10 Object Oriented Programming

8/13/2019 10 Object Oriented Programming

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An OO Case Study Copy Constructor and Operators Inheritance virtual Operations static C++ vs. Java

10. Object-oriented Programming

7. Juli 2011


Einf uhrung in die ProgrammierungIntroduction to C/C++, Tobias Weinzierl page 1 of 47
http://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierlhttp://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierl


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Outline

Object Case Study

Brain Teaser

Copy Constructor & Operators

Object-oriented Programming,i.e. Inheritance

Dynamic and Static Polymorphism

The Keyword Static




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10.1. An OO Case Study

Create a Struct Vector

Create a struct Vector with three doubles and an additional double holding thelength of the vector.

s t r u c t Vector {double x [ 3 ] ;double l eng th ;

} ;

Create three vectors a = ( 0 . 2 , 0 . 4) , b = ( 1 . 0 , 0 . 0), and c = ( 1 . 2 , 0 . 0) in yourmain routine. The rst two instances shall be variables, the latter instance shall becreated on the heap.

Vector a , b ;Vector c = new Vector ; / / ass ign va lues now




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Compute the Length

Assume there is an operation void computeLength( Vector & vector) that

computes the length of the passed vector and sets it accordingly.

# i n c l u d e < cmath >

s t r u c t Vector {double x [ 3 ] ;double l eng th ;

} ;

vo id computeLength( Vector& vecto r ) {v e c to r . l e n g t h = s t d : : s q r t ( v e ct o r . x [ 0 ] v e c t o r . x [ 0 ]

+ v e c to r . x [ 1 ] . . . ) ;}

. . .computeLength (a ) ;computeLength( c ) ;

Make void computeLength( Vector & vector) a method of Vector , i.e. changethis piece of code into its object-based variant.




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Methods Instead of Functions


s t r u c t Vector {double x [ 3 ] ;double l e n g t h ;void computeLength ( ) ;

} ;

void Vecto r : : computeLength ( ) {l e n g th = s td : : s q r t ( x [ 0 ] x [ 0 ] + x [ 1] . . . ) ;

}

a . computeLength ( ) ;c> computeLength ( ) ;

Add an operation toTerminal() writing the vector to the terminal.




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Methods Instead of Functions


s t r u c t Vector {double x [ 3 ] ;double l e n g t h ;void computeLength ( ) ;

} ;

void Vecto r : : computeLength ( ) {l e n g th = s td : : s q r t ( x [ 0 ] x [ 0 ] + x [ 1] . . . ) ;

}

a . computeLength ( ) ;c> computeLength ( ) ;

Add an operation toTerminal() writing the vector to the terminal.

vo id Vector : : toTerminal ( ) {s td : : cout


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A Constructor

Add a constructor to Vector which accepts three doubles and automatically

ensures that length holds the right value. Do not use initialisation lists (makes it alittle bit simpler).




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A Constructor

Add a constructor to Vector which accepts three doubles and automatically

ensures that length holds the right value. Do not use initialisation lists (makes it alittle bit simpler).


s t r u c t Vector {. . .Vect or ( double x1 , double x2 , double x3 ) ;

} ;

Vector : : Vector ( double x1 , double x2 , double x3 ) {x [ 0 ] = x1 ; . . .computeLength ( ) ;

}

Make your struct a class and try to manipulate the attribute length of a manuallywithin the main function.

Add a function scale(double value) that scales the vector and automaticallyalso updates the length.

Make computeLength() private, too. Does your code still work? Why?




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Setters and Getters

Write an operation getLength() .

Add a function void getAngle( const Vector& a, const Vector& b) . It is notdened within a class/object but is a plain (old-fashioned) function.




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Setters and Getters

Write an operation getLength() .

Add a function void getAngle( const Vector& a, const Vector& b) . It is notdened within a class/object but is a plain (old-fashioned) function.

Make the operation getLength() a const operation.




