Multiple Dispatching

Alex Tritthart

WS 12/13

Outline

1 Introduction

2 Dynamic DispatchSingle DispatchDouble Dispatch

3 Multiple DispatchExample

4 Evaluation

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Introduction

What is it all about?

Assume

c l a s s Animal { shout ( ) = 0}c l a s s L ion : Animal { shout ( ) {” r o a r ”}}

vo id shoutMethod ( Animal &a ){ a . shout ( ) }

Call

L ion l i o n ( ) ;shoutMethod ( l i o n ) ; // shou ld output ” r o a r ”

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Introduction

What do we need?

We need

no compiling time resolution, but runtime function resolution

no function overloading, but function overriding

This is called:

Dynamic Dispatch

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Introduction

What do we need?

We need

no compiling time resolution, but runtime function resolution

no function overloading, but function overriding

This is called:

Dynamic Dispatch

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Dynamic Dispatch

Dynamic Dispatch

Decision at runtime

Allows instance behaviour

Implemented through virtual tables

Dynamic Dispatch is every Dispatch with:

More than one possibility

Same method signature in super class and subclasses

Choosing method depending on the real types of arguments

There exist single dispatch, double dispatch, triple dispatch, ...

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Dynamic Dispatch Single Dispatch

Single Dispatch

Definition

”The Object-Oriented term for overriding, allowing different methods tobe used depending on the type of a single object.”

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Dynamic Dispatch Single Dispatch

Single Dispatch

Dynamic dispatch with a single type

Used mostly in object oriented languages

Simple to implement

Languages

Java, C#, JS, Python, Ruby can handle single dispatch

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Dynamic Dispatch Single Dispatch

But how to realize?

Virtual Methods

Virtual Tables

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Dynamic Dispatch Single Dispatch

But how to realize?

Virtual Methods

Virtual Tables

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Dynamic Dispatch Single Dispatch

Virtual Methods

Methods which behaviour can get overridden

Methods where the vector address gets determined at runtime

Requires: same class hierarchy and same signature

Example

c l a s s Animal : p u b l i c Animal{ v i r t u a l vo id shout ( ) = 0 ; }

Languages

In C++, Object Pascal an explicit declaration is necessary

In Java, Smalltalk, Python all methods are virtual

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Dynamic Dispatch Single Dispatch

Virtual Function Table

Each function gets an index

Each object gets a hidden pointer to the table

The compiler only generates the routing - so calls can be dissolved

Alternatives

Binary tree dispatch

String in a hash table

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Dynamic Dispatch Single Dispatch

Virtual Function Table

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Dynamic Dispatch Double Dispatch

Double Dispatch

Considers behavior between two objects with subclasses

Examples

Adaptive collision

Event handling systems

Printing figures on different printers

Not straight supported by OO languages like java and C++

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Dynamic Dispatch Double Dispatch

Print all figures on all printers

vo id p r i n tA l l E v e r ywh e r e ( F i gu r e [ ] f i g u r e s , P r i n t e r [ ] p r i n t e r s )f o r ( i : f i g u r e s ) {F i gu r e f i g u r e = f i g u r e s [ i ] ;f o r ( j : p r i n t e r s ) {P r i n t e r p r i n t e r = p r i n t e r s [ j ] ;f i g u r e . p r in tOn ( p r i n t e r ) ;}}

must work for any printer or figure !

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Dynamic Dispatch Double Dispatch

The Problem with Double Dispatch

Assume

F i gu r e f i g u r e ; // d e f a u l t : c i r c l e −f i g u r eSqua r eF i gu r e s q u a r eF i g u r e ;P r i n t e r p r i n t e r ; // d e f a u l t : b lack−white−p r i n t e rCo l o r P r i n t e r c o l o r P r i n t e r ;

Call

s q u a r eF i g u r e . p r in tOn ( p r i n t e r ) ; // squa r e i n b l a c k and wh i t es q u a r eF i g u r e . p r in tOn ( c o l o r P r i n t e r ) ; // squa r e i n c o l o r

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Dynamic Dispatch Double Dispatch

