Design PatternsPart IV

(TIC++V2:C10)

Yingcai Xiao

10/01/08

Design Patterns (What?)

• The original paper by the gang of four http://www.cse.msu.edu/~cse870/Materials/Patterns/Docs/orig-patterns-paper.pdf

• Design Patterns are devices that allow designers to share knowledge about their design. Design patterns identify, name, and abstract common themes in object-oriented design.

• idea reuse vs. code reuse.

Characteristics of a Design Pattern

• smart: elegant solutions that would not occur to a novice immediately

• generic: independent of specific system characteristics

• well-proven: identified from successful real, object-oriented systems

• simple: involve only a handful of classes

More Characteristics

• reusable: reuse at the design level, generic, well-documented

• object-oriented: uses classes, objects, generalization, and polymorphism

Constructing a Design Pattern

• name

• problem description: when pattern is to be used and which problem it attempts to solve

• solution: classes and objects, their structure, and dynamic collaboration

• consequences: results and trade-offs of applying the pattern

Types of Patterns (Purpose)

Categorize patterns by what they do.

• creational: deal with the process of object creation

• structural: deal with the composition of classes or objects

• behavioral: describe ways in which classes or objects interact and allocate responsibilities

Types of Patterns (Scope)

• Scope specifies whether a pattern applies primarily to classes or to objects.

• Class scope:: deals with relationship between classes, established through inheritance. (static)

• Object scope: deals with object relationship, established through inclusion and usage. (dynamic)

Design Pattern Space

Example: Proxy Pattern• Proxy: provides a surrogate to hide the real object behind.• Applications:

1. remote proxy: to represent a remote object locally for easy and efficient coding (e.g. Java RMI and .NET Remoting)

2. protection proxy: to control the access to the real object (e.g. a proxy server hides the real server behind the firewall)

3. virtual proxy: to defer the expansive actions creating the real object.

4. smart reference: replacement for bare pointer that performs additional actions when an object is accessed

Example: Virtual Proxyfrom the original paper by the gang of four

using OMT Notation (object diagram)

Referes to Referes to (virtual representation)

Class Diagram of the Virtual Image Proxy

General Structure of a Proxy

Class Diagram

Object Diagram

Components of a Proxy

• Proxy:– maintains a reference to let proxy access the

real subject– provides an interface identical to Subject’s so a

proxy can be substituted for the real subject– controls access to the real subject; may be

responsible for creating and deleting it

More Participants

• Subject:– defines the common interface for RealSubject

and Proxy so a Proxy can be used anywhere a RealSubject is expected

• RealSubject:– defines the real object that the proxy represents

Collaborations

• Proxy forwards request to RealSubject when appropriate, depending on the kind of proxy

Sequence Diagram

Collaboration Diagram

Consequences

• proxy pattern introduces a level of indirection when accessing an object– 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– protection proxies and smart references allow

additional tasks when an object is referenced

ImplementationA proxy can exploit the following features:• Java: use interface and implementation.• C++: use virtual functions and overloading

the member access operators. • Smalltalk: use doesNotUnderstand, which

supports automatic forwarding of requests• Proxy doesn’t have to know the type of the

real object (upcasted to Object in Java)

Sample Code in Javapublic class Proxy implements Subject {

RealSubject refersTo;

public void Request ( ) {

if (refersTo = = null)

refersTo = new RealSubject ( );

refersTo.Request ( );

}

}

Known Uses

• Stubs in Java RMI.

• Proxy server in networking

• NEXTSTEP uses proxies as local representatives for objects that may be distributed

• Proxies in Smalltalk to access remote objects

Related Patterns

• adapter: provides a different interface to the object it adapts; proxy provides the same interface as its subject

• decorator: adds one or more responsibilities to an object; proxy controls access to an object

• a protection proxy might be implemented exactly like a decorator

Proxy Example in C++

class ProxyBase {public: virtual void f() = 0; virtual void g() = 0; virtual void h() = 0; virtual ~ProxyBase() {}}; class Implementation : public ProxyBase {public: void f() { cout << "Implementation.f()" << endl; } void g() { cout << "Implementation.g()" << endl; } void h() { cout << "Implementation.h()" << endl; }};

Proxy Example in C++

class Proxy : public ProxyBase { ProxyBase* implementation;public: Proxy() { implementation = new Implementation(); } ~Proxy() { delete implementation; } // Forward calls to the implementation: void f() { implementation->f(); } void g() { implementation->g(); } void h() { implementation->h(); }}; int main() {

Proxy p; p.f(); p.g(); p.h();

} ///:~