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Brain Teaser

c l as s S t r i n g {p r i v a t e :

/ / a i n t a 0 t er mi na t ed s t r i n g char data ; / / l e n g t h o f s t r i n g i n t l e n g t h ;

p u b l i c :S t r i n g ( ) ;void copyFromOther String ( const S t r in g& s t r i n g ) ;

} ;

. . .

S t r i n g : : S t r i n g ( ) :data (0) ,l e n g t h ( 0 ) {}

void St r i ng : : copyFromOtherStr ing ( const S t r i ng& s t r i n g ) {da ta = s t r i n g . data ;l e ng t h = s t r i n g . l e ng t h ;

}




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10.2. Copy Constructor and Operators

C++ generates a couple of default operations for any class/struct to allow you touse these datatypes as you use a built-in datatype.

i n t a ( 1 0 ) ;Date moated( anotherDate ) ;

Sometimes, we have to rewrite these default operations. For the copy constructor, it is very simple.




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Copy Constructor for a String Class

c l as s S t r i n g {p r i v a t e :

/ / a i n t a 0 t er mi na t ed s t r i n g char data ; / / l e n g t h o f s t r i n g i n t l e n g t h ;

p u b l i c :S t r i n g ( ) ;S t r i n g ( const S t ri ng& s t r i n g ) ;

} ;

. . .

S t r i n g : : S t r i n g ( ) :data (0) ,l e n g t h ( 0 ) {}

S t r i n g : : S t r i n g ( const S t ri ng& s t r i n g ) :d at a ( 0 ) ,l e ng t h ( s t r i n g . l e ng t h ) {data = new . . . ;

f o r ( . . . ) {. . .

}}




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Excursus: C++ Strings

Good news: You now know how to write a copy constructor for your personal string

class. Bad news: Such a class does already exist.

# i n c l u d e < s t r i n g >

s t d : : s t r i n g m yS tr in g0 ;s td : : s t r i ng mySt ring1 ( h e l l o w or l d ) ;

s td : : s t r i ng myStr ing2 ( myStr ing1 ) ;s t d : : s t r i n g m yS tr in g3 = h e l l o MPG ;s td : : s t r i ng mySt ring4 = mySt ring3 ;myString . leng th ( ) ;myString4 += myString1 ;



A OO C S d C C d O I h i i l O i i C J


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Default Operations

The following operations are generated automatically as public operations if we

dont provide our own implementation: Default constructor (attributes contain garbage). Destructor (does nothing). Copy constructor (bit-wise copying). Assignment (bit-wise copying).

However, theres still another pitfall/missing thing:

MyStr ing a ; / / do something w i t h a ; M yS tr in g b ( a ) ; / / works M yS tr i ng c ;i f (c==a) { / / does no t work c o r r e c t l y

s td : : cout


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Operators

In C++, operators such as +,-,+=,=,


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Operators RedenedPart I

c l a s s Vec t or {p u b l i c :

/ / copy c o n s t r u c t o r Vecto r ( const Vec to r& v e c t o r ) ;

Vec to r& opera to r=( const Vec to r& v e c to r ) ; / / unary o p e ra to r s Vecto r& opera to r +=( const Vec to r& vec to r ) ;Vecto r& opera to r =( cons t double & v al u e ) ;

void toS tream( s td : : o st ream& ou t ) const ;} ;

/ / b i n a r y o p e ra to r s Vector& opera to r +( const Vector& lhs , const Vec to r& rhs ) ;s td : : o st ream& operato r


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Operators RedenedPart II

c l a s s Vec t or {p u b l i c :

/ / copy c o n s t r u c t o r Vecto r ( const Vec to r& v e c t o r ) ;

Vec to r& opera to r=( const Vec to r& v e c to r ) ; / / unary o p e ra to r s Vecto r& opera to r +=( const Vec to r& vec to r ) ;Vecto r& opera to r =( cons t double & v al u e ) ;

void toS tream( s td : : o st ream& ou t ) const ;} ;

