Delegates: Difference between revisions
Added Rust |
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puts a.delegated # prints "Delegate" |
puts a.delegated # prints "Delegate" |
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</lang> |
</lang> |
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=={{header|Rust}}== |
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Requiring delegates to implement Thingable: |
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<lang Rust>trait Thingable { |
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fn thing(&self) -> &str; |
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} |
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struct Delegator<T>(Option<T>); |
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struct Delegate {} |
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impl Thingable for Delegate { |
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fn thing(&self) -> &'static str { |
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"Delegate implementation" |
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} |
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} |
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impl<T: Thingable> Thingable for Delegator<T> { |
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fn thing(&self) -> &str { |
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self.0.as_ref().map(|d| d.thing()).unwrap_or("Default implmementation") |
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} |
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} |
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fn main() { |
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let d: Delegator<Delegate> = Delegator(None); |
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println!("{}", d.thing()); |
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let d: Delegator<Delegate> = Delegator(Some(Delegate {})); |
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println!("{}", d.thing()); |
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}</lang> |
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{{Out}} |
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<pre>Default implmementation |
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Delegate implementation</pre> |
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Using nightly-only specialization feature: |
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<lang Rust>#![feature(specialization)] |
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trait Thingable { |
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fn thing(&self) -> &str; |
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} |
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struct Delegator<T>(Option<T>); |
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struct Delegate {} |
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impl Thingable for Delegate { |
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fn thing(&self) -> &'static str { |
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"Delegate implementation" |
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} |
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} |
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impl<T> Thingable for Delegator<T> { |
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default fn thing(&self) -> &str { |
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"Default implementation" |
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} |
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} |
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impl<T: Thingable> Thingable for Delegator<T> { |
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fn thing(&self) -> &str { |
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self.0.as_ref().map(|d| d.thing()).unwrap_or("Default implmementation") |
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} |
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} |
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fn main() { |
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let d: Delegator<i32> = Delegator(None); |
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println!("{}", d.thing()); |
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let d: Delegator<i32> = Delegator(Some(42)); |
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println!("{}", d.thing()); |
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let d: Delegator<Delegate> = Delegator(None); |
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println!("{}", d.thing()); |
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let d: Delegator<Delegate> = Delegator(Some(Delegate {})); |
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println!("{}", d.thing()); |
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}</lang> |
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{{Out}} |
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<pre>Default implementation |
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Default implementation |
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Default implmementation |
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Delegate implementation</pre> |
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=={{header|Scala}}== |
=={{header|Scala}}== |
Revision as of 01:32, 5 February 2019
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
Aime
<lang aime>text thing(void) {
return "delegate implementation";
}
text operation(record delegator) {
text s;
if (r_key(delegator, "delegate")) { if (r_key(delegator["delegate"], "thing")) { s = call(r_query(delegator["delegate"], "thing")); } else { s = "default implementation"; } } else { s = "default implementation"; }
return s;
}
integer main(void) {
record delegate, delegator;
o_text(operation(delegator)); o_byte('\n');
r_link(delegator, "delegate", delegate); o_text(operation(delegator)); o_byte('\n');
