OOP, Inheritance and Polymorphism Lecture 6

Object relations  Inheritance is ‘a kind of’, ‘a type of’ e.g. a revolver is a type of gun  Aggregation ‘comprises’, ‘consists’, ‘ is made of’ etc. e.g. Our gun has a chamber, Our player has an inventory, Our map consists of locations  Association is related to by some action e.g. musician plays the piano, cowboy fires a gun

Composition  Clock comprises of Case Clock face Minute hand Hour hand Works  It is not a clock without these objects!

Inheritance  Inheritance allows code reuse by enabling the creation of sub-classes based on base classes  Syntax of inheritance: class subclass: access_level baseclass1,access_level baseclass2{};  Class Dog : public Mammal, public Canine  Avoid multiple inheritance

Inheritance  When a sub class is instantiated the base class constructor is called by default  The access level describes the maximum access level that members of the sub class will have  The default access level is private for classes and public for structs

Sub classes  Do not inherit Constructors Destructors Copy constructors Friends

Protected  Private members are only accessible in the base class  Protected members are accessible in the child class  Non-static protected variables are accessible to: Friends Methods of a child class

Base class  Virtual functions Anything that is allowed to be overridden Destructor Tells the compiler to check the virtual function table  Pure virtual function

Derived class  Virtual functions As someone might want to use your class  Initializer lists  MyClass : ParentClass(), x(a), y(b)

Advantages  We can store an array of Parent class objects  We don’t have to rewrite code

Access levels  Public Inherit all parent’s public attributes and methods that are public AS public  Private Inherit all parent’s public attributes and methods that are public AS private  Protected Inherit all parent’s public attributes and methods that are public AS protected

Pure virtual methods  virtual void show () = 0;  No implementation of function in the.cpp  Allows us to create an interface for a class  The interface can not be instantiated

 Aircraft is an interface for types of Aircraft

 We still use parent class pointers even if the class is abstract i.e. Aircraft* plane = NULL;  These statements are permitted plane = new Fighter(); plane = new Bomber();  Compiler error plane = new Aircraft();

Polymorphism  A base class pointer can reference a derived class  However it can only access members the derived class inherits  Bar is a Foo  Foo f has been instantiated (Bar*) f; dynamic_cast (f); f->NextLine(); //Calls Bar’s function!

Let’s dive into some code  Time to see some epic typos!

Inheritance architecture  Clock is a base object  Clock has a Hand class  Is Hand also a base object?

Plane is a BaseObject

The issue with inheritance

Component architecture

BaseObject* clock = new BaseObject(); clock->addComponent(BigHand); clock->addComponent(SmallHand); clock->addComponent(Face); clock->addComponent(Engine);

Component abstract class