/ / b i n a r y o p e ra to r s Vector& opera to r +( const Vector& lhs , const Vec to r& rhs ) ;s td : : o st ream& operato r


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An OO Case Study Copy Constructor and Operators Inheritance virtual Operations static C vs. Java

10.3. Inheritance

A Case Study I

Image you write a galaxy simulation program, i.e. Gadget-2 does not exist. You have Planet s and Sun s. Due to your brilliant C++ course, you decide to model both as classes. Both of them have a mass, a diameter, and the sun additionally has a temperature.

The planet holds a boolean ag that indicates whether theres life on this planet.

class Sun {p r i v a t e :

double mass ;double d iameter ;double temperature ;

} ;

c l a s s P l a ne t {p r i v a t e :double mass ;double d iameter ;bool l i f e ;

} ;





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y py p p

A Case Study II

class Sun {

p r i v a t e :double mass , d iamete r ;double temperature ;

} ;

c l a s s P l a ne t {p r i v a t e :

double mass , d iamete r ;

bool l i f e ;} ;

Write a member function (method)getWeight() const , and

10. Object-oriented ProgrammingEinf uhrung in die ProgrammierungIntroduction to C/C++, Tobias Weinzierl page 17 of 47



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A Case Study II

class Sun {


} ;



bool l i f e ;} ;


write a function doublegetForce(xxx, xxx) with either a sunor a planet as input parameters. Keepin mind that these parameters shouldbe passed as const references andthat you can use overloading.




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A Case Study II

class Sun {


} ;



bool l i f e ;} ;


write a function doublegetForce(xxx, xxx) with either a sunor a planet as input parameters. Keepin mind that these parameters shouldbe passed as const references andthat you can use overloading.

Is it good code (code duplication)?




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Case Study Rewritten

The planet and the sun have something in common. They both are celestialbodies.

Imagine there is something like a celestial body in C. A Planet then is an extension (specialisation) of CelestialBody . A S then is an extension (specialisation) of CelestialBody , too. Sun and Planet are two different things. Both have in common that they have a weight and a diameter.




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is aRelations

Planet has all operations and attributes CelestialBody has. Sun has all operations and attributes CelestialBody has.

Every operation expecting an instance of CelestialBody could also be passed aninstance of Sun . Every operation expecting an instance of CelestialBody could also be passed an

instance of Planet . Again, we could image this to be an extension of a cut-n-paste mechanism.




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is aRelations in Source Code (Part I)

c l a s s C e l e s t ia l B o d y {p r i v a t e :

double mass , d iamete r ;p u b l i c :

double getMass () const ;} ;

c l as s Sun : p u b l i c C e le s ti a l Bo d y {p r i v a t e :

double temperature ;} ;

c l as s P l an et : p u b l i c C e le s ti a l Bo d y {p r i v a t e :

bool l i f e ;} ;

. . .

P l a ne t e a r t h ;ear th . getMass ( ) ; / / t h at s f i n e




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is aRelations in Source Code (Part II)

c l a s s C e l e s t ia l B o d y {

p r i v a t e :double mass , d iamete r ;p u b l i c :

double getMass () const ;} ;

c l as s Sun : p u b l i c C e le s ti a l Bo d y {p r i v a t e :

double temperature ;p u b l i c :void explode ( ) ;

} ;

c l as s P l an et : p u b l i c C e le s ti a l Bo d y {p r i v a t e :

bool l i f e ;

} ;void foo ( const C e l e s t i a l B od y& a ) {

a . getMass ( ) ; / / t h at s f i n e a . explode ( ) ; / / a r rg h

}




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Object-oriented Programming

is-a relations are mapped to inheritance .

Object-based programming becomes object-oriented programming due to thisinheritance concept.

