [[a operation] isEqualToString:@"delegate implementation"]);

   // With a delegate that implements "thing":
   Delegate *d = [[Delegate alloc] init];
   [a setDelegate:d];
   NSLog(@"%d\n", [[a operation] isEqualToString:@"delegate implementation"]);

   return 0;

}</lang>

Oz

Translation of: Python

<lang oz>declare

 class Delegator from BaseObject
    attr

delegate:unit

    meth set(DG)

{Object.is DG} = true %% assert: DG must be an object delegate := DG

    end

    meth operation($)

if @delegate == unit then {self default($)} else try {@delegate thing($)} catch error(object(lookup ...) ...) then  %% the delegate did not understand the message {self default($)} end end

    end

    meth default($)

"default implementation"

    end
 end

 class Delegate from BaseObject
    meth thing($)

"delegate Implementation"

    end
 end

 A = {New Delegator noop}  

in

 {System.showInfo {A operation($)}}

 {A set({New BaseObject noop})}
 {System.showInfo {A operation($)}}

 {A set({New Delegate noop})}  
 {System.showInfo {A operation($)}}</lang>

Pascal

See Delphi

Perl

Translation of: Python

<lang perl>use strict;

package Delegator; sub new {

  bless {}

} sub operation {

  my ($self) = @_;
  if (defined $self->{delegate} && $self->{delegate}->can('thing')) {
     $self->{delegate}->thing;
  } else {
     'default implementation';
  }

} 1;

package Delegate; sub new {

  bless {};

} sub thing {

  'delegate implementation'

} 1;

package main;

1. No delegate

my $a = Delegator->new; $a->operation eq 'default implementation' or die;

1. With a delegate that does not implement "thing"

$a->{delegate} = 'A delegate may be any object'; $a->operation eq 'default implementation' or die;

1. With delegate that implements "thing"

$a->{delegate} = Delegate->new; $a->operation eq 'delegate implementation' or die; </lang>

Using Moose.

<lang perl> use 5.010_000;

package Delegate::Protocol use Moose::Role;

1. All methods in the Protocol is optional
2. optional 'thing';
3. If we wanted to have a required method, we would state:
4. requires 'required_method';

package Delegate::NoThing; use Moose; with 'Delegate::Protocol';

package Delegate; use Moose;

1. The we confirm to Delegate::Protocol

with 'Delegate::Protocol'; sub thing { 'delegate implementation' };

package Delegator; use Moose;

has delegate => (

    is      => 'rw',
   does => 'Delegate::Protocol', # Moose insures that the delegate confirms to the protocol.
  predicate => 'hasDelegate'

);

sub operation {

   my ($self) = @_;
   if( $self->hasDelegate  && $self->delegate->can('thing') ){
       return $self->delegate->thing() . $postfix; # we are know that delegate has thing.
   } else {
       return 'default implementation';
  }

};

package main; use strict;

1. No delegate

my $delegator = Delegator->new(); $delegator->operation eq 'default implementation' or die;

1. With a delegate that does not implement "thing"

$delegator->delegate( Delegate::NoThing->new ); $delegator->operation eq 'default implementation' or die;

1. With delegate that implements "thing"

$delegator->delegate( Delegate->new ); $delegator->operation eq 'delegate implementation' or die;

</lang>

Perl 6

<lang perl6>class Non-Delegate { }

class Delegate { method thing { return "delegate implementation" } }

class Delegator { has $.delegate is rw;

method operation { $.delegate.^can( 'thing' ) ?? $.delegate.thing !! "default implementation" } }

my Delegator $d .= new;

say "empty: "~$d.operation;

$d.delegate = Non-Delegate.new;

say "Non-Delegate: "~$d.operation;

$d.delegate = Delegate.new;

say "Delegate: "~$d.operation;</lang>

PHP

Translation of: Python

<lang php>class 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" ;</lang>

PicoLisp

<lang PicoLisp>(class +Delegator)

1. delegate

(dm operation> ()

  (if (: delegate)
     (thing> @)
     "default implementation" ) )

(class +Delegate)

1. thing

(dm T (Msg)

  (=: thing Msg) )

(dm thing> ()

  (: thing) )

(let A (new '(+Delegator))

  # Without a delegate
  (println (operation> A))

  # With delegate that does not implement 'thing>'
  (put A 'delegate (new '(+Delegate)))
  (println (operation> A))

  # With delegate that implements 'thing>'
  (put A 'delegate (new '(+Delegate) "delegate implementation"))
  (println (operation> A)) )</lang>

Output:

"default implementation"
NIL
"delegate implementation"