r_put(delegate, "thing", thing); o_text(operation(delegator)); o_byte('\n');
return 0;
}</lang>
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>
ALGOL 68
As Algol 68 doesn't have classes, we supply a non-OO approximation, similar to the C version. <lang algol68># An Algol 68 approximation of delegates #
- The delegate mode - the delegate is a STRUCT with a single field #
- that is a REF PROC STRING. If this is NIL, it doesn't implement #
- thing #
MODE DELEGATE = STRUCT( REF PROC STRING thing );
- A delegator mode that will invoke the delegate's thing method #
- - if there is a delegate and the delegate has a thing method #
MODE DELEGATOR = STRUCT( REF DELEGATE delegate
, PROC( REF DELEGATE )STRING thing );
- constructs a new DELEGATE with the specified PROC as its thing #
- Algol 68 HEAP is like "new" in e.g. Java, but it can't take #
- parameters, so this PROC does the equivalent #
PROC new delegate = ( REF PROC STRING thing )REF DELEGATE:
BEGIN REF DELEGATE result = HEAP DELEGATE; thing OF result := thing;
result END # new delegate #
- constructs a new DELEGATOR with the specified DELEGATE #
PROC new delegator = ( REF DELEGATE delegate )REF DELEGATOR:
HEAP DELEGATOR := ( delegate , # anonymous PROC to invoke the delegate's thing # ( REF DELEGATE delegate )STRING: IF delegate IS REF DELEGATE(NIL) THEN # we have no delegate # "default implementation"
ELIF thing OF delegate IS REF PROC STRING(NIL) THEN # the delegate doesn't have an implementation # "default implementation"
ELSE # the delegate can thing # thing OF delegate
FI )
- invokes the delegate's thing via the delagator #
- Because the PROCs of a STRUCT don't have an equivalent of e.g. Java's #
- "this", we have to explicitly pass the delegate as a parameter #
PROC invoke thing = ( REF DELEGATOR delegator )STRING:
# the following is Algol 68 for what would be written in Java as # # "delegator.thing( delegator.delegate )" # ( thing OF delegator )( delegate OF delegator )
main: (
print( ( "No delegate : " , invoke thing( new delegator( NIL ) ) , newline , "Delegate with no thing: " , invoke thing( new delegator( new delegate( NIL ) ) ) , newline , "Delegate with a thing : " , invoke thing( new delegator( new delegate( HEAP PROC STRING := STRING: ( "delegate implementation" ) ) ) ) , newline ) )
) </lang>
- Output:
No delegate : default implementation Delegate with no thing: default implementation Delegate with a thing : delegate implementation
C
As best you can do, without support for classes. <lang c>#include <stdio.h>
- include <stdlib.h>
- 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#
<lang csharp>using System;
interface IOperable {
string Operate();
}
class Inoperable { }
class Operable : IOperable {
public string Operate() { return "Delegate implementation."; }
}
class Delegator : IOperable {
object Delegate;
public string Operate() { var operable = Delegate as IOperable; return operable != null ? operable.Operate() : "Default implementation."; }
static void Main() { var delegator = new Delegator(); foreach (var @delegate in new object[] { null, new Inoperable(), new Operable() }) { delegator.Delegate = @delegate; Console.WriteLine(delegator.Operate()); } }
}</lang> Output:
Default implementation. Default implementation. Delegate implementation.
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>
- include <tr1/memory>
- include <string>
- include <iostream>
- 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>
Clojure
<lang clojure>(defprotocol Thing
(thing [_]))
(defprotocol Operation
(operation [_]))
(defrecord Delegator [delegate]
Operation (operation [_] (try (thing delegate) (catch IllegalArgumentException e "default implementation"))))
(defrecord Delegate []
Thing (thing [_] "delegate implementation"))</lang>
- Output:
; without a delegate => (operation (Delegator. nil)) "default implementation" ; with a delegate that does not implement "thing" => (operation (Delegator. (Object.))) "default implementation" ; with a delegate that implements "thing" => (operation (Delegator. (Delegate.))) "delegate implementation"
CoffeeScript
<lang coffeescript> class Delegator
operation: -> if @delegate and typeof (@delegate.thing) is "function" return @delegate.thing() "default implementation"
class Delegate
thing: -> "Delegate Implementation"
testDelegator = ->
# Delegator with no delegate. a = new Delegator() console.log a.operation() # Delegator with delegate not implementing "thing" a.delegate = "A delegate may be any object" console.log a.operation() # Delegator with delegate that does implement "thing" a.delegate = new Delegate() console.log a.operation()
testDelegator() </lang> output <lang>
> coffee foo.coffee
default implementation default implementation Delegate Implementation </lang>
Common Lisp
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 creating an additional Delegate object and pass a real delegate directly to Delegator.
<lang d>class Delegator {
string delegate() hasDelegate;
string operation() { if (hasDelegate is null) return "Default implementation"; return hasDelegate(); }
typeof(this) setDg(string delegate() dg) { hasDelegate = dg; return this; }
}
void main() {
import std.stdio; auto dr = new Delegator; string delegate() thing = () => "Delegate implementation";
writeln(dr.operation()); writeln(dr.operation()); writeln(dr.setDg(thing).operation());
}</lang>
- Output:
Default implementation Default implementation Delegate implementation
Version using 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()
- value: "default implementation"
? def delegator := makeDelegator(def doesNotImplement {}) > delegator.operation()
- value: "default implementation"
? def delegator := makeDelegator(def doesImplement { > to thing() { return "delegate implementation" } > }) > delegator.operation()
- value: "default implementation"</lang>
Elena
ELENA 3.4 :
Using multi methods: <lang elena>import extensions. import system'routines.
interface IOperable {
abstract operate []
}
class Operable :: IOperable {
constructor new [] operate = "delegate implementation".