Whenever an object of type A is expected somewhere, we can also pass an objectwith a subtype of B . This concept is called polymorphism .

Theres a new visibility mode protected : These attributes and operations behavelike private , i.e. they are not visible from outside, but they are visible within

subclasses.




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Super Constructors

c l a s s A {

p u b l i c :A( i n t v ar i ab le ) ;} ;

c la ss B : p u bl i c A {p u b l i c :

B ( ) ;} ;

. . .

B : : B ( ) :A ( 2 3 ) {

}

This is the reason, C++ introduced initialisation lists, and treats both attributes andsuper types the same way. Please take care of the order: First, you have to call thesupertypes constructor, then you have to initialise your attributes in the right order.




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Case Study RevisitedWrite Your First Inheritance


p r i v a t e :double mass , d iamete r ;p u b l i c :

Celes t ia lBody ( double mass);double getMass ( ) const ;

} ;c l as s Sun : p u b l i c C e l es t i al B o dy {

p r i v a t e :

double temperature ;p u b l i c :Sun( double mass);

} ;c l as s P l an et : p u b l i c C e l es t i al B o dy {

p r i v a t e :bool l i f e ;

p u b l i c :

Planet ( double mass);} ;double computeForce (

const C e l e s t i a l B o d y & a ,const C e l e s t i a l B od y& b ) { / / your jo b

}




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Inheritance Summarised

Inheritance represents is-a relations.

It enables us to represent/model the real world more accurately than just plainstructs and functions/methods. It helps us to avoid code duplication. We can (without overloading) write functions for a whole set of different structs. We can extend existing structs without toughing them due to an additional

subclass.




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10.4. virtual Operations

c l a s s A {p u b l i c :

void f oo ( ) ;} ;


void f oo ( ) ;} ;

c la ss C : p u bl i c B {p u b l i c :void f oo ( ) ;

} ;

A o bj ec t1 ;B o bj ec t2 ;C o bj ec t3 ;

A o b j e c t4 = new A ( ) ;A o b j e c t5 = new B ( ) ;B o b j e c t6 = new C ( ) ;

o b j ec t 1 . f oo ( ) ; o b je c t2 . f oo ( ) ; o b je c t 3 . f oo ( ) ;object4 > f oo ( ) ; o bj ec t5 > f oo ( ) ; o bj ec t6 > f oo ( ) ;




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Static Polymorphism

c l a s s A {

p u b l i c :void f oo ( ) ;} ;


void f oo ( ) ;} ;

A o b j e c t4 = new A ( ) ;

object4 > f oo ( ) ;

C++ implements a static polymorphism by default.

It is called static, as the (known) object type determines which operation is to beinvoked. There is a variant called dynamic polymorphism , too. Java, C# support only dynamic polymorphism .




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Dynamic Polymorphism

c l a s s A {

p u b l i c :void f oo ( ) ;v i r t u a l void b ar ( ) ;

} ;


void f oo ( ) ;

v i r t u a l void b ar ( ) ;} ;

A o b j e ct 1 = new A ( ) ;B o b j e ct 2 = new B ( ) ;A o b j e ct 3 = new B ( ) ;

object1 > f o o ( ) ;

object2 > f o o ( ) ;object3 > f o o ( ) ;object1 > b ar ( ) ;object2 > b ar ( ) ;object3 > b ar ( ) ;




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Dynamic Destructors

If you use inheritance, always make your destructor virtual .




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Abstract Operations

c l a s s A {

} ;c la ss B : p u bl i c A {} ;

c la ss C : p u bl i c A {} ;

Let B and C be classes that belong together logically. An example: B writesmeasurements into a database, C plots data to Matlab.

However, B and C have to implementation in common. They share solely thesignature.

Consequently, all operations (in A) have to be virtual.

However, it does not make sense to provide any default implementation of anoperation in A.




