introduction to programming - bs.informatik.uni-siegen.de fileadrian kacso / r. wismu¨ller / t....

Adrian Kacso / R. Wismuller / T. KielmannBetriebssysteme / verteilte Systeme Introduction to Programming (1) i

Adrian Kacso, Univ. Siegenadriana.dkacsoa@duni-siegena.dde

Tel.: 0271/740-3966, Office: H-B 8406

Stand: June 21, 2010

Introduction to ProgrammingSS 2010

Adrian Kacso / R. Wismuller / T. KielmannBetriebssysteme / verteilte Systeme Introduction to Programming (2) 255

Introduction to ProgrammingSS 2010

9 Inheritance and Polymorphism

9 Inheritance and Polymorphism ...

Contents:

➥ Inheritance

➥ Polymorphism

➥ Abstract classes

➥ Yet another example

9.1 Inheritance

Reminder: classes

➥ A class defines a new type:

➥ a collection of variables (data members , attributes )

➥ a set of related operations (methods , member functions )

➥ they define the interface to objects of the class

➥ i.e., the data members can (usually) be accessed only viathese methods

➥ An object is an instance (a variable) of a class

9.1 Inheritance ...

Often Classes are quite similar

➥ Assume we need classes for cats and dogs:

Cat Dogage age

Attributes weight weight

eat() eat()

Methods Speak() Speak()

purr() wagTail()

➥ Why are these classes so similar?

➥ Because both cats and dogs are special kinds of mammals!

➥ the common attributes / methods are those of mammals

9.1 Inheritance ...

Refinement / specialization

Mammal

DogCatbreed

wagTail()purr()

weightage

Speak()

➥ Class Mammal defines thecommon attributes andmethods

➥ Cat and Dog are derivedfrom Mammal

➥ They inherit all attributesand methods of Mammal➥ they also can define

additional ones

9.1 Inheritance ...

Extending the hierarchy of animal classes ...

Animal

Mammal Fish

CatDog

ageweight

nurseBaby()spawn()

wagTail()purr()

Speak()

9.1 Inheritance ...

Relations between classes

➥ Mammal inherits from Animal,and Dog inherits from Mammal

➥ A Dog is a Mammal which is an Animal

➥ Animal is a (direct) base class (or superclass ) of Mammal,Animal and Mammal are base classes (or superclasses ) of Dog

➥ Mammal is a derived class (or subclass ) of Animal,Dog and Mammal are derived classes (or subclasses ) of Animal

9.1 Inheritance ...

Uses of inheritance

➥ Conceptual Modelling:

➥ define a hierarchy of classes that represent the concepts of thedata➥ specialization / generalization, e.g., cats and dogs as

special kinds of mammals

➥ this mostly defines hierarchies of interfaces

➥ Code re-use:

➥ take a given class, and extend it by a little code to get what youneed

9.1 Inheritance ...

A Mammal class

class Mammal {

public:

Mammal();

~Mammal();

int getAge() const;

void setAge(int);

int getWeight() const;

void setWeight(int);

void Speak() const;

protected: // allow access by derived classesint itsAge;

int itsWeight;

9.1 Inheritance ...

Private vs. protected vs. public

Class members can be private, protected, or public:

➥ private members can be accessed only within methods of theclass itself

➥ protected members can be accessed from within methods of theclass itself, and of its derived classes

➥ public members can also be accessed from other classes (“codeoutside the class itself”)

9.1 Inheritance ...

A Cat class

class Cat : public Mammal {

public:

Cat();

~Cat();

void purr() const;

➥ public class inheritance means that the public members of thebase class are also public for the derived class

➥ (there is also private and protected inheritance, but we will notuse it in this course)

9.1 Inheritance ...

A Dog class

enum Breed {

YORKIE, DANDIE, SHETLAND, DOBERMAN, LABRADOR

class Dog : public Mammal {

public:

Dog();

Dog(Breed breed);

~Dog();

Breed getBreed() const;

void wagTail() const;

protected:

Breed itsBreed;

9.1 Inheritance ...

The interface of Dog (accumulated)

➥ The following methods can be called for a Dog object:

Inherited from Mammal Defined by Dog

int getAge() const; Dog();

void setAge(int); Dog(Breed breed);

int getWeight() const; ~Dog();

void setWeight(int); Breed getBreed() const;

void Speak() const; void wagTail() const;

➥ The constructor and destructor is not inherited !!