}
class Delegator {
object theDelegate. set Delegate:object [ if(nil == object) [ theDelegate := nilValue. ]; [ theDelegate := object. ] ] private operate(object operable) = "default implementation". private operate(IOperable operable) = operable operate. operate <= operate(theDelegate).
}
public program [
var delegator := Delegator new. (nil, Object new, Operable new) forEach(:o) [ delegator Delegate := o. console printLine(delegator operate). ].
]</lang> Generic solution: <lang elena>import extensions. import system'routines.
class Operable {
operable = self. operate = "delegate implementation".
}
class Delegator {
object prop Delegate :: theDelegate. constructor new [ theDelegate := nil. ] operate [ // if the object does not support "get&operable" message - returns nil var anOperable := theDelegate operable \ back:nil. if (nil == anOperable) [ ^ "default implementation". ]; [ ^ anOperable operate. ]. ]
}
public program [
var delegator := Delegator new. (nil, Object new, Operable new) forEach(:o) [ delegator Delegate := o. console printLine(delegator operate). ].
]</lang>
- Output:
default implementation default implementation delegate implementation
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>
Forth
Works with any ANS Forth
Needs the FMS-SI (single inheritance) library code located here: http://soton.mpeforth.com/flag/fms/index.html <lang forth>include FMS-SI.f
- class delegate
:m thing ." delegate implementation" ;m
- class
delegate slave
- class delegator
ivar del \ object container :m !: ( n -- ) del ! ;m :m init: 0 del ! ;m :m default ." default implementation" ;m :m operation del @ 0= if self default exit then del @ has-meth thing if del @ thing else self default then ;m
- class
delegator master
\ First, without a delegate master operation \ => default implementation
\ then with a delegate that does not implement "thing" object o o master !: master operation \ => default implementation
\ and last with a delegate that implements "thing" slave master !: master 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>
Io
<lang Io>Delegator := Object clone do(
delegate ::= nil operation := method( if((delegate != nil) and (delegate hasSlot("thing")), delegate thing, "default implementation" ) )
)
Delegate := Object clone do(
thing := method("delegate implementation")
)
a := clone Delegator a operation println
a setDelegate("A delegate may be any object") a operation println
a setDelegate(Delegate clone) a operation println</lang>
- Output:
default implementation default implementation delegate implementation
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>
<lang java>package delegate;
@FunctionalInterface public interface Thingable {
public String thing();
}</lang>
<lang java>package delegate;
import java.util.Optional;
public interface Delegator {
public Thingable delegate(); public Delegator delegate(Thingable thingable);
public static Delegator new_() { return $Delegator.new_(); } public default String operation() { return Optional.ofNullable(delegate()) .map(Thingable::thing) .orElse("default implementation") ; }
}</lang>
<lang java>package delegate;
@FunctionalInterface /* package */ interface $Delegator extends Delegator {
@Override public default Delegator delegate(Thingable thingable) { return new_(thingable); }
public static $Delegator new_() { return new_(() -> null); }
public static $Delegator new_(Thingable thingable) { return () -> thingable; }
}</lang>
<lang java>package delegate;
public final class Delegate implements Thingable {
@Override public String thing() { return "delegate implementation"; }
}</lang>
<lang java>package delegate;
// Example usage // Memory management ignored for simplification public interface DelegateTest {
public static String thingable() { return "method reference implementation"; }
public static void main(String... arguments) { // Without a delegate: Delegator d1 = Delegator.new_(); assert d1.operation().equals("default implementation");
// With a delegate: Delegator d2 = d1.delegate(new Delegate()); assert d2.operation().equals("delegate implementation");
// Same as the above, but with an anonymous class: Delegator d3 = d2.delegate(new Thingable() { @Override public String thing() { return "anonymous delegate implementation"; } }); assert d3.operation().equals("anonymous delegate implementation");
// Same as the above, but with a method reference: Delegator d4 = d3.delegate(DelegateTest::thingable); assert d4.operation().equals("method reference implementation");
// Same as the above, but with a lambda expression: Delegator d5 = d4.delegate(() -> "lambda expression implementation"); assert d5.operation().equals("lambda expression implementation"); }
}</lang>
JavaScript
<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>
Julia
Module: <lang julia>module Delegates
export Delegator, Delegate
struct Delegator{T}
delegate::T
end
struct Delegate end
operation(x::Delegator) = thing(x.delegate) thing(::Any) = "default implementation" thing(::Delegate) = "delegate implementation"
end # module Delegates</lang>
Main: <lang julia>using .Delegates
a = Delegator(nothing) b = Delegator("string")
d = Delegate() c = Delegator(d)
@show Delegates.operation(a) @show Delegates.operation(b) @show Delegates.operation(c)</lang>
- Output:
Delegates.operation(a) = "default implementation" Delegates.operation(b) = "default implementation" Delegates.operation(c) = "delegate implementation"
Kotlin
Whilst Kotlin supports class delegation 'out of the box', the delegate and delegator both have to implement a particular interface and the delegate cannot be optional or null.
