Object Oriented Programming Development - Polymorphism

1
Object Oriented ProgrammingDevelopment -
Polymorphism

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

2
Module Outline

• Inheritance
• Aggregation
• Polymorphism
• Multifile Development

• Introduction
• Non object oriented basics
• Classes

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The Meaning of the word.

• From the Greek
• Polus Morphe Polumorphos
• (many ) (shape/form)
• The English word "polymorphe" dates from the 19th
  century and was applied to different animal forms
  arising in the the same species.

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The Meaning of the word.

• In object-oriented computing it means different
  forms of data being handled by the same type of
  process.
• Example The operator has a different meaning
  in the expression 2 3 (add two integers) than
  in 1.7 3.3 (add two floating point numbers)

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Types of Polymorphism

• In Object Oriented Programming there are three
  types of polymorphism
• method overloading, with the special and
  important case of operator overloading
• method overriding
• run-time polymorphism

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Types of Polymorphism

• In Object Oriented Programming there are three
  types of polymorphism
• method overloading, with the special and
  important case of operator overloading
• method overriding
• run-time polymorphism

• Method overloading can also be applied in
  non-object oriented contexts and refers boths to
  functions and methods.

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Types of Polymorphism

• In Object Oriented Programming there are three
  types of polymorphism
• method overloading, with the special and
  important case of operator overloading
• method overriding
• run-time polymorphism

• Method overriding and run-time polymorphism is
  specific to inheritance hierarchies and object
  oriented programming

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Types of Polymorphism

• In Object Oriented Programming there are three
  types of polymorphism
• method overloading, with the special and
  important case of operator overloading
• method overriding
• run-time polymorphism

• Run-time polymorphism, also called dynamic
  binding, or late binding is often considered as
  the object oriented feature of C.

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Method Function Overloading

• Overloading a function simply means, that a
  function is not only defined its name but by its
  name and parameter types.
• The following functions are different in C
• int makeBreakfast(int i, int k)
• void makeBreakfast(Creature who)
• float makeBreakfast()

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Example The Creature class

• class Creature
• private
• int yearOfBirth
• public
• void setYearOfBirth(int year)
• yearOfBirth year
• void setYearOfBirth(Creature other)
• yearOfBirth other.yearOfBirth
• int getYearOfBirth()
• return yearOfBirth

born1997
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Example The Creature class

• class Creature
• private
• int yearOfBirth
• public
• void setYearOfBirth(int year)
• yearOfBirth year
• void setYearOfBirth(Creature other)
• yearOfBirth other.yearOfBirth
• int getYearOfBirth()
• return yearOfBirth

These two methods are different.
born1997
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Example The Creature class
These two methods are different because they have
different argument types.

• class Creature
• private
• int yearOfBirth
• public
• void setYearOfBirth(int year)
• yearOfBirth year
• void setYearOfBirth(Creature other)
• yearOfBirth other.yearOfBirth
• int getYearOfBirth()
• return yearOfBirth

born1997
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Operator Overloading - Motivation

• Question How many function calls are involved in
  the following statement?
• a 2 3

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Operator Overloading - Motivation

• Question How many function calls are involved in
  the following statement?
• a 2 3
• There are two functions implicitly involved
  and .
• Look at this statement as
• assign(a, add(2,3))

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Operator Overloading

• So, operators as , -, , ltlt, , etc. can be seen
  as functions as well. That means we can
  overload operators.
• The C syntax uses function names prefixed
  with operator for overloading operators.

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Overloading Operators - Example

• A Sandwich filling.
• may contain bacon (yes/no).
• a fraction of a lettuce-leaf.
• a number of tomato slices.

• class BLT
• public
• bool bacon
• float lettuce
• int tomatoes
• // Constructor
• BLT(bool b, float l, int t)
• // (more code)

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Overloading Operators - Example

• BLT filling1(true,0.5,2)
• BLT filling2(false,0.2,0)
• ...
• BLT filling3 filling1 filling2
• ...
• / Should give a filling with bacon, 0.7 lettuce
  and 2 tomatoes/

• class BLT
• public
• bool bacon
• float lettuce
• int tomatoes
• // Constructor
• BLT(bool b, float l, int t)
• // (more code)

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Overloading Operators - Example

• BLT filling1(true,0.5,2)
• BLT filling2(false,0.2,0)
• BLT filling3 filling1 filling2
• ...
• / Should give a filling with 3 bacon slices,
  0.7 lettuce and 2 tomatoes /

• class BLT
• public
• bool bacon
• float lettuce
• int tomatoes
• // Constructor
• BLT(bool b, float l, int t)
• // (more code)

This is the operator we want to overload
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Overloading Operators - Example

• // The C Syntax
• BLT operator(BLT y, BLT z)
• bool b y.bacon z.bacon
• float l y.lettuce z.lettuce
• int t y.tomatoes z.tomatoes
• BLT result(b,l,t)
• return result

• class BLT
• public
• bool bacon
• float lettuce
• int tomatoes
• // Constructor
• BLT(bool b, float l, int t)
• // (more code)

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Operator Overloading

• Operators can also be overloaded as methods, e.g.
  the operator
• class BLT
• //
• BLT operator(BLT other)
• bacon other.bacon
• tomatoes other.tomatoes
• lettuce other.lettuce
• //

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Operator Overloading
BLT filling1(true,0.5,2) BLT filling2(false,0.2,0
) filling1 filling2 ... /
Should give a filling with bacon, 0.7 lettuce
and 2 tomatoes/

• Operators can also be overloaded as methods, e.g.
  the operator
• class BLT
• //
• BLT operator(BLT other)
• bacon other.bacon
• tomatoes other.tomatoes
• lettuce other.lettuce
• //

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Operator Overloading

• Operators can also have other types as parameter
  
• class BLT
• //
• BLT operator(int factor)
• tomatoes 2
• lettuce 2
• //

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Operator Overloading
BLT filling1(false,0.5,2) filling1
2 ... / Should give a filling with no
bacon, 1 lettuce and 4 tomatoes /

• Operators can also have other types as
  parameters
• class BLT
• //
• BLT operator(int factor)
• tomatoes factor
• lettuce factor
• //

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Operator Overloading

• The following operators can be overloaded
• new, delete, , -, , /, , , , , , !, , lt,
  gt, , -, , /, , , , , ltlt, gtgt, gtgt,
  ltlt, , !, lt, gt, , , , --, , , -gt. -gt,
  (),
• Note that "" has already a default behaviour.
  When "overloaded" it will be in fact overridden.

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Operator Overloading - Interesting Observation

• cout ltlt Hello World\n

Overloaded ltlt operator
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Operator Overloading - Summary

• Operators may be overloaded to work with user
  defined data types (objects).
• The syntax for overloading involves the
  'operator' keyword and the operator.
• Note In a good design it is important, that the
  normal meanings of operators are not distorted
  (don't subtract with a operator)

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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

• 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)

• 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

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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)

• dynamic binding?

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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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Overriding Methods - The keyword virtual

• Methods in the parent class can be redifined 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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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).
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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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Important stuff to remember

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