9.1 Inheritance ...

The data members of Dog (accumulated)

➥ A Dog object has the following data members:

Inherited from Mammal Defined by Dog

int itsAge; Breed itsBreed;

int itsWeight;

9.1 Inheritance ...

Constructors and destructors

➥ What happens when a Dog is created?

➥ first, the Mammal constructor is called➥ it initializes the data members inherited from Mammal

➥ then, the Dog constructor is called➥ it initializes the data members added by Dog

➥ And when a Dog is deleted?

➥ first, the Dog destructor is called,

➥ then, the Mammal destructor

9.1 Inheritance ...

Passing arguments to base constructors

➥ Assume we have the following constructors for Mammal:

Mammal();

Mammal(int age);

➥ And we want the following ones for Dog:

Dog();

Dog(int age, Breed breed);

Dog(int age, int weight, Breed breed);

➥ How to do this?

9.1 Inheritance ...

Passing arguments to base constructors ...

➥ The Mammal constructors could be implemented like this:

Mammal::Mammal()

: itsAge(1),

itsWeight(5)

Mammal::Mammal(int age)

: itsAge(age),

itsWeight(5)

9.1 Inheritance ...

Passing arguments to base constructors ...

Dog::Dog()

: Mammal(), // could be omitteditsBreed(YORKIE)

Dog::Dog(int age, BREED breed)

: Mammal(age), // base class initializationitsBreed(breed)

Dog::Dog(int age, int weight, BREED breed): Mammal(age),

itsBreed(breed)

itsWeight = weight; // cannot initialize itsWeight}

9.1 Inheritance ...

Overriding methods

➥ Implementation of the Speak method in Mammal:

void Mammal::Speak()

cout << "Mammal sound" << endl;

➥ Mammal m; m.Speak(); ⇒ Mammal sound

➥ Dog d; d.Speak(); ⇒ Mammal sound

➥ Dog inherits the implementation of Speak

➥ but shouldn’t a dog say “Woof”?

➥ Dog should implement the Speak method differently

9.1 Inheritance ...

Overriding methods ...

➥ This can be done by overriding the method:

class Dog : public Mammal {

public:

void Speak() const; // redefine the method// in class Dog

void Dog::Speak() const // and provide a new{ // implementation

cout << "Woof!" << endl;

➥ Dog d; d.Speak(); ⇒ Woof!

9.1 Inheritance ...

Overriding vs. overloading

➥ Overloading➥ define multiple methods with the same name, but different

signature

➥ Overriding➥ define a method in a derived class with the same name and

signature as in the base class

➥ Caution: overriding means hiding!

➥ if you override one of many, overloaded methods (with thesame name), you hide all of them!

9.1 Inheritance ...

Hiding, calling base methods

void Mammal::Move(int distance) {...}

void Mammal::Move(int distance, int angle) {...}

void Dog::Move(int distance) {...}

➥ Now, with Dog d; you can not call d.Move(2,4) anymore

➥ But, you can call d.Mammal::Move(2,4)

➥ And even d.Mammal::Move(2), which calls Mammal’simplementation of Move(int distance)

9.2 Polymorphism

➥ Often we want an object to do something, without knowing theexact class of the object

➥ E.g., if the Cat class implements

void Cat::Speak() const {

cout << "Meow!" << endl;

can we do something like

Mammal *myPets[3]; int i;

myPets[0] = new Dog(DOBERMAN);

myPets[1] = new Cat();

myPets[2] = new Dog(LABRADOR);

cin >> i; myPets[i]->Speak(); // ==> Woof! or Meow!

9.2 Polymorphism ...

Pointers, references and inheritance

➥ If Sub is a subclass of Super, then the following assignments areallowed:

Sub sub;

Super *p = ⊂

Super &r = sub;

➥ I.e., a superclass pointer (reference) can also point (refer) to anobject of a subclass

➥ but not the other way round!

➥ Quiz: what happens in this case?

Sub sub; Super super;

super = sub;

Virtual methods

➥ In C++, if you want the polymorphism to work, the base classmust declare the methods in question as being virtual!

class Mammal {

public:

virtual void Speak() const;

➥ Otherwise, in our example, you would just get Mammal sound

➥ i.e., the method of class Mammal is invoked

How does it work?

itsAgeitsWeight

itsBreed

Dog object

Mammal part

in memory:A Dog object

➥ An object of a derived class consists of

➥ data members of the base class

➥ additional data members of the derived class

How does it work? ...