The first two scenarios are not therefore strictly possible though the second can be simulated by passing a 'responds' parameter to the delegate class constructor. <lang scala>// version 1.1.51
interface Thingable {
fun thing(): String?
}
class Delegate(val responds: Boolean) : Thingable {
override fun thing() = if (responds) "delegate implementation" else null
}
class Delegator(d: Delegate) : Thingable by d {
fun operation() = thing() ?: "default implementation"
}
fun main(args: Array<String>) {
// delegate doesn't respond to 'thing' val d = Delegate(false) val dd = Delegator(d) println(dd.operation())
// delegate responds to 'thing' val d2 = Delegate(true) val dd2 = Delegator(d2) println(dd2.operation())
}</lang>
- Output:
default implementation delegate implementation
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>
M2000 Interpreter
Delegates in M2000 used with Event Objects. An Event object has a signature for arguments and has a list of functions which we can add or remove, and may have a default function. For adding a function(s) we use Event NameOfEvent New. To remove functions we use Event NameofEvent Drop and the names of functions. Functions insert to list as code, and maybe with a weak reference to object where function belong (a static group). So there is no name to compare, only code. To drop a function we have to use a function with same code.
In first example we make a Group from a Class (a function which return a group). We make two external events and we call methods in group using events objects by reference. All handling done from outside.
The task is to show a call to event when no function defined in event list, then a call to default function (as the second event has a default function), then adding new functions to events we will see how we get feedback using a function from another group.
We cab make Global events so we can use them without passing by reference, but code have global names references and that isn't for universal use.
<lang M2000 Interpreter> Module CheckIt {
Event Alfa { Read X, &Z } Event Beta { Read A$ Function { Print "Default:"; A$ } } Class ObjectA { Private: Aname$="No Name Yet" mX=10, mY=20 Public: Remove { Print "Delete:";.Aname$ } Module MethodA (&eventA){ oldy=.mY Call Event EventA .mX, &oldy If oldy=.mY then Print "Nothing Happen" } Module MethodB (&eventB) { Call Event eventB .Aname$ } Module SetName (.Aname$) {} Class: Module ObjectA { if match("S") then Read .Aname$ } } A=ObjectA("Master") A.MethodA &Alfa 'Nothing Happen A.MethodB Beta 'Default:Master Group CountOne { counter=0 Function ToDelegateA (a, &b){ b+=a .counter++ } } \\ Now add a function to ALfa, twice Event Alfa New CountOne.ToDelegateA(), CountOne.ToDelegateA() For i=1 to 5 : A.MethodA &Alfa :Next i Print CountOne.counter=10 ' true Function DelegateB (a$){ Print "Delegate:";a$ } Event Beta New DelegateB() A.MethodB &Beta 'Default:Master & Delegate:Master Event Beta Drop DelegateB() A.MethodB &Beta 'Default:Master Event Alfa Hold For i=1 to 5 : A.MethodA &Alfa :Next i Print CountOne.counter=10 Event Alfa Release For i=1 to 5 : A.MethodA &Alfa :Next i Print CountOne.counter=20 Event Alfa Clear A.MethodA &Alfa 'Nothing Happen Clear A
} Checkit </lang> In second example we make a Group with event objects as members. One event is private so we have to make some methods (modules inside group) to handle it. The other event is public so we can handle it from outside the object. We can use a module B to copy this (supposed we have in module A the above program), so we can run A and B, from M2000 console and see that they have the same output.
