Object Oriented Programming Development - Week8

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Object Oriented ProgrammingDevelopment - Week8

• Rob Manton
• Email
• Rob.Manton_at_luton.ac.uk
• Room D104

2
Module Outline

• Introduction
• Non object oriented basics
• Classes

• Inheritance
• Aggregation
• Polymorphism
• Multifile Development

3
Today

• Some advice for next weeks test
• Inheritance recap
• Aggregation

4
Practical Test in Week 9

• Tuesday 26th November in two sittings
• 5-6pm and 6.15-7.15pm
• Will involve writing a simple class
• instantiating objects
• other C constructs as practised in the lab
  sheets to date
• inheritance and or aggregation

5
Last week Inheritance

• Inheritance allows classes to inherit attributes
  and methods from other classes in a
  classification hierarchy

6
Last week Inheritance

• Inheritance allows
• specialisation
• (extending the functionality of an existing
  class)
• generalisation
• (sharing commonality between two or more classes)

7
Last week Inheritance

• Inheritance is appropriate where a class can be
  said to be a kind of other class

A car is a kind of vehicle car class can
inherit from vehicle class
Vehicle
Car
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Last week Inheritance

• Inheritance is appropriate where a class can be
  said to be a kind of other class

A wheel isnt a kind of car. A wheel is a part
of a car - this is dealt with by
aggregation which is this weeks topic
?
Car
Wheel
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Last week Inheritance

• Inheriting from a class doesnt affect the
  integrity of that class - objects of the original
  base class can still be created
• Generalisation allows removal of redundancy and
  duplication among classes
• Some base classes are abstract - they are not
  specific enough to be instantiated but act as
  holders for common attributes and methods of
  derived classes

10
Last week C Syntax

• The colon () operator to denote inheritance
• Public and private derivation of object methods
  from a base class
• The protected keyword to allow derived classes
  to access inherited attributes

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C Syntax public derivation

• Class DerivedClass public BaseClass
• private
• int y
• public
• void setY(int y_in)
• int getY()

A derived class. The colon operator means the
derived class inherits from the base class
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C Syntax public derivation

• Class DerivedClass public BaseClass
• private
• int y
• public
• void setY(int y_in)
• int getY()

the public derivation means that objects of the
derived class can access the public methods and
attributes of the base class This is the most
usual type of derivation
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C Syntax public derivation

• class BaseClass
• private
• int x
• public
• void setX(int x_in)
• int getX()
• class DerivedClass public BaseClass
• private
• int y
• public
• void setY(int y_in)
• int getY()

Main() BaseClass base_object DerivedClass
derived_object base_object.setX(7) derived_obj
ect.setX(12) derived_object.setY(1) return 0
Object of the derived class can access methods of
the derived class and also methods of the base
class
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C Syntax public derivation
Publicly derived class objects of this class
are able to call public methods of the base
class Public derivation is the most common type
15
C Syntax private derivation
Changed to a Privately derived class objects of
this class are NOT able to call public methods of
the base class - this wont compile!
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Last week public/private inheritance

• Public inheritance allows all public methods of
  ancestor classes to be available to objects of a
  derived class

Line
Class ColouredLine public Line . main()
ColouredLine myLine myLine.draw()
Attributes start position end position
Methods draw
Coloured Line
Attributes colour
Object of derived class can access method of base
class
Methods setColour
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Last week public/private inheritance

• Private inheritance prevents objects of the
  derived class from calling the public methods of
  the ancestor class

Line
Class ColouredLine private Line . main()
ColouredLine myLine // myLine.draw()
Attributes start position end position
Methods draw
?
Coloured Line
Attributes colour
Methods setColour
Object of privately derived class cant access
method of base class
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private/public/protected attributes

• Now a quick comparison between public, private
  and protected attributes..

