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Stéphane DucasseStephane.Ducasse@univ-savoie.frhttp://www.listic.univ-savoie.fr/~ducasse/

Proxy

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License: CC-Attribution-ShareAlike 2.0http://creativecommons.org/licenses/by-sa/2.0/

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Proxy•Category

• Structural

• Intent• Provide a surrogate or placeholder for another

object to control access to it

•Also known as• Surrogate

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Proxy Pattern Motivation

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Proxy Applicability

• A remote proxy provides a local representative for an object in a different address space

• A virtual proxy creates expensive objects on demand

• A protection proxy controls access to the original object, in order to enforce access rights

• A smart reference is a replacement for a bare pointer that performs additional actions when an object is accessed

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Proxy Pattern Structure

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Proxy Pattern Participants• Proxy (ImageProxy)

• maintains a reference that lets the proxy access the real subject

• provides an interface identical to Subject’s so that a proxy can be substituted for the real subject

• controls access to the real subject and may be responsible for creating and deleting it

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Proxy Participants (continued)•Subject (Graphic)

• defines the common interface for RealSubject and Proxy so that a Proxy can be used anywhere a RealSubject is expected

•RealSubject (Image)• defines the real object that the proxy

represents

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Proxy Consequences•Consequences

• A remote proxy can hide the fact that an object resides in a different address space

• A virtual proxy can perform optimizations such as creating an object on demand

• Both protection proxies and smart references allow additional housekeeping tasks when an object is accessed