Pop11

<lang pop11>uses objectclass; define :class Delegator;

   slot delegate = false;

enddefine;

define :class Delegate; enddefine;

define :method thing(x : Delegate);

  'delegate implementation'

enddefine;

define :method operation(x : Delegator); if delegate(x) and fail_safe(delegate(x), thing) then

  ;;; Return value is on the stack

else

  'default implementation'

endif; enddefine;

Default, without a delegate

lvars a = newDelegator(); operation(a) =>

a delegating to itself (works because Delegator does not

implement thing)

a -> delegate(a); operation(a) =>

delegating to a freshly created Delegate

newDelegate() -> delegate(a); operation(a) =></lang>

Python

<lang python>class Delegator:

  def __init__(self):
     self.delegate = None
  def operation(self):
      if hasattr(self.delegate, 'thing') and callable(self.delegate.thing):
         return self.delegate.thing()
      return 'default implementation'

class Delegate:

  def thing(self):
     return 'delegate implementation'

if __name__ == '__main__':

  # No delegate
  a = Delegator()
  assert a.operation() == 'default implementation'

  # With a delegate that does not implement "thing"
  a.delegate = 'A delegate may be any object'
  assert a.operation() == 'default implementation'

  # With delegate that implements "thing"
  a.delegate = Delegate()
  assert a.operation() == 'delegate implementation'</lang>

Ruby

Translation of: Python

<lang ruby>class Delegator

  attr_accessor :delegate
  def operation
     if @delegate.respond_to?(:thing)
        @delegate.thing
     else
        'default implementation'
     end
  end

end

class Delegate

  def thing
     'delegate implementation'
  end

end

if __FILE__ == $PROGRAM_NAME

  # No delegate
  a = Delegator.new
  puts a.operation # prints "default implementation"

  # With a delegate that does not implement "thing"
  a.delegate = 'A delegate may be any object'
  puts a.operation # prints "default implementation"

  # With delegate that implements "thing"
  a.delegate = Delegate.new
  puts a.operation # prints "delegate implementation"

end</lang>

Using Forwardable lib

<lang ruby>require 'forwardable'

class Delegator; extend Forwardable

 attr_accessor :delegate
 def_delegator :@delegate, :thing, :delegated

 def initialize
   @delegate = Delegate.new()
 end

end

class Delegate

 def thing
   'Delegate'
 end

end

a = Delegator.new puts a.delegated # prints "Delegate" </lang>

Tcl

Works with: Tcl version 8.6

or

Library: TclOO

Uses Assertions#Tcl <lang tcl>package require TclOO

oo::class create Delegate {

   method thing {} {
       return "delegate impl."
   }
   export thing

}

oo::class create Delegator {

   variable delegate
   constructor args {
       my delegate {*}$args
   }
   
   method delegate args {
       if {[llength $args] == 0} {
           if {[info exists delegate]} {
               return $delegate
           }
       } elseif {[llength $args] == 1} {
           set delegate [lindex $args 0]
       } else {
           return -code error "wrong # args: should be \"[self] delegate ?target?\""
       }
   }
   
   method operation {} {
       try {
           set result [$delegate thing]
       } on error e {
           set result "default implementation"
       }
       return $result
   }

}

1. to instantiate a named object, use: class create objname; objname aMethod
2. to have the class name the object: set obj [class new]; $obj aMethod

Delegator create a set b [Delegator new "not a delegate object"] set c [Delegator new [Delegate new]]

assert {[a operation] eq "default implementation"}  ;# a "named" object, hence "a ..." assert {[$b operation] eq "default implementation"}  ;# an "anonymous" object, hence "$b ..." assert {[$c operation] ne "default implementation"}

1. now, set a delegate for object a

a delegate [$c delegate] assert {[a operation] ne "default implementation"}

puts "all assertions passed"</lang>

To code the operation method without relying on catching an exception, but strictly by using introspection: <lang tcl>method operation {} {

   if { [info exists delegate] &&
        [info object isa object $delegate] &&
        "thing" in [info object methods $delegate -all]
   } then {
       set result [$delegate thing]
   } else {
       set result "default implementation"
   }

}</lang>

Vorpal

Delegate objects can be an array of delegates or as a single delegate. <lang vorpal>a = new() a.f = method(){

       .x.print()

}

c = new() c.g = method(){

       (.x + 1).print()

}

1. array of delegates

b = new() b.delegate = new() b.delegate[0] = a b.delegate[1] = c b.x = 3 b.f() b.g()

1. single delegate

d = new() d.delegate = a d.x = 7 d.f()</lang>

The resulting output:

3
4
7