Object Oriented Programming Development - Multi File Development

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Object Oriented ProgrammingDevelopment - Multi
File Development

• By
• Marc Conrad Rob Manton
• University of Luton
• Email
• Marc.Conrad_at_luton.ac.uk
• Rob.Manton_at_luton.ac.uk
• Room
• D104

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Module Outline

• Inheritance
• Aggregation
• Polymorphism
• Multifile Development

• Introduction
• Non object oriented basics
• Classes

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Todays lecture

• Polymorphism II recap
• Polymorphic Pointers
• Overriding Methods
• Static typing Dynamic binding
• Multi File Development
• Referring to classes not yet defined
• .H and .CPP files
• Libraries

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Polymorphic Pointers

• In C a pointer of a parent class is allowed to
  point to an object of the child class. E.g.
• class Vehicle
• // ...
• class Car public Vehicle
• // ...
• // ...
• Vehicle vp new Car()

• Valid C syntax!

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Overriding Methods

• Three ways for a derived class to implement a
  polymorphic method
• Inherit it unchanged
• Replace with a different implementation
• Add to the existing implementation

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Overriding Methods

• Methods in the parent class can be redefined in
  the child class.
• class Vehicle
• void move(int i)
• class Car public Vehicle
• void move(int i)
• // ...
• Vehicle vp new Car()
• vp-gtmove(100)

• Valid C syntax!

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Overriding Methods

• Methods in the parent class can be redifined in
  the child class.
• class Vehicle
• void move(int i)
• class Car public Vehicle
• void move(int i)
• // ...
• Vehicle vp new Car()
• vp-gtmove(100)

• BUT
• Which of these two move() methods will be called?

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Overriding Methods

• Methods in the parent class can be redifined in
  the child class.
• class Vehicle
• void move(int i)
• class Car public Vehicle
• void move(int i)
• // ...
• Vehicle vp new Car()
• vp-gtmove(100)

• static typing!
• In C, static typing is the default behaviour.

As vp is of type pointer to a Vehicle, the method
of the Vehicle is called.
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As vp is of type pointer to a Vehicle, the method
of the Vehicle is called.
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Overriding Methods - The keyword virtual

• Methods in the parent class can be redefined in
  the child class.
• class Vehicle
• virtual void move(int i)
• class Car public Vehicle
• virtual void move(int i)
• // ...
• Vehicle vp new Car()
• vp-gtmove(100)

• dynamic binding!
• The keyword virtual allows the use of dynamic
  binding.

As vp points to a Car object the method of the
Car is called
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(No Transcript)
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Dynamic Binding As vp points to a Car object
the method of the Car is called
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Abstract Methods Classes

• Abstract methods are methods without any
  implementation (pure virtual methods).
• class Vehicle
• virtual void move(int i) 0
• class Car public Vehicle
• virtual void move(int i)
• // ...
• Vehicle vp new Car()
• vp-gtmove(100)

• Syntax for declaring abstract methods.

Note that Vehicle objects cannot be instantiated
(but Car objects can).
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Syntax for declaring abstract methods.
We are not allowed to declare an object of an
abstract type...
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Static typing Dynamic binding

• Static typing means that the legality of a member
  function invocation is checked at the earliest
  possible moment by the compiler at compile time.
  The compiler uses the static type of the pointer
  to determine whether the member function
  invocation is legal.

• Dynamic binding means that the address of the
  code in a member function invocation is
  determined at the last possible moment based on
  the dynamic type of the object at run time. It is
  called "dynamic binding" because the binding to
  the code that actually gets called is
  accomplished dynamically (at run time).

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Why is dynamic binding useful?

• If we have a variety of classes based on the same
  base class then each type can be pointed to by a
  pointer to the base class
• animal pAnimal
• pAnimalnew sheep()
• pAnimalnew crocodile()

Animal
mammal
reptile
human
sheep
crocodile
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Why is dynamic binding useful?

• Any method declared in the base class..

Animal
MakeNoise()
mammal
reptile
human
sheep
crocodile
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Why is dynamic binding useful?

• Any method declared in the base class... ..can be
  overridden by any derived class

Animal
MakeNoise()
mammal
reptile
MakeNoise()
MakeNoise()
human
sheep
crocodile
MakeNoise()
MakeNoise()
MakeNoise()
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Why is dynamic binding useful?

• With static binding, the version of the method
  that gets called is the one defined in the base
  class (not so useful!)
• animal pAnimal
• pAnimalnew sheep()
• pAnimal-gtMakeNoise()

Animal
MakeNoise()
mammal
reptile
MakeNoise()
MakeNoise()
human
sheep
crocodile
MakeNoise()
MakeNoise()
MakeNoise()
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Why is dynamic binding useful?

