CIS05 Comparative Languages Inheritance and Polymorphism

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CIS05 Comparative LanguagesInheritance and
Polymorphism

• Marc Conrad
• D104a (Park Square Building)
• Marc.Conrad_at_luton.ac.uk
• These slides
• What is Inheritance?
• C example of inheritance.
• Static binding and dynamic linking.

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What is Inheritance?

• http//www.cs.nmsu.edu/rth/cs/cs177/map/inheritd.
  html
• Inheritance is a way of relating two classes so
  that one class may use another class's members
  without redefining them.
• http//www.parashift.com/c-faq-lite/basics-of-in
  heritance.html
• Inheritance is what separates abstract data type
  (ADT) programming from OO programming.

• The basic principle is simple
• A class get the state and behaviour of another
  class and adds additional state and behaviour.

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What is the purpose of Inheritance?

• Specialisation
• Extending the functionality of an existing class
• Generalisation
• Sharing commonality between two or more classes
• Polymorphism
• Implementation of different behaviour to the same
  message (depending on the type of the object).

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Terminology

• Derived class or subclass or child class.
• A class which inherits some of its attributes and
  methods from another class.
• Base class or superclass or parent class.
• A class from which another class inherits.
• Ancestor.
• A classs ancestors are those from which its own
  superclasses inherit.
• Descendant.
• A classs descendants are those which inherit
  from its subclasses.

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(Reminder the Person class)
include ltiostreamgt class Person private
char name20 int yearOfBirth public
Person(char theName, int theYear)
yearOfBirth theYear sprintf(name,
theName) void jump()
stdcout ltlt name ltlt
" jumps" ltlt stdendl int
getAge(int thisYear) return thisYear
- yearOfBirth

• State
• name
• year of birth
• Behaviour
• jump
• get age

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Inheritance - Example

• include "Person.h"
• class Athlete public Person
• private
• int fitness
• public
• Athlete(char theName, int yearOfBirth)
• Person(theName, yearOfBirth)
• fitness 0
• void jump()
• Personjump()
• fitness fitness 1
• stdcout ltlt
• "The Athlete became fitter" ltlt
  stdendl
• void displayFitness()
• stdcout ltlt "The fitness is "
• ltlt fitness ltlt stdendl

• The Athlete inherits state and behaviour from the
  Person class.

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Inheritance - Syntax

• include "Person.h"
• class Athlete public Person
• private
• int fitness
• public
• Athlete(char theName, int yearOfBirth)
• Person(theName, yearOfBirth)
• fitness 0
• void jump()
• Personjump()
• fitness fitness 1
• stdcout ltlt
• "The Athlete became fitter" ltlt
  stdendl
• void displayFitness()
• stdcout ltlt "The fitness is "
• ltlt fitness ltlt stdendl

• The colon indicates an inheritance
  relationship.
• In C an inheritance relationship can be public
  or private.
• A class can inherit more than one class
• class C public B, public A

Further reading multiple inheritance
http//www.parashift.com/c-faq-lite/multiple-in
heritance.htmlfaq-25.5
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Inheritance - Constructors

• include "Person.h"
• class Athlete public Person
• private
• int fitness
• public
• Athlete(char theName, int yearOfBirth)
• Person(theName, yearOfBirth)
• fitness 0
• void jump()
• Personjump()
• fitness fitness 1
• stdcout ltlt
• "The Athlete became fitter" ltlt
  stdendl
• void displayFitness()
• stdcout ltlt "The fitness is "
• ltlt fitness ltlt stdendl

• There is a special syntax for passing constructor
  arguments to the parent class(es).
• Note that the base class is constructed first.

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Inheritance - Overriding of methods

• include "Person.h"
• class Athlete public Person
• private
• int fitness
• public
• Athlete(char theName, int yearOfBirth)
• Person(theName, yearOfBirth)
• fitness 0
• void jump()
• Personjump()
• fitness fitness 1
• stdcout ltlt
• "The Athlete became fitter" ltlt
  stdendl
• void displayFitness()
• stdcout ltlt "The fitness is "
• ltlt fitness ltlt stdendl

• The jump() method is overridden (re-implemented).
  
• Using the ltparent_classgtmethod_name() syntax we
  can call the original method of the parent class.

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Inheritance - Protected Attributes

• include "Person.h"
• class Athlete public Person
• private
• int fitness
• public
• Athlete(char theName, int yearOfBirth)
• Person(theName, yearOfBirth)
• fitness 0
• void jump()
• Personjump()
• fitness fitness 1
• stdcout ltlt
• "The Athlete became fitter" ltlt
  stdendl
• void displayFitness()
• stdcout ltlt "The fitness is "
• ltlt fitness ltlt stdendl

• Note that we cannot call the attribute name of
  the parent class in the child class.
• There is a solution to this make the attributed
  of the parent class protected instead of private.

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Test program for an inherited athlete object.

• include "Athlete.h
• void doExercise(Athlete who, int howMuch)
• for( int i 0 i lt howMuch i )
• who-gtjump()
• int main()
• Athlete fabian("Fabian", 1998)
• doExercise(fabian, 3)
• fabian.displayFitness()
• stdcout ltlt "Fabian is "
• ltlt fabian.getAge(2004) ltlt
• " years old" ltlt stdendl
• return 0

• jump() is defined in Person and overridden in the
  Athlete.
• displayFitness() is defined in Athlete (only).
• getAge() is inherited from Person.

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Dynamic binding vs. Static linkingExample 1

• Consider the following code
• Person pFabian new Athlete("Tom",2000)
• pFabian-gtjump()
• Which jump() method is called?
• a) The jump() method of Person?
• b) The jump() method of Athlete?
• Dynamic binding vs. Static linking

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Dynamic binding vs. Static linkingExample 2

• include "Athlete.h
• void doExercise( Person who, int howMuch)
• for( int i 0 i lt howMuch i )
• who-gtjump()
• int main()
• Athlete fabian("Fabian", 1998)
• doExercise(fabian, 3)
• fabian.displayFitness()
• stdcout ltlt "Fabian is "
• ltlt fabian.getAge(2004) ltlt
• " years old" ltlt stdendl
• return 0

• jump() is defined in Person and overridden in
  Athlete.
• Question 1 which jump() method is called?
• Question 2 Why?

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The keyword virtual
include ltiostreamgt class Person private
char name20 int yearOfBirth public
Person(char theName, int theYear)
yearOfBirth theYear sprintf(name,
theName) virtual void jump()
stdcout ltlt name ltlt
" jumps" ltlt stdendl
int getAge(int thisYear) return
thisYear - yearOfBirth

• The keyword virtual enables dynamic linking of
  the jump() method.

Note that dynamic linking is the default in
Java. In C it must be enabled explicitly.
Further reading www.cs.wustl.edu/schmidt/PDF/C
-dynamic-binding4.pdf
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Polymorphism

• Dynamic linking implements polymorphic behaviour.
  
• Not the compiler decides how a method is executed
  but the object that receives the message.
• Objects of different type respond differently to
  the same message.