The Problem with Double Dispatch

Assume

F i gu r e& f i g u r e R e f e r e n c e = squa r eF i g u r e ;

Call

f i g u r e R e f e r e n c e . p r in tOn ( p r i n t e r ) ; // squa r e i n b l a c k and wh i t ef i g u r e R e f e r e n c e . p r in tOn ( c o l o r P r i n t e r ) ; // squa r e i n c o l o r

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Dynamic Dispatch Double Dispatch

The Problem with Double Dispatch

Assume

F i gu r e& f i g u r e R e f e r e n c e = squa r eF i g u r e ;P r i n t e r& p r i n t e r R e f e r e n c e = c o l o r P r i n t e r ;

Call

// note the type o f the p o i n t e r and the type o f the o b j e c t .f i g u r e . p r in tOn ( p r i n t e r R e f e r e n c e ) ; // c i r c l e i n c o l o r ?f i g u r e R e f e r e n c e . p r in tOn ( p r i n t e r R e f e r e n c e ) ; // squa r e i n c o l o r ?

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Multiple Dispatch

Multiple Dispatch

Uses runtime types of more than one argument

Generic function chooses the appropriate method

The choice depends on the signature

Multiple Dispatch is a kind of nested instance-of checks

Languages

Common Lisp, Haskell, Dylan, Nice, Perl 6, C# 4.0

With extensions: Python, Perl, Java, C#, .NET

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Multiple Dispatch

Multiple Dispatch Pattern

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Multiple Dispatch Example

Example in C# 4.0:

vo id pr i n tOn Imp l ( F i gu r e x , P r i n t e r y ) {// p r i n t c i r c l e i n b l a c k and wh i t e

}vo id pr i n tOn Imp l ( Squa r eF i gu r e x , P r i n t e r y ){

// p r i n t squa r e i n b l a c k and wh i t e}. . .

vo id pr in tOn ( TopClass x , TopClass y ) {dynamic a = x ;dynamic b = y ;p r i n tOn Imp l ( a , b ) ;

}

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Evaluation

Advantages

Less code to write

no argument preparation

Avoiding asymmetry of single dispatch

Generic function is relevant for several classes

Extending a class becomes nearly trivial

Languages with multiple dispatch use efficient algorithms to resolvecalls

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Evaluation

Disadvantages

Single dispatch occuring most, already useable by most objectoriented languages

Median overhead of 13.7% to the maximum of 47% (study 1996)[4]

There are not that many real cases where double or triple dispatch isused

Different generic functions for one class

But: Development tools help finding all relevant functions

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Evaluation

Thank you!

When should I use multiple dispatch?

Whenever you are implementing equal methods which work with a numberof different objects

Question:

Is there a workaround to get double dispatch in languages with using singledispatch?

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Evaluation

Workaround: VisitorPattern

Add method to printers:

v i r t u a l vo id pr i n tOn Imp l ( F i gu r e& f i g u r e )f i g u r e . p r in tOn (∗ t h i s ) ;

Call

P r i n t e r& p r i n t e r R e f e r e n c e = c o l o r P r i n t e r ;F i gu r e& f i g u r e R e f e r e n c e = squa r eF i g u r e ;p r i n t e r R e f e r e n c e . p r in tOn ( f i g u r e ) ;p r i n t e r R e f e r e n c e . p r in tOn ( f i g u r e R e f e r e n c e ) ;

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Evaluation

Quellenangabe

http://c2.com/cgi/wiki?MultipleDispatch

http://c2.com/cgi/wiki?DoubleDispatchExample

Radu Muschevici, Alex Potanin, Ewan Tempero, James Noble:Multiple Dispatch in Practice http://homepages.mcs.vuw.ac.nz/

~alex/files/MuscheviciPotaninTemperoNobleOOPSLA2008.pdf

The multiple dispatch pattern in its detail

Karel Driesen, Urs Holzle: The Direct Cost of Virtual Function Calls inC++ http://homepages.mcs.vuw.ac.nz/~alex/files/

MuscheviciPotaninTemperoNobleOOPSLA2008.pdf

http:

//java.dzone.com/articles/multiple-dispatch-fix-some

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