• With dynamic binding, the version of the method
  that gets called is the one defined in the class
  of the object that the pointer points to
• animal pAnimal
• pAnimalnew sheep()
• pAnimal-gtMakeNoise()

Animal
virtual MakeNoise()
mammal
reptile
MakeNoise()
MakeNoise()
human
sheep
crocodile
MakeNoise()
MakeNoise()
MakeNoise()
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Why is dynamic binding useful?

• At compile time we can create a pointer to the
  base class and call a method defined in the base
  class. This should compile properly.
• At run time we can make that pointer point to an
  object of any class in the same inheritance
  hierarchy and the correct version of the method
  gets called.
• Very useful for handling situations where the
  number or composition or lifespan of objects
  cannot be predicted at compile time

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Practical example

• In a computer game we might have an inheritance
  hierarchy containing many different characters
  (old man, woman, alien, spider, goblin, martial
  arts expert etc etc) all derived from a base
  character class.

Character
Update() Draw()
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Practical example
Character
Human
Animal
Old man
Martial arts expert
Alien
spider
Each derived class can override the base class
methods and provide its own update() and draw()
methods
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Practical example

• We dont need to know at compile time how many of
  each type of character will exist at any time
  during the game
• We can have an array of pointers or a linked list
  or other container object to store a variable
  number of character objects
• Character characters50

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Practical example

• At run time we can allocate dynamically created
  objects of any of the classes in the hierarchy to
  the pointers
• eg characters1new Alien()
• characters2new Spider()
• characters3new OldMan()

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Practical example

• Because they are all derived from a standard base
  class we can call the same methods of each
  derived character
• for (i0ilt50i)
• if (Charactersi!NULL)
• Characteri-gtUpdate()
• Characteri-gtDraw()

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Practical example

• Polymorphism in action
• a single method name being called from different
  types of object (old man, spider, martial arts
  expert, alien etc)
• Caters well with situations where number or
  composition or life time of objects cant be
  predicted at compile time..

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Important stuff to remember

• Overriding methods
• Polymorphic pointers
• Static binding
• Dynamic binding - the virtual keyword
• Abstract methods

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Referring to classes not yet defined

1. If a class contains or references another class
   (aggregation or association) then the compiler
   needs the enclosed class to be defined first -
   what happens if we have a circular relationship?

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Referring to classes not yet defined

• what happens if we have a circular relationship -
  which class goes first?
• class Car
• private
• Person myDriver
• class Person
• private
• Car myCar

Car has a pointer to person and person has a
pointer to car
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Referring to classes not yet defined
Person object not defined yet so we get an error
message
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Referring to classes not yet defined

• Solution is to provide a forward declaration of
  the class which is being used but hasnt been
  declared yet
• class Person
• class Car
• private
• Person myDriver
• class Person
• private
• Car myCar

Forward declaration of Person class means that
compiler lets you refer to Person class even
though Person class hasnt been declared yet
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Referring to classes not yet defined
Car has a pointer to person and person has a
pointer to car forward declaration makes
this legal!
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.H and .CPP files

• On larger projects class definitions and
  implementations are usually split up
• definitions go into a .H file
• implementations into a .CPP file
• can do this automatically in the compiler with
  Insert/New Class

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.H and .CPP files
Name of class
Insert/New Class
Optional base class
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.H and .CPP files
.h and .cpp files now appear in project
Skeleton for class implementation
Skeleton for class definition
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.H and .CPP files
If we refer to a class defined in separate file
we get an error
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.H and .CPP files
Need to include the .h header file then all is
OK!
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Using libraries

• Rapid application development - using existing
  modules or libraries rather than writing your own
• Ready made libraries like DirectX, OpenGL,
  Renderware etc comprise of a set of Header files
  (.H) and a set of library files (.LIB)
• A static library (.LIB) is a file containing
  objects that is linked into your program when the
  executable file is built.

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Using libraries

• Dont need to recompile the library files each
  time - saves time
• Preserves security of source code (commercial
  reasons)
• User only needs to know about the interface to
  the classes which is defined in the header file
• follows C notion of separation of interface
  from implementation

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Using libraries

• How do you make a library file?

Static library is an option in the usual File/New
Dialog
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Using libraries

• How do you use an existing library file?
• Insert both the .lib file and the.h header file
  into your project using Project/Add to
  project/Files
• Add the folder that contains the library/header
  files to the list of paths in Tools/Options/Direct
  ories

One list of folders relates to include files
(.h) another to library (.lib) files
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Thats all for today

• First session after Christmas will be revision
  for the exam
• See you then and have a good one..