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Abstract Operations

c l a s s A {

v i r t u a l void f oo ( ) = 0 ;} ;

c la ss B : p u bl i c A {v i r t u a l void f oo ( ) ;

} ;

c la ss C : p u bl i c A {

v i r t u a l void f oo ( ) ;} ;

virtual operations can be made abstract . A class with at least one abstract method is an abstract class . We cannot instantiate abstract types. This way, we enforce subclasses to implement them. Classes with solely abstract operations are called interface .




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Case Study RevisitedAbstract Operations


. . .p u b l i c :v i r t u a l void plotToCout ( ) const = 0 ;

} ;c l as s Sun : p u b l i c C e le s ti a l Bo d y {

. . .p u b l i c :

v i r t u a l void plotToCout ( ) const ;

} ;c l as s P l an et : p u b l i c C e le s ti a l Bo d y {. . .

p u b l i c :v i r t u a l void plotToCout ( ) const ;

} ;

Ce les t i a lBody a = new C e l e st i a l B o d y ( ) ;

Ce les t i a lBody b = new Sun ( . . . ) ;a> p lo tToCout ( ) ;b> p lo tToCout ( ) ;

Implement plotToCout() for both subclasses. Does the code snippet from abovework?




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10.5. static

Variable Lifecycle

void foo ( ) {i n t myInt ;. . .

}

void bar ( ) {s t a t i c i n t myInt ;

. . .}

Variables belong to a scope. Whenever we encounter a variable denition, we reserve memory. At the end of the scope, the variable is destroyed (invoke destructor, free memory).




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Static Variables

void foo ( ) {

i n t myInt ;. . .}

void bar ( ) {s t a t i c i n t myInt ;. . .

}

Variables belong to a scope. Whenever we encounter a static variable denition for the rst time , we reserve

memory. A static variable is freed when the application terminates (invoke destructor, free

memory).



l f bl


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An Example for Static Variables

void bar ( ) {

s t a t i c i n t m y In t = 2 0;myInt++;

}

. . .b ar ( ) ; / / f i r s t c a l l o f b ar ( ) e ve r b ar ( ) ;

b ar ( ) ;

Static variables are bind to the application, not to the scope. However, they are still accessible only within the scope. Usually, (good?) procedural programmers do not use static.



Obj V i bl


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Object Variables

c l a s s A {

i n t f i e l d ;void f o o ( ) ;} ;

void A : : f o o ( ) { f i e l d + +; }

A ins tance1 , in s tance2 ;ins tance1 . foo ( ) ;

ins tance2 . foo ( ) ;

Both instances work on different instantiations of field .



Cl V i bl (St ti )


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Class Variables (Static)

c l a s s A {

s t a t i c i n t f i e l d ;void f o o ( ) ;} ;

void A : : f o o ( ) { f i e l d + +; }

A ins tance1 , in s tance2 ;ins tance1 . foo ( ) ;

ins tance2 . foo ( ) ;A : : f i e l d ++;

Here, static binds the attribute to the class. It is a class attribute . Depending on thevisibility, one can either access the static attribute within any operation of A, or evenoutside of the class (not recommended as it violates the encapsulation idea). If the eld

is public, the class acts (from a syntax point of view) similar to a namespace.



E l I t C t


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Example: Instance Counter

c l a s s A {

p r i v a t e :s t a t i c i n t ins tance s ;p u b l i c :

A ( ) ;} ;

void A : : A ( ) {in s tances++;

}

This code keeps track how many instances of A are created.



Static Operations


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Static Operations

c l a s s A {

p u b l i c :void f oo ( ) ;s t a t i c voi d b ar ( ) ;

} ;

void A : : f o o ( ) { . . . }void A : : bar ( ) { . . . }

We can also declare methods as static . Static methods (class methods) belong to the class, not to an object. Static methods may work on class attributes only.