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C Syntax private attributes
We have a privately defined attribute x in the
base class
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C Syntax private attributes
Here we have a method in the derived class
(getIfYEqX) that needs to access an attribute (x)
declared in the base class
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C Syntax private attributes
Because x is declared as private in the base
class, the derived class is not allowed to access
it directly so we get an error and the code wont
compile
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C Syntax public attributes
If we change the attribute to be public then the
method in the derived class can access it but...
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C Syntax public attributes
Doing this goes against the idea of encapsulation
and data hiding users of the class can access
the attributes directly which is a bad idea
usually!
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C Syntax protected attributes
If we change the attribute to be protected then
the method in the derived class can access the
attribute in the base class..
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C Syntax protected attributes
And the notion of encapsulation and data hiding
is preserved because users of the class arent
allowed to access the protected attribute directly
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C Syntax the protected keyword

• So in summary...Protected attributes share some
  features of public and some of private
• Derived classes can access the protected
  attributes of their ancestor classes
• (as if they were publicly defined)
• but external users of the class cant access the
  protected attributes
• (as if they were privately defined)

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C Syntax inheriting constructors

• A derived class always inherits the constructor
  of the base class. The base class constructor is
  called first.
• If the base class constructor takes no parameters
  then the inheritance is implicit - you dont need
  to do anything!
• If the base class constructor takes parameters
  then each derived class needs to declare a
  constructor with the same parameters. You can
  pass the arguments given to the derived class
  constructor to the constructor for the base class

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C Syntax inheriting constructors
Base class declares a constructor that takes a
char pointer parameter

• Class Customer
• Customer (char name_in)
• Class AccountCustomerpublic Customer
• AccountCustomer(char name_in)
• ..

Derived class declares a constructor that takes
the same char pointer parameter
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C Syntax inheriting constructors

• AccountCustomer AccountCustomer(char name_in)
• Customer (name_in)
• //main body of constructor..

In the implementation of the constructor for the
derived class, the parameter passed to
the derived class constructor is passed down to
the base class constructor. Note use of the
colon () syntax once again
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C Syntax inheriting constructors

• class creature
• private
• int yearOfBirth
• public
• creature(int YOB)
• int getYearOfBirth()
• int main()
• creature myCreature(1985)
• cout ltlt "my creature was born in " ltlt
  myCreature.getYearOfBirth() ltltendl
• return 0

This class has a constructor that takes an
integer argument.
When instantiating an object of this class you
pass a parameter to the constructor.
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C Syntax inheriting constructors
Dog class derived from creature class

• class dogpublic creature
• public
• void bark()
• int main()
• creature myCreature(1985)
• dog myDog(1985)
• cout ltlt "my creature was born in " ltlt
  myCreature.getYearOfBirth() ltltendl
• return 0

At the moment we cant do this there is no
constructor for the dog class that takes an
integer argument
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C Syntax inheriting constructors

• class dogpublic creature
• public
• dog(int YOB)
• void bark()
• //implementation for dog constructor
• dogdog(int YOB)
• creature(YOB)
• //other constructor stuff goes here

Now we have defined a constructor that does
take an integer argument
The argument sent to the dog constructor gets
sent to the creature constructor so the
YearOfBirth attribute of the base class gets
set properly
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C Syntax inheriting constructors
Now we do have an appropriate constructor for the
dog class which correctly initialises
the attributes defined in the base class

• class dogpublic creature
• public
• dog(int YOB)
• void bark()
• int main()
• creature myCreature(1985)
• dog myDog(1985)
• cout ltlt "my creature was born in " ltlt
  myCreature.getYearOfBirth() ltltendl
• cout ltlt "my dog was born in " ltlt
  myDog.getYearOfBirth() ltltendl
• return 0

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C Syntax inheriting destructors

• A derived class always inherits the destructor of
  the base class. The derived class destructor is
  called first. This is the reverse of the sequence
  for constructors
• Because destructors never take an argument there
  is no issue with inheritance of destructor
  parameters.

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This weeks new topic

• Associations
• links between objects
• Aggregations
• objects composed wholly or partly of others

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Associations

• Up to now we have made single objects and called
  their methods from the main() function.
• In practice it is likely that we will have many
  objects that will need to communicate with each
  other..

