Introduction to Design Patterns This lecture will introduce the idea of design patterns What they are and how they are structured We’ll talk about specific patterns several times this semester Today: Iterator and Factory Method After we’ve studied C++ objects: Adapter, Memento, Observer After we’ve studied C++ memory: Singleton, Prototype, Visitor

What’s 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 don’t 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

Iterator Pattern Problem Context Solution core Consequences Example Want to access aggregated elements sequentially E.g., traverse a container of values, objects etc. and print them out Context Don’t want to know/manage details of how they’re 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 list<int>::iterator

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 we’ll use) Consequences Type that’s created matches type(s) it’s used with Example vector<double> v; vector<double>::iterator i = v.begin();