Introduction to Design Patterns

1
Introduction to Design Patterns

• This lecture will introduce the idea of design
  patterns
• What they are and how they are structured
• Well talk about specific patterns several times
  this semester
• Today Iterator and Factory Method
• After weve studied C objects Adapter,
  Memento, Observer
• After weve studied C memory Singleton,
  Prototype, Visitor

2
Whats a Design Pattern?

• A design pattern has a name
• So when someone says Adapter you know what they
  mean
• So you can communicate design ideas as a
  vocabulary
• A design pattern describes the core of a solution
  to a recurring design problem
• So you dont have to reinvent known design
  techniques
• So you can benefit from others (and your) prior
  experience
• A design pattern is capable of generating many
  distinct design decisions in different
  circumstances
• So you can apply the pattern repeatedly as
  appropriate
• So you can work through different design problems
  using it

3
Iterator Pattern

• Problem
• Want to access aggregated elements sequentially
• E.g., traverse a container of values, objects
  etc. and print them out
• Context
• Dont want to know/manage details of how theyre
  stored
• E.g., could be in an array, list, vector, or
  deque
• Solution core
• Provide a common interface for iteration over a
  container (1) start (2) access (3)
  increment (4) termination
• Consequences
• Frees user from knowing details of how elements
  are stored
• Decouples containers from algorithms (crucial in
  C STL)
• Example
• listltintgtiterator

4
Factory Method Pattern

• Problem
• You want a type to create a related type
  polymorphically
• E.g., a container should create appropriate begin
  and end iterators
• Context
• Each type knows which related type it should
  create
• Solution core
• Polymorphic creation
• E.g., abstract method that different types
  override
• E.g., provide traits and common interface (as in
  the STL well use)
• Consequences
• Type thats created matches type(s) its used
  with
• Example
• vectorltdoublegt v vectorltdoublegtiterator i
  v.begin()