Case Study Revisited Static Operations


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Case Study RevisitedStatic Operations


. . .p u b l i c :s t a t i c i n t getNumberOfCallsToGetMass ( ) ;double getMass () const ;

} ;c l as s Sun : p u b l i c C e le s ti a l Bo d y {

. . .} ;

c l as s P l an et : p u b l i c C e le s ti a l Bo d y {. . .} ;

Ce les t i a lBody a = new P l an e t ( ) ;Ce les t i a lBody b = new Sun ( . . . ) ;s td : : cout


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10.6. C++ vs. Java

Multiple Inheritance


void f oo ( ) ;} ;c l a s s B {

p u b l i c :void b ar ( ) ;

} ;

c l as s C: p u b l i c A , B {} ;

C myC;myC. foo ( ) ; / / works myC. bar ( ) ; / / works

Multiple inheritance is supported by C++. Java does not have such a concept. Often, it is considered to be a bad smell , while inheriting from multiple interfaces is

not a bad smell. However, it is sometimes useful; in particular if it reects ideas of (AOP).




Private Inheritance


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Private Inheritance


void f oo ( ) ;} ;

c la ss B : p r i v at e A {p u b l i c :

void b ar ( ) ;} ;

void B : : bar ( ) { f oo ( ) ; } / / works

B myB;myB. bar ( ) ; / / works myB. foo ( ) ; / / doesn t work

Private inheritance is supported by C++. Private downgrades the visibility of the super class. Here, the is-a relationship does not hold anymore, i.e. it is a pure implementation

inheritance, not a behavioural. However, today it is considered to be a bad smell . Java only supports public inheritance.




Explicit Call of Supertype


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Explicit Call of Supertype



c la ss B : p r i v at e A {p u b l i c :


void A : : f o o ( ) { / / do something }

void B : : f o o ( ) { A : : f oo ( ) ; }

B myB;myB. foo ( ) ;

We still can access the supertypes implementation, but only inside the subclass not from the outside. This is sometimes useful if we extend a function, i.e. add behaviour to an already

existing function (such as tracing).




Ambiguity I


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Ambiguity I

c l a s s B {i n t m y A t t r i b u t e ;void foo ( ) {

/ / wo rk s w i t h m y A t t r i b u t e }

} ;c l a s s C {

i n t m y A t t r i b u t e ;void bar ( ) {

/ / wo rk s w i t h m y A t t r i b u t e }

} ;c l as s D: p u b l i c B ,C {

void t a r ( ) {myAt t r i bu te ++; / / doesn t work

B : : m y A t t r i b u t e + +; C : : m y A t t r i b u t e + +; / / sh ou ld work }

} ;

D myD;myD. foo ( ) ;myD. bar ( ) ;

Class D holds two myAttribute instances.




Ambiguity III


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Ambiguity III

c l a s s A {i n t m y A t t r i b u t e ;

} ;c la ss B : p u bl i c A {

void foo ( ) { / / wo rk s w i t h m y A t t r i b u t e

}} ;c la ss C : p u bl i c A {

void bar ( ) { / / wo rk s w i t h m y A t t r i b u t e

}} ;c l as s D: p u b l i c B ,C {} ;


Question of the day: How many myAttribute instances does an object of type D have?




Virtual Inheritance


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c l a s s A {i n t m y A t t r i b u t e ;

} ;c l as s B : v i r t u a l p ub li c A {

void foo ( ) { / / wo rk s w i t h m y A t t r i b u t e

}} ;c l as s C: v i r t u a l p ub li c A {

void bar ( ) { / / wo rk s w i t h m y A t t r i b u t e

}} ;c las s D: v i r t u a l p ub l i c B ,C {} ;


Theres also a concept called virtual inheritance , i.e. at runtime the system identies which attributes do exist.




Missing C++ Topics


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g p

Generic programming Idioms of the standard library (iterators, e.g.) Standard library RTTI Exception handling (Design) patterns Outlook: Parallel programming

And then theres due to the course open issues, questions, and feedback.



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10 Object Oriented Programming

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