C++/Qt Design Patterns

Ynon Perekynon@ynonperek.comhttp://ynonperek.com

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Good Code Bad Code

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When OO Goes Wrong

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Java Input Validation

• In the past, Java Swing provided only JTextField for input texts

• JNumericTextField quickly appeared by the community

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Java Input Validation

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OO Fail

• Too many classes

• Hard to maintain

• Frustrated Developers

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Enter Design Patterns

• Reusable and tested solutions

• Sustainable for the long run

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Enter Design Patterns

• Started by Christopher Alexander in 1977

• And he was an Architect

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Example: Street Caffe“The street cafe provides a unique setting, special to cities: a place where people can sit lazily, legitimately, be on view, and watch the world go by...”

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Patterns Are Not

• Not a detailed solution

• Not a guide to help you build it

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Patterns Are

• Describing existing and working solutions

• Your job: Apply to your world

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Patterns In Software

• Defined by GoF

• Categorized:

• Creational

• Behavioral

• Structural

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Using Patterns

• Understand

• Memorize

• Apply Repeatedly

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Patterns In Qt

• Plenty !

• We’ll show 2:

• Strategy

• Proxy

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Strategy Pattern

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The Problem

• An input text provides different validation techniques

• Only one is used at any given time

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Approach #1

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Why Is It Bad

• Class Explosion

• Hard to reuse validation code

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The Better Way

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Why Is It Awesome

• Can reuse validators code

• Reduce class explosion

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The Pattern

• A validator encapsulates the validation algorithm

• The general pattern is called Strategy

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Intent

• Define a family of algorithms, encapsulate each one, and make them interchangeable.

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Motivation

• Client code is simpler if the algorithm is external to it

• No need to support an algorithm we don’t use

• We need to make it easy to add new algorithms

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Applicability

• Configure a class with one of many behaviors

• Use different variants of an algorithm

• Eliminate Switch blocks

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Structure

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Participants

• Strategy

• ConcreteStrategy

• Context

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Collaborations

• Strategy + Context = Algorithm

• Context forwards requests to Strategy

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Consequences• Families of related algorithms

• Eliminate switch blocks

• Allow different implementations

• Strategy and Context interface can get complicated

• Increased number of objects

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More Examples

• Layout management

• QAbstractNetworkCache

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Keep In Mind

• A Default Strategy will make everyone happy

• Allow clients to create their own strategies and pass as parameters

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Demo

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Q & A

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Proxy Pattern

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______________ < Let's do COW > -------------- \ ^__^ \ (oo)\_______ (__)\ )\/\ ||----w | || ||

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Example Code

QString s1 = getText();QString s2 = getText();QString s3 = s1;

// "Hello World" is kept in memory only once

s1.append(" And now it's copied");

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The Proxy Way

• A QString is just a proxy to the data

• Implicitly shared

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class MyCow {public:

MyCow &operator=(const MyCow &other) { m_data = other.m_data; } void write() { m_data = new Data( m_data ); m_data.write(); }

private: Data *m_data;};

The Proxy Way

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Proxy Types

• Virtual Proxy

• Protection Proxy

• Remote Proxy

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Proxy: Pattern Details

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Intent

• Provide a surrogate or placeholder for another object

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Motivation

• Save time by lazy loading heavy resources

• Easy Cows

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Applicability

• Access remote resources

• Create resources on demand

• Enforce per-object access control

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Structure

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Participants

• Proxy

• Subject

• Real Subject

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Collaborations

• Proxy forwards requests to Real Subject

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Consequences

• Hide the fact that an object is remote

• Optimize load times (virtual proxy)

• Can use for Copy-On-Write

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Qt Cow

• Not really “By The Book”

• QString has a Data struct

• Does all work by itself

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Qt Patterns

• Patterns are part of any framework (Qt Included)

• Understanding patterns helps us better understand the framework

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Thanks For Listening !

Slides at: http://ynonperek.com

Questions / Comments:ynon@ynonperek.com

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Thanks For Listening

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