Try this too: Copy Alfa and Beta events in module Checikt below, and change A definition using this A=ObjectA("Master", Alfa, Beta) so now we provide two events (passed by copy), from outside but for this group only.
<lang M2000 Interpreter> Module CheckIt {
Class ObjectA { Private: Aname$="No Name Yet" mX=10, mY=20 Event EventA {} Public: Remove { Print "Delete:";.Aname$ } Event EventB {} Module Hold { Event .EventA Hold } Module Release { Event .EventA Release } Module Clear { Event .EventA Clear } Function AddDelegate { while not empty { read def$ Event .EventA New def$ } =true } Module MethodA { oldy=.mY Call Event .EventA .mX, &oldy If oldy=.mY then Print "Nothing Happen" } Module MethodB { Call Event .eventB .Aname$ } Module SetName (.Aname$) {} Class: Module ObjectA { if match("S") then Read .Aname$ if match("EE") then { Read .EventA, .EventB } Else { Event LocalAlfa { Read X, &Z } Event LocalBeta { Read A$ Function { Print "Default:"; A$ } } .EventA<=LocalAlfa .EventB<=LocalBeta } } } A=ObjectA("Master") A.MethodA 'Nothing Happen A.MethodB 'Default:Master Group CountOne { counter=0 Function ToDelegateA (a, &b){ b+=a .counter++ } } If A.AddDelegate(&CountOne.ToDelegateA(), &CountOne.ToDelegateA()) Then { For i=1 to 5 : A.MethodA : Next i Print CountOne.counter=10 ' true } Function DelegateB (a$){ Print "Delegate:";a$ } Event A.EventB New DelegateB() A.MethodB 'Default:Master & Delegate:Master Event A.EventB Drop DelegateB() A.MethodB 'Default:Master A.Hold For i=1 to 5 : A.MethodA :Next i Print CountOne.counter=10 A.Release For i=1 to 5 : A.MethodA :Next i Print CountOne.counter=20 A.Clear A.MethodA 'Nothing Happen Clear A \\ call Remove function by default
} Checkit </lang>
Mathematica / Wolfram Language
<lang Mathematica>delegator[del_]@operate :=
If[StringQ[del@operate], del@operate, "default implementation"];
del1 = Null; del2@banana = "phone"; del3@operate = "delegate implementation"; Print[delegator[#]@operate] & /@ {del1, del2, del3};</lang>
- Output:
default implementation default implementation delegate implementation
NGS
<lang NGS>{ type Delegator
F init(d:Delegator) d.delegate = null
F default_impl(d:Delegator) 'default implementation'
F operation(d:Delegator) default_impl(d)
F operation(d:Delegator) { guard defined thing guard thing is Fun try { d.delegate.thing() } catch(e:ImplNotFound) { # Might be unrelated exception, so check and optionally rethrow e.callable !== thing throws e default_impl(d) } }
F operation(d:Delegator) { guard d.delegate is Null default_impl(d) }
a = Delegator()
echo(a.operation())
# There is no method thing(s:Str) a.delegate = "abc" echo(a.operation())
# ... now there is method thing(s:Str) F thing(s:Str) 'delegate implementation' echo(a.operation())
}</lang>
- Output:
default implementation default implementation delegate implementation
Objective-C
Classic Objective-C
<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>
Objective-C 2.0, modern runtime, Automatic Reference Counting, Autosynthesize (LLVM 4.0+)
<lang objc>#import <Foundation/Foundation.h>
// Formal protocol for the delegate @protocol DelegatorDelegatingProtocol
- (NSString *)thing;
@end
@interface Delegator : NSObject
@property (weak) id delegate; - (NSString *)operation;
@end @implementation Delegator
- (NSString *)operation { if ([self.delegate respondsToSelector: @selector(thing)]) return [self.delegate thing];
return @"default implementation"; }
@end
@interface Delegate : NSObject
<DelegatorDelegatingProtocol>
@end @implementation Delegate
- (NSString *)thing { return @"delegate implementation"; }
@end
// Example usage with Automatic Reference Counting int main() {
@autoreleasepool { // Without a delegate: Delegator *a = [Delegator new]; NSLog(@"%@", [a operation]); // prints "default implementation"
// With a delegate that does not implement thing: a.delegate = @"A delegate may be any object"; NSLog(@"%@", [a operation]); // prints "default implementation"
// With a delegate that implements "thing": Delegate *d = [Delegate new]; a.delegate = d; NSLog(@"%@", [a operation]); // prints "delegate implementation" } return 0;
}</lang>
Oforth
<lang Oforth>Object Class new: Delegate1
