A Design Pattern Dictionary

Section 

2

 - The Dictionary

Foil 

9

 - Double Dispatch Idiom

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    Page published 2003/12/11, updated 2009/8/2. Copyright © 2016 by Avner Ben. All rights reserved.

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"Double Polymorphism", "Multi-method" requirement: Functionality distributed between two objects of unknown type (e.g. finding the intersection area between two shapes).

Challenge:

To resolve the multi-method efficiently, (e.g. without runtime searching).

Skill:
View Wirechart

The system shall have the capability to resolve multiple-object functionality among objects...

    UNCONDITIONAL (from outside) UNCONDITIONAL

    Participants:

    Subject:

    The skill to dispatch the operation to the right hand object plus the set of all skills to resolve the operation by the right hand object. (In a dynamically scoped language: May also implement the dispatch.)

    Concrete Left-Side Subject:

    (In a statically scoped language: Dispatches the operation to the right hand object.) The collection of all methods for a particular operation involving concrete subject a on the right hand and all other known subject types.

    Concrete Right-Side Subject:

    (The same, respectively.)

    Signature:

    Left-side Subject responds to messages involving the entire known range of right-side Subject types. Concrete subjects implement this interface.

    Used by:

    Visitor, efficient multi-method implementations.

    Known issues:

    Limited to a "stable hierarchy". (In a statically scoped language:) creates cyclic namespace dependency.

    Scope:

    Languages that do not support multi-methods.