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Types of associations

• Associations can take many forms
• one to one
• one to many
• many to many
• bidirectional
• unidirectional

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Types of associations

• Associations can take many forms
• one to one
• one object of a class has a link to one other
  object of a class
• one to many
• many to many
• bidirectional
• unidirectional

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Types of associations

• Associations can take many forms
• one to one
• one to many
• one object of a class has many links with objects
  of a particular class
• many to many
• bidirectional
• unidirectional

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Types of associations

• Associations can take many forms
• one to one
• one to many
• many to many
• many objects of one class have links with many
  objects of a particular class
• bidirectional
• unidirectional

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Types of associations

• Associations can take many forms
• one to one
• one to many
• many to many
• bidirectional
• messages can be sent in both directions
• unidirectional

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Types of associations

• Associations can take many forms
• one to one
• one to many
• many to many
• bidirectional
• unidirectional
• messages only need to be sent in one direction.
• One object is the actor - acts upon another
• the other is a server - it is only acted upon.
  Booch,1994

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Aggregation

• Aggregations
• Up to now we have made classes that use simple
  data types for their attributes - (int, float,
  char etc)
• Aggregations use objects of other classes to
  define their state

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Terms used to describe aggregation

• Aggregation
• Composition
• Part-Whole
• A part of (APO)
• Has-a
• Containment

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Aggregation

• Aggregation

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Composition vs Aggregation

• Composition Implies that the internal objects are
  not seen from the outside.
• Aggregation implies that some objects have some
  visibility or existence outside of the hierarchy

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Containment

• Containment
• a composition hierarchy defines how an object is
  composed of other objects in a fixed
  relationship. Aggregate object cannot exist
  without its components.
• Eg a car and its engine have a containment
  relationship
• A Container
• an object of a container class which is able to
  contain other objects. Its existence is
  independent of whether it actually contains
  anything. Contained objects will probably be
  dynamic and vary over time.
• Eg a car boot is a container which is able to
  contain some or no objects

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Aggregations

• Aggregations can be
• fixed
• - like in composition - the number and identity
  of contained objects is always the same
• variable
• - as with a container object - the number of
  contained objects may vary at runtime
• recursive
• may contain components of its own type

49
Fixed Aggregation in C

• Fixed aggregation - declare attributes as objects
  rather than standard data types
• class car
• private
• engine myEngine
• light left_headlight
• light right_headlight

Class attributes now include objects of other
classes
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Fixed Aggregation in C

• aggregated class can then call methods of the
  component objects
• void
• carLightsOn()
• left_headlight.switchOn()
• right_headlight.switchOn()

Car class methods can now call methods of the
contained objects
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Fixed Aggregation with parameters

• What if the contained objects require parameters
  for their construction?
• class wheel
• private
• int diameter
• public
• wheel(diameter_in)
• int getDiameter()

Constructor for wheel class takes an
integer argument to define its diameter
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Fixed Aggregation with parameters

• What if the contained objects require parameters
  for their construction?
• class car
• private
• wheel l_front, r_front, l_back, r_back
• engine theEngine
• public
• car (int diameter_in,int enginecc_in)

Constructor for car class takes enough parameters
to construct all the constituent objects
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Fixed Aggregation with parameters

• What if the contained objects require parameters
  for their construction?
• carcar (int diameter_in,int enginecc_in)
• l_front(diameter_in),
• r_front(diameter_in),
• l_back(diameter_in),
• r_back(diameter_in),
• theEngine(enginecc_in)
• //rest of constructor code goes here

Note use of colon() operator to pass parameters
sent to constructor of aggregated object to the
constructors of the constituent objects
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Variable Aggregation in C

• Variable aggregation - declare attributes as
  object pointers rather than standard data types.
  Pointers can be set to NULL to represent no
  object
• class car
• private
• person theDriver NULL
• ..

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Variable Aggregation in C

• Variable aggregation - declare attributes as
  object pointers rather than standard data types.
  Pointers can be set to NULL to represent no
  object
• class car
• private
• person theDriver NULL
• ..

Class attributes now include pointers to objects
of other classes
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Variable Aggregation in C

• caraddDriver()
• theDrivernew person()

We could have a method which instantiates an
object of the contained class and allocates it to
the pointer like this..
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Variable Aggregation in C

• caraddDriver()
• theDrivernew person()
• carremoveDriver()
• delete the Driver
• theDriverNULL

..and deletes it like this
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Variable Aggregation in C

• However, some objects in an aggregation may have
  a lifetime outside of the contained object. In
  this case a pointer to the previously existing
  object needs to be passed to the containing
  object
• caraddDriver(person driver_in)
• theDriverdriver_in

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Variable Aggregation in C

• However, some objects in an aggregation may have
  a lifetime outside of the contained object. In
  this case a pointer to the previously existing
  object needs to be passed to the containing
  object
• caraddDriver(person driver_in)
• theDriverdriver_in

A pointer to a previously existing object is now
available to the aggregated object
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Variable Aggregation in C