(seen by d)

Dog *d itsAgeitsWeight

itsBreed

Dog object

Mammal part

➥ A derived class pointer “sees” everything

(seen by m)

Mammal *m

(seen by d)

Dog *d itsAgeitsWeight

itsBreed

Dog object

Mammal part

➥ A base class pointer “sees” just the base class part at thebeginning

v−table

Speak()

Move()...

Mammal::Move(...) { ...}

Dog::Speak() { cout << "Woof!"...}

itsAgeitsWeight

itsBreed

Mammal *m

Dog *d

➥ The object contains a pointer to a virtual method table

➥ this table has an entry for each virtual method, which points tothe proper implementation

➥ Thus, each object “knows” its virtual methods

How does it work? (now with a Cat object)

v−table

Speak()

Move()...

Cat::Speak() { cout << "Meow! ...

itsAgeitsWeight

Mammal *m

Cat *c

in memory:A Cat object

➥ In a Cat object’s virtual method table, the entry for Speak() pointsto Cat::Speak()

➥ Thus, m->Speak() prints: Moew!

How does it work? (now with a Mammal object)

v−table

Speak()

Move()...

Mammal::Speak() { cout << "Mamm ...

itsAgeitsWeight

Mammal *min memory:A Mammal object

➥ In a Mammal object’s virtual method table, the entry for Speak()points to Mammal::Speak()

➥ Thus, m->Speak() prints: Mammal sound

Virtual destructors

➥ When an object is destroyed, we always want to call the destructorof the object’s actual class

➥ e.g., call ~Dog(), even with a pointer to Mammal

➥ In principle, destructors should always be declared as virtual

➥ but virtual makes objects larger and method calls slower

➥ Rule of thumb :

➥ if a class contains a virtual method, the destructor also shouldbe virtual

(assumption: in this case, you refer to objects via a base classpointer or reference)

9.3 Abstract Classes

Reminder: the drawing example

➥ Remember the classes of our drawing program example:

Point Line Rectangle

draw()move()rotate()getX()getY()

rotate()move()draw()

➥ Ideally, we would like to use the methods draw(), move(), androtate() in a polymorphic way, e.g.:

drawingElements[i]->draw();

9.3 Abstract Classes ...

A class hierarchy for the drawing example

Point Rectangle

DrawingElement

getX()getY()

draw()move()rotate()

➥ But, implementing draw(), move(), and rotate() in the baseclass doesn’t really make sense!

The animal hierarchy revisited

Animal

Mammal Fish

CatDog

ageweight

nurseBaby()spawn()

wagTail()purr()

Speak()

➥ Animal and Mammal provideuseful interfaces

➥ But implementing themethods only makes sensefor the bottommost classes➥ think e.g. of this artificial

“Mammal sound” inMammal::Speak()

➥ Animal or Mammal objectsalso don’t make too muchsense

Pure virtual methods

➥ Instead of providing an interface and its implementation, a baseclass also can provide only the interface:

class Mammal {

public:

Mammal();virtual ~Mammal();

...virtual void Speak() const = 0; // pure virtual...

➥ That is, we leave the implementation to the derived classes:

void Dog::Speak() const {

cout << "Woof!\n";}

Abstract classes

➥ In C++, a class is abstract , if it has at least one pure virtualmethod

➥ abstract C++ classes may also implement some methods

➥ An abstract class can not be instantiated

➥ you cannot create an object of an abstract class

➥ But (of course) polymorphism still works

➥ if AC is an abstract class,you can still use pointers (or references ) to AC

to point to objects of (non-abstract) subclasses of AC

➥ (In Java, interfaces serve a similar purpose)

Hierarchies of abstract / concrete classes

➥ We can define Animal and Mammal as abstract classes

➥ they now simply provide common interfaces for all theirsubclasses

➥ If we derive Dog and Cat from Mammal, we make these classesconcrete ,so we can have Dog objects that implement the Mammal interface

The abstract / concrete hierarchy of animals

(defineinterfaces)

weight

Mammal Fish

spawn()Speak()

nurseBaby()

Cat Dog

wagTail()purr()

Animal

ConcreteClasses

(provideimplementations)

ClassesAbstract

9.4 Yet Another Example

A hierarchy of graphics elements

➥ From a previous exam:

Define an abstract class Element for graphics elements.