Object Class new: Delegate2 Delegate2 method: thing "Delegate implementation" println ;
Object Class new: Delegator(delegate) Delegator method: initialize := delegate ;
Delegator method: operation
@delegate respondTo(#thing) ifTrue: [ @delegate thing return ] "Default implementation" println ;</lang>
Usage :
<lang Oforth>Delegator new(null) operation Default implementation
Delegator new(Delegate1 new) operation Default implementation
Delegator new(Delegate2 new) operation Delegate implementation</lang>
ooRexx
<lang ooRexx> delegator = .delegator~new -- no delegate say delegator~operation -- an invalid delegate type delegator~delegate = "Some string" say delegator~operation -- a good delegate delegator~delegate = .thing~new say delegator~operation -- a directory object with a thing entry defined d = .directory~new d~thing = "delegate implementation" delegator~delegate = d say delegator~operation
-- a class we can use as a delegate
- class thing
- method thing
return "delegate implementation"
- class delegator
- method init
expose delegate use strict arg delegate = .nil
- attribute delegate
- method operation
expose delegate if delegate == .nil then return "default implementation"
-- Note: We could use delegate~hasMethod("THING") to check -- for a THING method, but this will fail of the object relies -- on an UNKNOWN method to handle the method. By trapping -- NOMETHOD conditions, we can allow those calls to go -- through signal on nomethod return delegate~thing
nomethod:
return "default implementation"
</lang>
OxygenBasic
<lang oxygenbasic> class DelegateA 'not implmenting thing() '============== ' string message
end class
class DelegateB 'implementing thing() '============== ' string message
method thing() as string return message end method ' end class
Class Delegator
'==============
'
has DelegateA dgA
has DelegateB dgB
'
method operation() as DelegateB
dgB.message="Delegate Implementation"
return @dgB
end method
method thing() as string return "not using Delegate" end method ' end class
'==== 'TEST '====
Delegator dgr let dg=dgr.operation print dgr.thing 'result "not using Delegate" print dg.thing 'result "Delegate Implementation" </lang>
Oz
<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($)}}</lang>
Pascal
See Delphi
Perl
<lang perl>use strict;
package Delegator; sub new {
bless {}
} sub operation {
my ($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;
- No delegate
my $a = Delegator->new; $a->operation eq 'default implementation' or die;
- With a delegate that does not implement "thing"
$a->{delegate} = 'A delegate may be any object'; $a->operation eq 'default implementation' or die;
- With delegate that implements "thing"
$a->{delegate} = Delegate->new; $a->operation eq 'delegate implementation' or die; </lang>
Using Moose.
<lang perl> use 5.010_000;
package Delegate::Protocol use Moose::Role;
- All methods in the Protocol is optional
- optional 'thing';
- If we wanted to have a required method, we would state:
- requires 'required_method';
package Delegate::NoThing; use Moose; with 'Delegate::Protocol';
package Delegate; use Moose;
- The we confirm to Delegate::Protocol
with 'Delegate::Protocol'; sub thing { 'delegate implementation' };
package Delegator; use Moose;
has delegate => (
is => 'rw', does => 'Delegate::Protocol', # Moose insures that the delegate confirms to the protocol. predicate => 'hasDelegate'
);
sub operation {
my ($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;
- No delegate
my $delegator = Delegator->new(); $delegator->operation eq 'default implementation' or die;
- With a delegate that does not implement "thing"
$delegator->delegate( Delegate::NoThing->new ); $delegator->operation eq 'default implementation' or die;
- 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>
Phix
Phix is not object orientated, instead I would just use a routine_id for this sort of thing.
I will admit that the whole concept of "no delegate/with one that does not implement" makes no sense whatsoever to me.
While I've shown this using a single rid, you could of course hold an entire sequence of them or even better a dictionary and use named lookups for rids in that.