• Removing the object from the aggregation simply
  means setting the pointer to null. (the object
  continues to exist in its own right but not as
  part of the aggregation)
• carremoveDriver()
• theDriverNULL

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Variable Aggregation in C

• Removing the object from the aggregation simply
  means setting the pointer to null. (the object
  continues to exist in its own right but not as
  part of the aggregation)
• carremoveDriver()
• theDriverNULL

The car object would now have no driver
pointer but the driver person object would
continue to exist outside of the car
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Implementing associations in C

• forms of Associations
• one to one
• one to many
• many to many
• bidirectional
• unidirectional

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Implementing associations in C

• forms of Associations
• one to one unidirectional
• eg a light switch and a light
• the switch is an actor - it acts upon another
  object Brooch
• the light is a server -it is told what to do
• switch needs a pointer to the light object to be
  able to control it

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Implementing associations in C

• forms of Associations
• one to one unidirectional
• main()
• light bigLight new light()
• button lightButtonnew button(bigLight)
• ..

Create a dynamic light object pass its pointer to
the constructor for the new button object button
object can now call methods of the light object
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Implementing associations in C

• forms of Associations
• one to one bidirectional
• eg between two people objects who become
  married
• two person objects of the same class
• each object needs a pointer to the other object
  (its partner)

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Implementing associations in C

• forms of Associations
• one to one bidirectional
• class person
• private
• person partner
• ..

The person class now contains a pointer
to another object of the person class. Initially
this will be set to NULL, but when the
person gets married the partner pointer
will point to another object of the person class
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Implementing associations in C

• forms of Associations
• one to one bidirectional
• main()
• person Frednew person()
• person Wilmanew person()
• Fred.marry(Wilma)
• Wilma.marry(Fred)
• ..

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Implementing associations in C

• forms of Associations
• one to one bidirectional
• main()
• person Frednew person()
• person Wilmanew person()
• Fred.marry(Wilma)
• Wilma.marry(Fred)
• ..

We can create two dynamic person objects and then
call the marry method from each. This
repetition is necessary to ensure both partners
know who they are married to!
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Implementing associations in C

• forms of Associations
• one to one bidirectional
• a method of establishing both ends of the
  bidirectional link simultaneously is to use the
  this keyword
• All objects in C have a this pointer which
  points to themselves

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Implementing associations in C

• forms of Associations
• one to one bidirectional
• personmarry(person partner_in)
• //establish one way link
• partnerpartner_in
• //now do the other way
• partner-gtmarry(this)
• ..

When the marry method of one object is called
it automatically calls the marry method of its
new spouse!
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Implementing associations in C
Need to avoid an infinite recursive call here!
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Implementing associations in C

• forms of Associations
• multiple associations
• Maintain an array of pointers and an integer
  attribute to record the current number of objects
  referenced.
• CaraddPassenger(person passenger_in)
• if (numPassengerslt5)
• passengernumPassengerspassenger_in

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Implementing associations in C

• forms of Associations
• multiple associations
• Maintain an array of pointers and an integer
  attribute to record the current number of objects
  referenced.
• caraddPassenger(person passenger_in)
• if (numPassengerslt5)
• passengernumPassengerspassenger_in
• ..

numPassengers is an integer attribute of the car
class If the car is not full then add the new
passenger
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Implementing associations in C

• forms of Associations
• multiple associations
• Maintain an array of pointers and an integer
  attribute to record the current number of objects
  referenced.
• caraddPassenger(person passenger_in)
• if (numPassengerslt5)
• passengernumPassengerspassenger_in
• ..

Here we use the postfix increment operator to
update the numPassengers variable
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Summary

• Associations
• links between objects
• Aggregations
• objects composed wholly or partly of others

76
Summary associations

• forms of Associations
• one to one
• one to many
• many to many
• bidirectional
• unidirectional

77
Summary Aggregation

• Composition Implies that the internal objects are
  not seen from the outside.
• Aggregation implies that some objects have some
  visibility or existence outside of the hierarchy

78
Summary Aggregation

• Aggregations can be
• fixed
• - like in composition - the number and identity
  of contained objects is always the same - use
  named automatic objects
• variable
• - as with a container object - the number of
  contained objects may vary at runtime - use
  pointers which can be NULL
• recursive
• may contain components of its own type