The abstract class shall have a method circumference thatreturns the circumference of the object as a double value.

9.4 Yet Another Example ...

Class Element

class Element {

public:

virtual ~Element() {}

virtual double circumference() const = 0;

Notes :

➥ No need to declare a constructor

➥ the compiler will create an empty one for us

➥ We are using a short form with ~Element(): you can also define amethod’s body inside the class declaration

➥ not recommended for real programming!

Derive real classes

➥ Implement three classes of graphics elements which are derivedfrom Element:

1. Circle, defined via its radius r.

2. Rectangle, defined via the lengths of its sides a and b.

3. Square, derived from Rectangle, using a = b.

For each class, you need to implement a constructor, the methodCircumference, and the data members.

➥ (Remark: In general, deriving a square class from a rectangleclass is a questionable approach: although each square is arectangle, not every operation on a rectangle is allowed on asquare, e.g., “set the length of one side”)

Class Circle

class Circle : public Element {

public:

Circle(double radius) : itsRadius(radius) {}

double circumference() const {

return 2 * PI * itsRadius;

private:

double itsRadius;

Class Rectangle

class Rectangle : public Element {

public:

Rectangle(double a, double b)

: itsA(a), itsB(b) {}

double circumference() const {

return 2 * (itsA + itsB);

private:

double itsA, itsB;

Class Square

class Square : public Rectangle {

public:

Square(double a) : Rectangle(a,a) {}

Use the Element interface

➥ Implement a function:

double sum(Element const *array[],

unsigned int len)

that computes the sum of the circumferences of the Element

objects in the array.

len gives the length of the array.

Function sum()

double sum(Element const *array[],

unsigned int len)

double retval = 0.0;

//unsigned int i;

for ( unsigned int i=0; i < len; i++) {

retval += array[i]->circumference();

return retval;

9.5 Summary

Inheritance

Mammal

DogCatbreed

wagTail()purr()

weightage

Speak()

➥ Class Mammal defines thecommon attributes andmethods

➥ Cat and Dog are derivedfrom Mammal

➥ They inherit all attributesand methods of Mammal➥ they also can define

additional ones

9.5 Summary ...

A Cat class derived from Mammal

class Mammal {

public:

Mammal(); ~Mammal();

int getAge() const;

protected: // allow access by derived classesint itsAge;

int itsWeight;

class Cat : public Mammal { // public members remain publicpublic:

Cat(); ~Cat();

void purr() const;

9.5 Summary ...

Polymorphism

➥ Often we want an object to do something, without knowing theexact class of the object

➥ E.g., we can write

Mammal *myPets[3]; int i;

myPets[0] = new Dog(DOBERMAN);

myPets[1] = new Cat();

myPets[2] = new Dog(LAB);

cin >> i; myPets[i]->Speak(); // ==> Woof! or Meow!

➥ Caveat : This only works if Mammal declares the Speak-Method asvirtual!

9.5 Summary ...

How do virtual methods work?

v−table

Speak()

Move()...

Dog::Speak() { cout << "Woof!"...}

itsAgeitsWeight

itsBreed

Mammal *m

Dog *d

➥ The object contains a pointer to a virtual method table

➥ This table has an entry for each virtual method, which points tothe proper implementation

➥ Thus, each object “knows” its virtual methods

9.5 Summary ...

Pure virtual methods

➥ Instead of providing an interface and its implementation, a baseclass also can provide only the interface:

class Mammal {

public:

Mammal();virtual ~Mammal();

...virtual void Speak() const = 0; // pure virtual...

➥ That is, we leave the implementation to the derived classes:

void Dog::Speak() const {

cout << "Woof!\n";}

9.5 Summary ...

Reminder: abstract classes

➥ In C++, a class is abstract , if it has at least one pure virtualmethod

➥ abstract C++ classes may also implement some methods

➥ An abstract class can not be instantiated

➥ you cannot create an object of an abstract class

➥ But (of course) polymorphism still works

➥ if AC is an abstract class,you can still use pointers (or references ) to AC

to point to objects of (non-abstract) subclasses of AC

➥ (In Java, interfaces serve a similar purpose)

introduction to programming - bs.informatik.uni-siegen.de fileadrian kacso / r. wismu¨ller / t....

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