<lang Phix>enum OTHER, OPERATION
function operation(object o)
integer rid = o[OPERATION] if rid!=NULL then return call_func(rid,{}) end if return "no implementation"
end function
function xthing()
return "default implementation"
end function
function newX()
return {1,routine_id("xthing"),2}
end function
function newY()
object res = newX() res[OTHER] = "something else" -- remove delegate: res[OPERATION] = NULL return res
end function
function zthing()
return "delegate implementation"
end function
function newZ()
object res = newX() -- replace delegate: res[OPERATION] = routine_id("zthing") return res
end function
object x = newX(),
y = newY(), z = newZ()
?operation(x) ?operation(y) ?operation(z)</lang>
- Output:
"default implementation" "no implementation" "delegate implementation"
Obviously, you can explictly test for rid=NULL as shown, or remove that test and catch exceptions, ie: <lang Phix>?operation(x) try -- (since rid=NULL check commented out)
?operation(y)
catch e
?"oops, no implementation"
end try ?operation(z)</lang>
PHP
<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)
- delegate
(dm operation> ()
(if (: delegate) (thing> @) "default implementation" ) )
(class +Delegate)
- 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>
Racket
[object-method-arity-includes?]
tests for the existence of the method in an object.
[is-a?]
can be used to test for a class instance or interface implementor; and is
probably more likely to be used in anger. But
object-method-arity-includes?
can be used generally; and actually
follows the requirement of the task better.
<lang racket>#lang racket
- Delegates. Tim Brown 2014-10-16
(define delegator%
(class object% (init-field [delegate #f]) (define/public (operation) (cond [(and (object? delegate) (object-method-arity-includes? delegate 'thing 0)) (send delegate thing)] [else "default implementation"])) (super-new)))
(define non-thinging-delegate% (class object% (super-new)))
(define thinging-delegate%
(class object% (define/public (thing) "delegate implementation") (super-new)))
(module+ test
(require tests/eli-tester) (define delegator-1 (new delegator%)) (define delegator-2 (new delegator%)) (define non-thinging-delegate (new non-thinging-delegate%)) (define thinging-delegate (new thinging-delegate%)) (test (send delegator-1 operation) => "default implementation" (send delegator-2 operation) => "default implementation" (set-field! delegate delegator-1 non-thinging-delegate) => (void) (set-field! delegate delegator-2 thinging-delegate) => (void) (send delegator-1 operation) => "default implementation" (send delegator-2 operation) => "delegate implementation" (send (new delegator% [delegate thinging-delegate]) operation) => "delegate implementation"))
</lang>
All the tests pass. Believe me.
Ruby
<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>
Rust
Requiring delegates to implement Thingable: <lang Rust>trait Thingable {
fn thing(&self) -> &str;
}
struct Delegator<T>(Option<T>);
struct Delegate {}
impl Thingable for Delegate {
fn thing(&self) -> &'static str { "Delegate implementation" }
}
impl<T: Thingable> Thingable for Delegator<T> {
fn thing(&self) -> &str { self.0.as_ref().map(|d| d.thing()).unwrap_or("Default implmementation") }
}
fn main() {
let d: Delegator<Delegate> = Delegator(None); println!("{}", d.thing()); let d: Delegator<Delegate> = Delegator(Some(Delegate {})); println!("{}", d.thing());
}</lang>
- Output:
Default implmementation Delegate implementation
Using nightly-only specialization feature: <lang Rust>#![feature(specialization)]
trait Thingable {
fn thing(&self) -> &str;
}
struct Delegator<T>(Option<T>);
struct Delegate {}
impl Thingable for Delegate {
fn thing(&self) -> &'static str { "Delegate implementation" }
}
impl<T> Thingable for Delegator<T> {
default fn thing(&self) -> &str { "Default implementation" }
}
impl<T: Thingable> Thingable for Delegator<T> {
fn thing(&self) -> &str { self.0.as_ref().map(|d| d.thing()).unwrap_or("Default implmementation") }
}
fn main() {
let d: Delegator<i32> = Delegator(None); println!("{}", d.thing()); let d: Delegator<i32> = Delegator(Some(42)); println!("{}", d.thing()); let d: Delegator<Delegate> = Delegator(None); println!("{}", d.thing()); let d: Delegator<Delegate> = Delegator(Some(Delegate {})); println!("{}", d.thing());
}</lang>
- Output:
Default implementation Default implementation Default implmementation Delegate implementation
Scala
- Output:
Best seen running in your browser either by ScalaFiddle (ES aka JavaScript, non JVM) or Scastie (remote JVM).
<lang Scala>trait Thingable {
def thing: String
}
class Delegator {
var delegate: Thingable = _
def operation: String = if (delegate == null) "default implementation" else delegate.thing
}
class Delegate extends Thingable {
override def thing = "delegate implementation"
}
// Example usage // Memory management ignored for simplification object DelegateExample extends App {
val a = new Delegator assert(a.operation == "default implementation") // With a delegate: val d = new Delegate a.delegate = d assert(a.operation == "delegate implementation") // Same as the above, but with an anonymous class: a.delegate = new Thingable() { override def thing = "anonymous delegate implementation" } assert(a.operation == "anonymous delegate implementation")
}</lang>
Sidef
<lang ruby>class NonDelegate { }
class Delegate {
method thing { return "delegate implementation" }
}
class Delegator (delegate = null) {
method operation {
if (delegate.respond_to(:thing)) { return delegate.thing }
return "default implementation" }
}
var d = Delegator() say "empty: #{d.operation}" d.delegate = NonDelegate() say "NonDelegate: #{d.operation}" d.delegate = Delegate() say "Delegate: #{d.operation}"</lang>
- Output:
empty: default implementation NonDelegate: default implementation Delegate: delegate implementation
Swift
Allowing the delegate to be any type and taking advantage of dynamism of method lookup: <lang swift>import Foundation
protocol Thingable { // prior to Swift 1.2, needs to be declared @objc
func thing() -> String
}
class Delegator {
weak var delegate: AnyObject? func operation() -> String { if let f = self.delegate?.thing { return f() } else { return "default implementation" } }
}
class Delegate {
dynamic func thing() -> String { return "delegate implementation" }
}
// Without a delegate: let a = Delegator() println(a.operation()) // prints "default implementation"
// With a delegate that does not implement thing: a.delegate = "A delegate may be any object" println(a.operation()) // prints "default implementation"
// With a delegate that implements "thing": let d = Delegate() a.delegate = d println(a.operation()) // prints "delegate implementation"</lang>
Alternately, requiring the delegate to conform to a given protocol:
<lang swift>protocol Thingable : class {
func thing() -> String
}
class Delegator {
weak var delegate: Thingable? func operation() -> String { if let d = self.delegate { return d.thing() } else { return "default implementation" } }
}
class Delegate : Thingable {
func thing() -> String { return "delegate implementation" }
}
// Without a delegate: let a = Delegator() println(a.operation()) // prints "default implementation"
// With a delegate: let d = Delegate() a.delegate = d println(a.operation()) // prints "delegate implementation"</lang>
Tcl
or
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 }
}
- to instantiate a named object, use: class create objname; objname aMethod
- 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"}
- 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()
}
- array of delegates
b = new() b.delegate = new() b.delegate[0] = a b.delegate[1] = c b.x = 3 b.f() b.g()
- single delegate
d = new() d.delegate = a d.x = 7 d.f()</lang>
The resulting output:
3 4 7
zkl
<lang zkl>class Thingable{ var thing; }
class Delegator{
var delegate; fcn operation{ if (delegate) delegate.thing; else "default implementation" }
}
class Delegate(Thingable){ thing = "delegate implementation" }</lang>
<lang zkl> // Without a delegate: a:= Delegator(); a.operation().println(); //--> "default implementation"
// With a delegate:
a.delegate = Delegate(); a.operation().println(); //-->"delegate implementation"</lang> A second example <lang zkl>class [static] Logger{ // Only one logging resource
var [mixin=File] dst; // File like semantics, eg Data, Pipe dst = File.DevNull; // initially, the logger does nothing fcn log(msg){dst.writeln(vm.pasteArgs())}
}</lang>
<lang zkl>Logger.log("this is a test"); //-->nada Logger.dst=Console; Logger.log("this is a test 2"); //-->writes to Console
class B(Logger){ log("Hello from ",self,"'s constructor"); } B(); //-->Hello from Class(B)'s constructor</lang>
The base class B was constructed at startup, so the first Hello went to DevNull as all base classes are created before code runs (base classes are used to create class instances, eg B()).
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